1. Introduction
During February 2019, a quasi-stationary amplified flow pattern across the North Pacific (NPAC) basin resulted in several linked extreme weather events (EWEs) across the western contiguous United States and Hawaii. These linked EWEs included 1) record cold over Hawaii and parts of the western United States; 2) three episodes of record snowfall across Washington, Oregon, Northern California, and parts of Arizona; and 3) two rounds of record rainfall across much of California and Oregon. For more details about the EWEs involved in this study, see Table 1. Select daily weather records are shown in Fig. 1, highlighting the spatial extent of the EWEs and the persistence of the large-scale flow pattern throughout the month.
All EWEs involved in this case study, the dates of each EWE, illustrative records broken during each EWE, and representative dynamical processes underpinning each EWE.
Time-mean 500-hPa geopotential height (contour; dam) and observed daily weather records for (a) 9–11 Feb, (b) 9–14 Feb, (c) 21–25 Feb, and (d) 26–28 Feb 2019. Daily records included are low temperature (dark blue), snowfall (white), and precipitation (green).
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
Many of the EWEs mentioned in this case study involve precipitation linked to atmospheric rivers (ARs). ARs are long filamentary structures of water vapor transport typically associated with the warm sector of an extratropical cyclone (e.g., Zhu and Newell 1998; Payne and Magnusdottir 2014; Mundhenk et al. 2016; Ralph et al. 2018). ARs vary in impacts from beneficial to deadly mainly based on the magnitude of the integrated vapor transport (e.g., Ralph et al. 2019). However, ARs in the aggregate account for most of the interannual variability in precipitation across western North America (e.g., Dettinger et al. 2011; Dettinger 2013). More specifically, AR families (e.g., Fish et al. 2019, 2022) comprise most landfalling ARs across California and frequently occur in persistent flow patterns conducive for extreme precipitation (e.g., Moore et al. 2021). Progress has been made in predicting AR activity on the subseasonal-to-seasonal (S2S) time scale (e.g., Mundhenk et al. 2018; DeFlorio et al. 2018; Castellano et al. 2023), but certain ARs exhibit low predictability due to the development of mesoscale frontal waves or secondary cyclones (e.g., Parker 1998), which can change the orientation of ARs relative to the coastline (e.g., Michaelis et al. 2021).
The month of February 2019 was noteworthy for being dominated by high-amplitude blocked flow over the NPAC. Blocking patterns are frequently associated with heavy precipitation (e.g., Amini and Straus 2019; Moore et al. 2021; Moore 2023) on the southern flanks of a block and anomalous cold surface temperatures (e.g., Lee et al. 2019) downstream of the block. Blocking patterns can be sustained through both barotropic (e.g., Dole and Gordon 1983; Simmons et al. 1983; Dole 1986, 1989) and baroclinic (e.g., Charney and DeVore 1979; Mullen 1986, 1987; Hoskins and Valdes 1990) instabilities upstream of the block. These instabilities usually manifest themselves as eddy vorticity and heat fluxes associated with a cyclone upstream of the block (e.g., Mullen 1986, 1987), as planetary-scale deformation processes leading to anticyclonic vorticity advection upstream of the block (e.g., Colucci 2001; Luo et al. 2019), or as preexisting Rossby wave trains (e.g., Lee and Held 1993) reinvigorating an antecedent block (e.g., Röthlisberger et al. 2018; Quinting and Vitart 2019). Blocking patterns generally feature lower predictability compared to nonblocked periods on both the synoptic (e.g., Winters et al. 2019) and S2S (e.g., Quinting and Vitart 2019; Gibson et al. 2020; Winters 2021a) time scales, suggesting that more research is needed to understand cases of extended blocking patterns.
The potential vorticity (PV) framework enables an alternative but complementary interpretation of the aforementioned blocking-related dynamics and flow amplification. Amplifying Rossby waves along a PV waveguide (i.e., the North Pacific jet stream) can become unstable and undergo irreversible overturning known as Rossby wave breaking (RWB, e.g., McIntyre and Palmer 1983, 1984). RWB can result in the formation of cutoff lows (e.g., Otkin and Martin 2004) or PV streamers (e.g., Wernli and Sprenger 2007; Papin et al. 2020) to the south and east of the RWB, both of which can lead to extreme precipitation (e.g., Simpson 1952; Martius et al. 2006). Periods of increased RWB frequency are associated with blocking patterns, as RWB can help deposit lower PV air into the vicinity of the block, allowing the block to persist for a longer period (e.g., Altenhoff et al. 2008).
One of the major sources of predictability on the S2S time scale is the Madden–Julian oscillation (MJO) (Madden and Julian 1971, 1972). Convection associated with the MJO can act as a Rossby wave source (e.g., Sardeshmukh and Hoskins 1988) with Rossby waves propagating into the midlatitudes along an effective waveguide, perturbing the midlatitude flow pattern (e.g., Hoskins and Karoly 1981; Hoskins and Ambrizzi 1993; Henderson et al. 2016, 2017). As a result of these perturbations, certain phases of the MJO are related to frequency anomalies of many midlatitude phenomena such as ridge and blocking occurrence (e.g., Henderson et al. 2016; Gibson et al. 2020), extratropical cyclones (e.g., Attard and Lang 2019), and ARs (e.g., Mundhenk et al. 2018; Wang et al. 2023). However, the extratropical response of the MJO can vary from case to case based on the strength and position of the North Pacific jet (e.g., Lin and Brunet 2018). In addition, anticyclonic wave braking (AWB) in the midlatitudes can impact the magnitude and propagation speed of the MJO itself (e.g., MacRitchie and Roundy 2016).
The purpose of this paper is to elucidate the formation and evolution of the flow regime over the NPAC basin, to elucidate the dynamical linkages between the flow pattern/flow regime and observed EWEs downstream, and to highlight the connections of the disparate EWEs through the evolving NPAC flow regime. Understanding how the large-scale flow pattern evolved to produce the observed EWEs will also motivate more systematic investigations of similar high-amplitude, persistent flow patterns across the NPAC and elsewhere across the globe.
The paper is organized as follows. Section 2 describes the data and methods used in analyzing this case. Section 3 describes the large-scale conditions conducive to the extended flow anomalies. Section 4 details the synoptic-scale interactions between the upstream flow pattern and the embedded EWEs over western North America. Section 5 summarizes the results of this paper.
2. Data sources and methodology
The primary dataset used in this study is the NOAA/National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) global gridded dataset, available at 0.5° and 6-h spatial and temporal resolution, respectively (Saha et al. 2010, 2014). Isobaric CFSR data are used for most synoptic maps, time-averaged summary maps, and relevant calculations in this study. Daily precipitation and temperature records were obtained from NOAA/NCEI’s online daily weather records tool (https://www.ncdc.noaa.gov/cdo-web/datatools/records). Outgoing longwave radiation (OLR) data were obtained from NOAA’s interpolated OLR dataset (Liebmann and Smith 1996). To define the phase and amplitude of the MJO, the real-time multivariate MJO (RMM) index was used as defined in Wheeler and Hendon (2004). To characterize the position of the North Pacific jet, real-time data on the position of the North Pacific jet (NPJ) were used as defined in Winters et al. (2019) and available from Winters (2021b). The NPJ is defined using the first two EOFs of a 21-day rolling average of the 250-hPa wind speed during the extended cool season (September–May) across the NPAC. PC1 represents zonal shifts in the position of the jet, while PC2 represents meridional shifts in the position of the jet.
3. Large-scale flow evolution in February 2019
Figure 2 contains two Hovmöller diagrams of (Fig. 2a) 500-hPa standardized height anomalies and 700-hPa eddy kinetic energy (EKE) and (Fig. 2b) OLR and manually derived cyclone tracks based on sea level pressure minima across the NPAC. Figure 2a shows a persistent, quasi-stationary ridge just east of the date line throughout February, a dominant feature seen in Fig. 1 as well. Periods when this persistent ridge retrogresses and intensifies are concomitant with periods of enhanced EKE, suggesting that upstream eddy activity is critical to the strength and evolution of the persistent ridge. There is also evidence of downstream baroclinic development (e.g., Simmons and Hoskins 1979; Orlanski and Chang 1993; Orlanski and Sheldon 1995) associated with the observed EWEs. A Rossby wave packet originating from a trough over eastern Siberia on 3 February resulted in the development of the strongest ridge anomaly near Alaska around 9 February and eventually a trough over the eastern North Pacific around 11 February that was linked to several of the EWEs listed above.
Hovmöller diagrams of (a) 500-hPa standardized geopotential height anomalies (shaded; sigma) and 700-hPa EKE (contour; m2 s−2) averaged from 40° to 60°N and (b) OLR anomalies (shaded; W m−2) averaged from 20°S to 20°N and manual cyclone tracks (yellow dots) for cyclones over the NPAC basin throughout the month of February 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
Figure 2b shows a quasi-stationary dipole of OLR anomalies throughout the month of February, with above-average OLR in the western NPAC (WPAC) and below-average OLR in the central NPAC (CPAC). This dipole is indicative of both the weak El Niño conditions and active MJO predominantly in phases 6, 7, and 8 throughout the month. Most of the cyclones are concentrated just to the west of the date line in the longitude range of the negative OLR anomalies. The location of the cyclones adds further evidence to the importance of ridge maintenance through upstream heat and vorticity fluxes associated with cyclone activity. Periods when cyclones are able to extend east of 150°W coincide with observed EWEs, further linking flow perturbations across the NPAC to EWEs.
Figure 3 shows four 7-day averaged standardized anomalies of 500-hPa geopotential height and precipitable water, along with 250-hPa anomalous wind speed. In general, the EWEs documented in this case study occurred when there is AWB associated with the persistent ridge over Alaska (Fig. 3b) or when the ridge builds and amplifies into higher latitudes (Fig. 3d), further indicating the important role of this persistent ridge during February 2019. Along with the high-latitude ridge, there is an equally persistent subtropical ridge in the western NPAC for the first 3 weeks of the month. This subtropical ridge is at the same longitude of the positive OLR anomalies in the tropics (Figs. 3a–c), suggesting a linkage between the tropical variability mentioned above and the anomalies of the extratropical NPAC flow pattern.
Mean 500-hPa standardized geopotential height anomalies (shaded; sigma), standardized precipitable water anomalies (contour; sigma), and 250-hPa anomalous wind speed (vector; m s−1) for (a) 1–7 Feb, (b) 8–14 Feb, (c) 15–21 Feb, and (d) 22–28 Feb 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
In the eastern NPAC, anomalous (≥1 sigma) precipitable water values are present both before and during the Valentine’s Day AR that impacted much of California (e.g., Hatchett et al. 2020; Hecht et al. 2022) as seen in Figs. 3a and 3b. Standardized precipitable water anomalies of +3 sigma (Fig. 3b) are associated with anomalous southwesterly flow, suggesting the potential tropical origins of the anomalous moisture. In contrast, EWEs later in the month are not associated with positive standardized precipitable water anomalies (Figs. 3c,d), suggesting that the snow and rainfall records are linked to precipitation with strong dynamical forcing but absent anomalous moisture.
Figure 4 shows the phase of the MJO and NPJ throughout the month of February 2019. The MJO was stalled in phases 6 and 7 for the first half of the month and then phase 8 for around 10 days, before quickly traversing through phases 1 and 2 by 28 February. Phases 6 and 7 of the MJO have been found to lead to increased likelihood of AR landfall across western North America (e.g., Wang et al. 2023; Mundhenk et al. 2018) and of blocking across the NPAC (e.g., Henderson et al. 2016; Attard and Lang 2019), suggesting that this case may be representative of a broader set of events with the same MJO progression. The NPJ was mostly retracted throughout the month, with near-climatological conditions at the beginning and end of the month. There were two periods with rapid jet retractions around 10 and 18 February followed by a gradual shift toward more climatological conditions. Both instances of shifts in the NPJ position preceded periods of EWEs over North America by around 4 days, which links well to the shift in eddy activity seen in Fig. 2b. While not explicitly shown, the MJO can heavily impact the position of the NPJ as OLR anomalies associated with the MJO impact the background PV gradient. Further work would be needed to more explicitly examine MJO–NPJ covariability and to determine how said covariability might impact large-scale regimes and EWE occurrence.
Phase and amplitude of the (a) MJO and (b) NPJ during February 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
4. Synoptic flow evolution in February 2019
a. Flow evolution and kona low formation, 1–11 February
The following sections of the paper delve into daily or subdaily map analysis in order to understand the complex interactions between synoptic- and planetary-scale features that contribute to the occurrence of EWEs over western North America. Figure 5 highlights the evolution of large-scale flow pattern over the NPAC in the first 9 days of the month. From 1 to 9 February, the main features across the NPAC are a time-mean ridge centered in the Gulf of Alaska labeled R1, a dominant planetary-scale trough in the WPAC, and synoptic-scale troughs propagating “up and over” the Gulf of Alaska ridge into the eastern NPAC (EPAC) and across western North America. A large-scale, intense (∼950 hPa) cyclone on 1 February off the coast of the Kamchatka Peninsula within the poleward jet exit region of the North Pacific jet stream, labeled C1, helped to create a storm track directed from the coast of Japan toward the Bering Strait (Fig. 5b). Subsequent cyclones formed in this poleward jet exit region around 165°E, as cold-air advection from eastern Asia behind a previous cyclone reinforced the baroclinicity in the WPAC to sustain the storm track through 5 February (Fig. 5f). On 7 February, the large-scale flow pattern in the WPAC reconfigured to include a slight latitudinal separation of the subtropical and polar jet streams, several jet streaks rather than one stronger jet maximum, and a 1024-hPa anticyclone shifting from southern China to just west of the date line. The change in jet characteristics is important, as shorter jet streaks are typically associated with higher differential cyclonic vorticity advection and upper-level divergence (Uccellini and Kocin 1987). As a result, the aforementioned jet streak may have helped to facilitate a rapidly developing cyclone on 9 February (C5) off the coast of the Kamchatka Peninsula (Fig. 5j). The development of this cyclone also resulted in the development of a narrow, meridionally oriented ridge along the date line (R2) on 9 February due to low-level warm-air advection and poleward-directed integrated vapor transport or IVT (Fig. 5j). A trough around 165°E became narrower and more meridionally elongated as it was sandwiched between the persistent Gulf of Alaska ridge R1 and rapidly developing ridge R2 further west, eventually forming a cutoff kona low (Fig. 5j).
Large-scale flow evolution preceding western U.S. EWEs. (left) The 1000–500-hPa thickness (contour; dam) and 250-hPa E-vector divergence (shaded; 10−3 m s−2) for 0000 UTC (a) 1 Feb, (c) 3 Feb, (e) 5 Feb, (g) 7 Feb, and (i) 9 Feb 2019. (right) Mean sea level pressure (contour; hPa), 250-hPa wind speed (shaded; m s−1), and IVT (vectors; kg m−1 s−1) for 0000 UTC (b) 1 Feb, (d) 3 Feb, (f) 5 Feb, (h) 7 Feb, and (j) 9 Feb 2019. Select cyclones and ridges within the NPAC are labeled for ease of reference.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
The overall E-vector divergence field on 1 February shows couplets of divergence and convergence along the North Pacific jet, along the Arctic coasts of Russia and Alaska, and to the east of Hawaii (Fig. 5a). As the large-scale trough became established after 1 February, much of the E-vector divergence was located east of the date line. The absence of E-vector divergence west of the date line highlights the stability of the WPAC trough and that frequent cyclone passages typically resulted in net-zero E-vector forcing when averaged over 48 h due to alternating υ-wind anomalies on either side of a passing cyclone. In the following days, Figs. 5c, 5e, and 5g show that much of the E-vector divergence was located east of the date line along the polar jet stream. E-vector convergence is primarily located on the western and eastern flanks of troughs where eddy westerly flow is increasing, while E-vector divergence is primarily located near the inflection points of troughs and ridges and climatological westerlies are expected. A couplet of E-vector divergence and convergence appears near the western coastline of Alaska in Fig. 5i, associated with the rapidly developing ridge and impactful cyclogenesis in the WPAC on 9 February. The E-vector convergence on the eastern flank of the elongated trough is especially noteworthy, suggesting strong dynamical forcing to decelerate the climatological westerly winds is occurring as the cutoff low is forming (Fig. 5i).
To better elucidate the dynamics of the kona low, Fig. 6 employs PV thinking with dynamic tropopause (DT) winds and potential temperature, as well as thermal vorticity to assess the structure of the kona low. Figure 6a depicts the previously mentioned rapid cyclogenesis and downstream ridge development just to the west of the date line on 9 February as concurrent CWB and AWB, respectively. While both instances of wave breaking initiated around the same time, the two wave breaks quickly begin to evolve differently. The AWB region (AWB 1) is blocked by the narrow downstream trough and persistent ridge in the Gulf of Alaska, preventing the eastward extension of this wave-breaking region (Fig. 6c). The CWB region (CWB 2), however, is able to expand through the Bering Strait and into the Arctic basin around 1200 UTC 9 February, fostering the enhancement of the persistent ridge as it builds into Alaska. At the same time, there is also evidence of embedded AWB within the northern portion of the persistent ridge near 145°W, labeled AWB 2. With higher DT theta values and stronger northeasterly winds on the eastern flank of the ridge, the persistent ridge R1 shifts toward northern Canada and takes on a strong positive tilt. This positively tilted ridge over Canada helps to shift a downstream trough from western Canada to off the coast of Washington, Oregon, and British Columbia (Figs. 6a–f). The base of the trough contains high values of thermal vorticity and a possible tropopause polar vortex (TPV, e.g., Cavallo and Hakim 2009), both of which contributed toward the record-breaking snowfall in Seattle on 11 February (Table 1).
Kona low development across the central NPAC basin. (left) DT potential temperature (shaded; K) and wind speed (vector; m s−1) for (a) 0000 UTC 9 Feb, (c) 1200 UTC 9 Feb, (e) 0000 UTC 10 Feb, (g) 1200 UTC 10 Feb, and (i) 0000 UTC 11 Feb 2019. (right) The 1000–500-hPa thickness (red/blue contours; dam) and thermal vorticity (shaded; 10−5 s−1) for (b) 0000 UTC 9 Feb, (d) 1200 UTC 9 Feb, (f) 0000 UTC 10 Feb, (h) 1200 UTC 10 Feb, and (j) 0000 UTC 11 Feb 2019. Select instances of RWB are labeled by their type for ease of reference.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
As this complex wave-breaking pattern evolved in the NPAC, an elongated trough around 160°W began to fracture and become cutoff from the main flow pattern, eventually forming a kona low (Figs. 6c,e). Previous work has identified high-latitude blocking as a primary precursor for kona low formation (e.g., Simpson 1952; Otkin and Martin 2004), suggesting that this case behaves similarly to other known kona low cases. The kona low becomes more zonally elongated as it enters the subtropics, with much of the thermal vorticity on the southern and eastern edge of the low (Figs. 6d,f). This kona low brought anomalously cold air to the subtropics due to the rapid advection of a polar air mass from around 50°N to just north of Hawaii, as evidenced by the DT theta below 300 K and 1000–500-hPa thickness below 540 dam through 0000 UTC 11 February (Figs. 6i,j). The kona low not only brought direct weather impacts (i.e., record cold) to Hawaii but also acted as a key player in subsequent flow perturbations linked to the Valentine’s Day AR in Southern California, discussed in the next section.
By 0000 UTC 10 February, the cyclone responsible for the previous flow perturbations and RWB in the WPAC had dissipated (Figs. 5j and 6e,f). Deep southerly flow near the date line both sheared the eastern flank of the antecedent cyclone and contributed to further high-latitude ridge building over Alaska and into the Arctic (Figs. 6e,f). The persistent ridge took on more of an omega block configuration as the ridge developed, with troughs flanking both sides of the ridge evident on the DT (Figs. 6g,h). The western trough associated with the omega block became increasingly sheared as AWB 1 near the date line continued, resulting in the western trough of the omega block being advected along the southern flank of the block in the Gulf of Alaska (Figs. 6i,j). This trough undercutting the block will be another feature relevant to the development of the Valentine’s Day AR toward the middle of the month.
b. Western U.S. EWEs, 11–15 February
The following section of the paper deals with the flow evolution responsible for the EWEs observed from 11 to 15 February 2019. Figures 7a and 7b match with the same time as Figs. 6i and 6j but with slightly different variables to show the developing AR in the subtropical EPAC. Figure 7a shows CWB 2 as a mature, intense cyclone with an AR in the warm sector of the cyclone. This CPAC AR extends from 30°N to around 55°N, with high levels (>2000 W m−2) of integrated moisture flux convergence along the warm front and near the southern end of the cold front. The kona low, labeled KL, has begun weakening from prolonged time in the subtropics but still exhibits moderate amounts (>500 kg m−1 s−1) of IVT primarily on the northern and southeastern edges of the low (Fig. 7a). Integrated moisture flux convergence associated with the kona low is primarily on the southeastern edge of the low along the anticyclonic shear side of an extensive subtropical jet streak (Fig. 7b). As previously mentioned, continued AWB just to the east of the date line led to the shearing and horizontal advection of the western trough (labeled T1) in an omega block configuration over the Bering Strait and Alaska, which brought the trough to around 150°W in the Gulf of Alaska. As this is occurring, the cyclone associated with CWB 2 continued to travel toward the northeast, helping to build the downstream ridge near the date line.
(left) The 1000–500-hPa thickness (red/blue contours; dam) and IVT (shaded; kg m−1 s−1) for (a) 0000 UTC 11 Feb, (c) 0000 UTC 12 Feb, (e) 0000 UTC 13 Feb, (g) 0000 UTC 14 Feb, and (i) 0000 UTC 15 Feb 2019. (right) The 250-hPa geopotential height (contour; dam), 250-hPa wind speed (shaded; m s−1), and integrated moisture flux convergence (shaded; W m−2) for (b) 0000 UTC 11 Feb, (d) 0000 UTC 12 Feb, (f) 0000 UTC 13 Feb, (h) 0000 UTC 14 Feb, and (j) 0000 UTC 15 Feb 2019. Select troughs and upper-level lows are labeled for ease of reference.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
By 12 February, the ridge had built into southern Alaska and took on a strong positive tilt, as the previous blocking high was advected to the northeast over the Arctic. Concurrently, the remnant western trough of the omega block T1 had merged with the eastern trough of the omega block over western Canada T2, creating an elongated positively tilted trough in the northeastern NPAC (Figs. 7c,d). On the eastern flank of the positively tilted trough, a jet streak develops off the coast of Northern California, with moderate integrated moisture flux convergence values over coastal Oregon and Washington (Fig. 7d). This integrated moisture flux convergence over Washington, along with the cold air mass implied by thickness values < 540 dam, allowed for a second period of snowfall for western Washington and additional record snowfall of 155 mm on 12 February (Table 1). Further south, the kona low was no longer a closed circulation in the 1000–500-hPa thickness field but was still visible as a trough in the thickness and 250-hPa height fields. Despite dissipating in overall strength, both IVT and integrated moisture flux convergence values increased between 11 and 12 February on the southeastern flank of the weakened kona low (Figs. 7c,d). The development and consolidation of this EPAC AR (Figs. 7e,g,i) was dependent on the initial strength of the kona low, as shown in the model sensitivity analysis by Chen et al. (2022). This relationship between the kona low and downstream AR highlights the nature of the complex synoptic interactions involved in this case study.
By 13 February, the positively tilted trough in the EPAC and the remnants of the kona low had merged into one extended trough (still referenced as T2) spanning 40° of latitude (Figs. 7e,f). The elongated trough allowed for the EPAC AR, which was developing on the eastern edge of the kona low to be pulled further north and toward the western coast of North America. Although both troughs had merged into one feature in both the thickness and 250-hPa height fields, there were still separate subtropical and polar jet streaks embedded within the trough (Fig. 7f). The configuration of these jet streaks had the poleward jet exit region of the subtropical jet and the equatorward jet entrance region of the polar jet both near the northern extent of the EPAC AR, creating a region favorable for strong vertical motion as expected from quasigeostrophic theory (Hecht et al. 2022). Within the dual jet streak region, there is a local maximum of integrated moisture flux convergence just off the coast of California and Oregon along the northern and eastern edge of the EPAC AR. Upstream in the WPAC, another cyclone began developing in the poleward jet exit region of a WPAC jet streak near 165°E (Fig. 7e). As noted in previous sections, much of the integrated moisture flux convergence associated with the developing WPAC cyclone was along the leading edge of the warm front and embedded AR (Fig. 7f). Integrated moisture flux convergence can also act as a proxy for total column latent heating, so the location of integrated moisture flux convergence in the WPAC on the southern end of a developing ridge downstream indicates that diabatic processes are aiding in ridge amplification into the CPAC going into 14 February.
Over the next 24 h, the EPAC AR made landfall along the coast of California and IVT increased over the western United States (Fig. 7g). As the AR progresses over the western United States, integrated moisture flux convergence values were locally maximized in longitudinally extended bands over Northern and Southern California where many of the noteworthy rainfall records were broken (Table 1 and Fig. 1). The trough just upstream of the AR became less positively tilted and has a closed low in the 1000–500-hPa thickness field. This change in the trough was coupled at the surface with secondary cyclogenesis from a mesoscale frontal wave as noted by Hecht et al. (2022). This secondary cyclone helped to extend a second plume of higher IVT further north and prolonged AR conditions into Northern California. Upstream, the cyclone which developed in the WPAC underwent CWB just off the coast of the Kamchatka Peninsula, further amplifying a downstream ridge near the date line through the advection of warm air and anticyclonic vorticity (Fig. 7h).
By 15 February, the trough associated with the AR became neutrally tilted and without the cutoff feature associated with the mesoscale frontal wave and secondary cyclone. As a result, the northern IVT maximum dissipated, leaving just the southern portion over the California–Mexico border. The largest integrated moisture flux convergence values were across the interior western United States, indicative of the heavy rainfall occurring across the region (Table 1). These integrated moisture flux convergence values are downstream of a pronounced subtropical jet (STJ) streak with wind speeds exceeding 100 m s−1 (Fig. 7j). The CWB in the WPAC had finished by 15 February, resulting in a newly enhanced ridge near the Aleutians and a shift toward a Rex block (Rex 1950) dominating the NPAC flow pattern for the days to come.
c. Subsequent NPAC flow evolution, 15–21 February
After the dissipation of the AR impacting California, there was a second period of substantive flow evolution from 15 to 21 February across the NPAC before additional EWEs occurred across the western United States later in the month. Figure 8 contains the same types of plots as Fig. 5 in order to describe similar changes to the NPAC flow pattern as those observed from 1 to 9 February. Figures 8a and 8b show a strong Rex block in the CPAC, the remnants of the landfalling AR across Southern California, and the WPAC cyclone which underwent CWB here labeled as C1. There is strong E-vector convergence in the diffluent exit region near the date line and E-vector divergence leading into the jet entrance region around 135°W (Fig. 8b). The E-vector forcing is mirrored in the northern and southern flanks of the Rex block, indicating that forcing from the eddy activity is helping to maintain the jet anomalies associated with the block. Upstream, a second cyclone (labeled C2) has developed on the southeastern flank of C1 and is aiding in reinforcing the Rex block (Fig. 8b).
Large-scale flow evolution preceding western U.S. EWEs. (left) The 1000–500-hPa thickness (contour; dam) and 250-hPa E-vector divergence (shaded; 10−3 m s−2) for (a) 0000 UTC 15 Feb, (c) 0000 UTC 17 Feb, (e) 0000 UTC 19 Feb, and (g) 0000 UTC 21 Feb 2019. (right) Mean sea level pressure (contour; hPa), 250-hPa wind speed (shaded; m s−1), and IVT (vectors; kg m−1 s−1) for (b) 0000 UTC 15 Feb, (d) 0000 UTC 17 Feb, (f) 0000 UTC 19 Feb, and (h) 0000 UTC 21 Feb 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
Over the next 2 days, the Rex block began to shift further east into the Gulf of Alaska as cyclone C2 further deepened and moved toward the Aleutians (Fig. 8d). A third cyclone (C3) developed in a favorable position between the equatorward entrance and poleward exit region of two jet streaks in the WPAC. In addition, the E-vector divergence field reflected the eastward shift of the Rex block, as the divergence and convergence couplets are approximately 10° further east in Fig. 8d than in Fig. 8b. Within the southern flank of the Rex block, the trough in Figs. 8c and 8d had become less visible in both the thickness and jet speed fields as compared to Figs. 8a and 8b. Instead of a pronounced upper-level trough signature, an inverted surface trough began to develop northeast of Hawaii associated with cyclonic IVT on the northern flank of the trough (Fig. 8d).
By 19 February, the persistent ridge had become more zonally elongated and less amplified, spanning from the date line to around 130°W (Fig. 8f). The western extent of the persistent ridge had shifted in response to ridge amplification downstream of yet another cyclone (C4) in the WPAC developing along the date line. Despite the broadening of the persistent ridge, the E-vector divergence field was not as amplified as in previous periods west of 150°W (Fig. 8e). Given that E vectors, by definition, represent eddy–mean flow interactions, the weaker E-vector divergence suggests the importance of eddy–eddy interactions for the observed ridge development in this period (e.g., Mullen 1987). Stronger E-vector divergence is found further east along the western coast of North America as a trough digs further to the south into Southern California (Fig. 8e). Along with a broadening of the persistent ridge, a similarly broad 1040-hPa surface anticyclone strengthened in the EPAC (Fig. 8f). The inverted trough shifted slightly toward the northwest while maintaining the south-southeasterly IVT on its eastern flank.
By 21 February, the intermediate period of flow configuration had ceased and the NPAC was once again primed for the occurrence of EWEs over western NA. A new cyclone, C5, had developed to the south of the Kamchatka Peninsula as C4 had entered the Bering Strait (Fig. 8h). As cyclone C4 progressed toward the northeast, it further amplified the persistent ridge in the Gulf of Alaska, once again reinforcing the dominant flow pattern within the NPAC. In association with the ridge development, the surface high in the EPAC became more meridionally elongated rather than zonally elongated. As the surface high broadened toward the west, the inverted trough became a closed low pressure system caught up in the polar jet stream on the western edge of the persistent ridge (Fig. 8h). Along with the surface low, the IVT associated with the inverted trough began to move poleward along the western edge of the surface high, adding stronger moisture transport to the periphery of the persistent ridge. The E-vector divergence pattern exhibited strong couplets of divergence and convergence along the edges of the main troughs and ridges, suggesting downstream propagation of eddies across the NPAC (Fig. 8g). One exception to this pattern is in the vicinity of the low north of Hawaii, where there is forcing to accelerate the mean winds from E-vector divergence on the eastern edge of the low without a corresponding deceleration of the mean winds further east.
Across this weeklong period of flow amplification and modification, the NPAC jet had become more amplified but with weaker embedded jet streaks. The persistent ridge had shifted further east during this period, aligning a trough directly over western North America without a component offshore. Finally, the STJ had remained active but was associated with shorter wavelength troughs and ridges embedded within the STJ as compared to the beginning of the period. In aggregate, these characteristics to the flow pattern helped to contribute to EWEs at the end of February 2019 as described in the subsequent sections of this paper.
d. Western U.S. extreme precipitation, 21–28 February
The following section of the paper describes a final period of three precipitation-based EWEs across the western CONUS from 21 to 28 February. As in previous sections, Figs. 9a and 9b overlap in time with Figs. 8g and 8h but with different variables most relevant to the EWEs. Figure 9b depicts two separate troughs centered around 120°W, one around 35°N and the other around 20°N. In the PV framework, the northern trough is meridionally elongated ending around 35°N, while the southern trough is a small, zonally elongated region of PV above 1 PVU (1 PVU = 10−6 K kg−1 m2 s−1) (Fig. 9a). A consolidated area of high thermal vorticity at the base of the southern trough aligns nicely with the higher PV seen in Fig. 9a. Upstream across the NPAC, there is a broad ridge extending from the western coast of North America to around 170°W. Within this ridge, there is an anticyclonically curved AR along the periphery of the ridge extending from Hawaii to Southern California (Fig. 9a). Within the AR, there is a meridionally extended region of integrated moisture flux convergence around 150°W from Hawaii to just south of Alaska. This AR configuration suggests the moisture in the EPAC originally came from the subtropics near Hawaii and was transported along the edge of the persistent ridge in the EPAC. Further upstream in the CPAC, a ridge is developing around 170°E immediately downstream of another cyclone in the WPAC.
(left) The 310-K PV (contour; PVU), IVT (vectors; kg m−1 s−1), and integrated moisture flux convergence (shaded; W m−2) for (a) 0000 UTC 21 Feb, (c) 0000 UTC 22 Feb, and (e) 0000 UTC 23 Feb 2019. (right) 1000–500-hPa thickness (red/blue contours; dam) and thermal vorticity (shaded; 10−5 s−1) for (b) 0000 UTC 21 Feb, (d) 0000 UTC 22 Feb, and (f) 0000 UTC 23 Feb 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
On 22 February, the CPAC ridge had amplified and moved to just east of the date line (Fig. 9c). A weak PV gradient evident across central North America inhibited Rossby wave propagation, leading to a slowdown of the ridge/trough couplet in the EPAC and resulted in the ridge/trough couplet narrowing and becoming positively tilted. The positive tilt of both the ridge and trough were the beginning stages of AWB and CWB, respectively. The trough began to dig further south across Southern California and absorbed the southern trough in the process (Fig. 9d). The trough merger led to an increase in the thermal vorticity, with the strongest values located on the western half of the base of the trough. The configuration of thermal vorticity on the western edge of the trough suggests, through the Sutcliffe–Trenberth approximation of the omega equation (Sutcliffe 1947; Trenberth 1978), a region favorable for strong ascent across the interior southwest United States (Fig. 9d). The AR embedded within the ridge began breaking up by 22 February, but two fragments remained near 150°W and along the southwestern edge of the positively tilted trough near Southern California and northern Mexico (Fig. 9c). Along the leading edge of the eastern “AR fragment,” integrated moisture flux convergence values increased over the previous 24 h over the southwest United States, overlapping with the favorable region for strong ascent.
RWB continued to occur through 23 February, as the ridge over western Canada became more positively tilted and progressed toward the east. The trough over the southwestern United States had become narrower as the upstream ridge progressed eastward, while a downstream ridge in the south-central United States remained stationary (Fig. 9e). On the eastern flank of the trough, integrated moisture flux convergence values increased and consolidated into a more organized region over Arizona and New Mexico, while IVT values decreased within this same region. Along with the consolidated integrated moisture flux convergence, thermal vorticity shifted from the western flank of the trough to the base of the trough along the U.S.–Mexico border (Fig. 9f). The consolidated integrated moisture flux convergence and the region favored for synoptic-scale ascent are collocated with 1000–500-hPa thickness values around 536 dam, all contributing to a favorable environment for the heavy snowfall observed in northern Arizona (Table 1). Further upstream, higher values of PV began to move to the southeast over far northwestern Canada. At the same time, the persistent ridge over the CPAC began to further amplify and take on a negative tilt just to the east of the date line (Fig. 9f). In the WPAC, a cyclone just off the coast of Japan began to rapidly intensify and developed a small-scale thermal ridge around 145°E, which was associated with very high integrated moisture flux convergence (>2800 W m−2) and IVT (∼1000 kg m−1 s−1) values (Fig. 9e).
After the heavy snowfall event over the southwest United States, another smaller-scale snowfall event occurred in Oregon during a period of rapid flow evolution in the NPAC. Much of the flow amplification and reconfiguration can be attributed to the rapidly developing cyclone in the WPAC mentioned in the previous paragraph. A lower latitude ridge developed in the region of high integrated moisture flux convergence, IVT, and warm-air advection immediately downstream of the cyclone around 165°E (Figs. 10a,b). The ridge development resulted in a negatively tilted PV streamer as the ridge impinged upon an antecedent trough in the northern WPAC (Fig. 10a). Downstream, the high PV values over northern Canada moved further south and developed into a positively tilted trough just off the coast of western Canada. The overall position of the two troughs flanking the persistent ridge still present in the CPAC resulted in an omega block configuration on 24 February. The configuration of the EPAC trough resulted in a nearly uniform zonally oriented PV gradient across the northwestern contiguous United States (Fig. 10a).
As in Fig. 9, but for (a),(b) 0000 UTC 24 Feb and (c),(d) 0000 UTC 25 Feb 2019.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
By 25 February, the WPAC cyclone aided in further perturbing the flow across the NPAC, resulting in an overall convoluted flow pattern and a weak meridional PV gradient east of the date line. The WPAC cyclone itself had undergone CWB near the date line, resulting in a negatively tilted trough and ridge around 45° and 55°N, respectively. The CPAC persistent ridge amplified as well, pushing PV values < 1 PVU well into the Arctic Ocean (Fig. 10c). Because of the stationary yet amplifying persistent ridge in the CPAC and the developing ridge north of the CWB in the WPAC, the positively tilted trough, which was once the western trough in the omega block, became elongated and sheared out into a PV streamer (Figs. 10c,d). The shearing of an upstream trough underneath a high-latitude block occurred earlier in the month from 10 to 11 February, which aided in eventually bringing the Valentine’s Day AR into Southern California, suggesting that similar dynamics are at play preceding the EWEs during February 2019 (Figs. 6i,j and 7). Along the coast of western North America, the eastern trough of the omega block configuration had narrowed slightly as a pronounced zonal PV gradient persisted in the region (Fig. 10c). While not associated with large values of thermal vorticity near the base of the trough, moderate IVT and integrated moisture flux convergence values extended from around 140° to 110°W along the southern edge of the PV gradient (Fig. 10d). Coupled with thickness values below 540 dam, there were sufficient antecedent conditions for heavy snowfall in regions favorable for orographic precipitation, mainly across Oregon (Table 1).
The final observed EWE across the western United States occurred from 26 to 28 February 2019, rounding out a monthlong, high-amplitude persistent flow pattern across the NPAC. On 26 February, the remnants of the sheared trough undercutting the blocking high (here labeled T1) had become a smaller-scale trough in both the 1000–500-hPa thickness and 250-hPa height field. Upstream, the WPAC cyclone had fully undergone CWB, resulting in a zonal trough on the western flank of the CPAC omega block as well as a southeasterly jet around 60°N (Fig. 11b). On the eastern side of the omega block, the trough (labeled T2) remained quasi-stationary, with an axis of moderate IVT on the southern and eastern edge of the trough (Fig. 11a). While IVT values were weak to moderate (<500 kg m−1 s−1), locally high integrated moisture flux convergence values were present across Northern California in a region favored for ascent from orographic lift and a favorable position within the equatorward jet entrance region of a jet streak (Fig. 11b).
(left) The 1000–500-hPa thickness (red/blue contours; dam) and IVT (shaded; kg m−1 s−1) for (a) 0000 UTC 26 Feb, (c) 0000 UTC 27 Feb, and (e) 0000 UTC 28 Feb 2019. (right) 250-hPa geopotential height (contour; dam), 250-hPa wind speed (shaded; m s−1), and integrated moisture flux convergence (shaded; W m−2) for (b) 0000 UTC 26 Feb, (d) 0000 UTC 27 Feb, and (f) 0000 UTC 28 Feb 2019. Select troughs and upper-level lows are labeled for ease of reference.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
By 27 February, the remnants of T1 had merged with T2, with only a small trough in the 546-dam thickness contour left of the original T1 (Fig. 11a). At the same time, the ridge portion of the omega block had become more positively tilted, with more of the ridge directly to the north of T2. The merger of T1 and T2, combined with the implied RWB embedded within the blocking high, helped to keep T2 quasi-stationary over the previous 24 h (Fig. 11d). IVT and integrated moisture flux convergence values had increased along the coast of California, potentially linked to the enhanced southerly geostrophic flow associated with the remnants of T1 merging with T2. Persistent IVT and integrated moisture flux convergence associated with the quasi-stationary trough on the eastern side of an omega block aided in the localized heavy rainfall observed in Northern California from 26 to 27 February (Table 1).
The final day of the month, 28 February, brought an end to the linked EWEs across much of the western United States. IVT values diminished to below the typical threshold for ARs (<250 kg m−1 s−1) and integrated moisture flux convergence drastically decreased across California and Oregon, coincident with the cessation of extreme rainfall in the region (Fig. 11e). Upstream in the CPAC, another round of transient ridge building downstream of a WPAC cyclone resulted in the western trough of the omega block cutting off and forming more of a Rex block configuration (Fig. 11f). This periodic ridge building in the CPAC helped to maintain the persistence of the CPAC ridge and to shift the axis of the persistent ridge slightly further west, emblematic of the dynamics at play throughout February which maintained the persistent ridge across the NPAC.
5. Discussion and conclusions
This paper has documented several EWEs across the western contiguous United States and Hawaii throughout the month of February 2019 and demonstrated the dynamical linkages between the EWEs through a persistent flow pattern upstream across the NPAC. Observed EWEs were concentrated in two periods: 9–15 February (record cold in HI, record snowfall in WA, record rainfall in southern CA) and 21–28 February (record snowfall in AZ and OR, record rainfall in CA and OR), while the persistent flow pattern developed from 1 to 9 February and evolved from 15 to 21 February, maintaining a linked period of extreme weather throughout February 2019.
From the planetary-scale perspective, both periods of EWEs are associated with rapid shifts in the position of the NPAC jet stream as defined by Winters et al. (2019). The period from 9 to 15 February is preceded by a rapid jet retraction, while the period of 21–28 February is preceded by a pronounced shift away from the jet retraction and toward a climatological position (Fig. 4b). The two periods of EWEs also appear to be associated with an increase in the eddy kinetic energy in the WPAC and CPAC immediately preceding and during the EWEs, suggesting the importance of upstream cyclones as precursors to downstream EWEs (Fig. 2a). In the tropics, negative OLR anomalies persisted around the date line for the first 3 weeks of the month in association with the MJO in phases 6–8 (Fig. 2b). The persistent MJO conditions likely aided in maintaining the persistence of the overall NPAC upper-level flow pattern throughout the month.
Figure 12 provides a schematic depiction of the primary synoptic drivers within the persistent flow pattern responsible for the EWE occurrences. From the synoptic perspective, the two main periods of EWEs occurred when the persistent ridge within the CPAC took on characteristics of an omega block with tilted troughs flanking either side of the ridge. The changes and movement of these associated flanking troughs, especially the easternmost trough of the omega block, are further linked to RWB within the blocking ridge. The local maxima of the standard deviation of the 500-hPa geopotential height field in the EPAC are located around the eastern flank of the persistent ridge and within the downstream troughs, emphasizing the variability in the ridge and trough position within the overall persistent pattern. In the WPAC, the jet entrance and exit regions also emerge as local maxima for 500-hPa geopotential height standard deviation, indicative of the continuous cyclogenesis in the WPAC and CPAC, as well as the RWB occurring to the north and south of the time-mean jet exit region.
Schematic depiction of key dynamical drivers for the observed EWEs in February 2019. Monthly averaged 500-hPa geopotential height (contour; m) and the standard deviation of the 500-hPa geopotential height (shaded; m) for February 2019, with annotations for the mean position of the jet, locations of RWB, and the observed EWEs included in this analysis.
Citation: Monthly Weather Review 152, 9; 10.1175/MWR-D-24-0040.1
This paper presents a new style of case study that extends the period of analysis from the synoptic time scale (e.g., Bosart et al. 1996, 2017) into the S2S time scale. Because this paper covered an entire month analyzed on a daily time scale, more detailed calculations pertaining to the EWEs on the mesoscale could not be included in this analysis as is typical for traditional synoptic case studies. However, the novelty of bridging the weather and climate time scales and documenting the dynamical linkage between observed EWEs spanning a monthlong period are the strengths of this style of the S2S case study. This study also highlights how examination of large-scale weather regimes on the monthly to intraseasonal time scale can overlook some of the imbedded synoptic variability driving the periodic EWEs during this month. For example, the classification of North American weather regimes by Lee et al. (2023a,b) considers February 2019 to be one continuous Pacific ridge regime, while Robertson et al. (2020) have brief interruptions in the Pacific ridge regime from around 14 to 18 February with a Pacific trough and West Coast ridge. While to first order this is the case, subtle shifts in the Pacific ridge regimes as a result of upstream cyclone activity, embedded RWB, and PV cutoff formation all govern the structure and evolution of the observed EWEs associated with this extended regime.
Current work by the authors aims to showcase how synoptic variability within an extended weather regime can heavily impact the observed weather associated with said weather regimes and to encourage further exploration of these interactions between synoptic and S2S variability on weekly to monthly time scales. Future S2S case studies should be conducted to assess other instances of dynamically linked EWEs and to establish additional mechanisms underpinning the linked EWEs. We hypothesize that RWB and anomalous activity within the NPAC storm track broadly speaking will prevail as the most common mechanisms in similar cases. Additional work should also connect Pacific and Atlantic activity in a similar manner, so that regional anomalies are considered within the context of the entire Northern Hemisphere. Recent work by Wenta et al. (2024) investigated extreme blocking in the North Atlantic and western Europe during February 2019, and additional EWEs during February 2019 occurred in the central and eastern United States, suggesting that similar cases most likely have hemispheric impacts. The aforementioned proposed work would also motivate investigations into model biases and representations of similar cases in both global numerical weather prediction models and nascent operational S2S models, in order to identify the predictability of such EWEs associated with persistent flow regimes. Last, examining the connections between similar flow regimes to the MJO, the NPJ, and other modes of variability could offer another method of increased predictability and situational awareness for various stakeholders.
Acknowledgments.
We thank Michael DeFlorio, Julie Kalansky, Brandt Maxwell, Benjamin Moore, Anna Wilson, and Alexander Mitchell for their research discussions about this case study and other related scientific topics. We would also like to thank Barbara Zampella for the logistical and administrative assistance, which makes research like this possible for both authors. This work was funded by NSF Grant AGS-1656406 and grants from the State of California Department of Water Resources associated with the California Atmospheric Rivers Program Phase 3 (4600014294) and Phase 4 (4600014942) to the Center for Western Weather and Water Extremes (CW3E) at the University of California San Diego Scripps Institution of Oceanography.
Data availability statement.
The CFSR data used in this study were accessed from the Research Data Archive at the National Center for Atmospheric Research (https://doi.org/10.5065/D69K487J). OLR data are available from the NOAA PSL website (https://psl.noaa.gov/data/gridded/data.olrcdr.interp.html). Daily weather records are available from NOAA/NCEI online daily weather records available at https://www.ncdc.noaa.gov/cdo-web/datatools/records. RMM data characterizing the MJO are available from the IRI/LDEO climate data library at https://iridl.ldeo.columbia.edu/SOURCES/.BoM/.MJO/.RMM/datasetdatafiles.html?Set-Language=en. Data characterizing the two leading EOFs of the NPAC jet are available from the University of Colorado, Boulder, as cited in Winters (2021b) (https://doi.org/10.25810/CKN0-GP39). All code used for all analysis was created in Python using Jupyter notebooks. All code written for the analysis in this paper is available upon request.
REFERENCES
Altenhoff, A. M., O. Martius, M. Croci-Maspoli, C. Schwierz, and H. C. Davies, 2008: Linkage of atmospheric blocks and synoptic-scale Rossby waves: A climatological analysis. Tellus, 60A, 1053–1063, https://doi.org/10.1111/j.1600-0870.2008.00354.x.
Amini, S., and D. M. Straus, 2019: Control of storminess over the Pacific and North America by circulation regimes. Climate Dyn., 52, 4749–4770, https://doi.org/10.1007/s00382-018-4409-7.
Attard, H. E., and A. L. Lang, 2019: The impact of tropospheric and stratospheric tropical variability on the location, frequency, and duration of cool-season extratropical synoptic events. Mon. Wea. Rev., 147, 519–542, https://doi.org/10.1175/MWR-D-18-0039.1.
Bosart, L. F., G. J. Hakim, K. R. Tyle, M. A. Bedrick, W. E. Bracken, M. J. Dickinson, and D. M. Schultz, 1996: Large-scale antecedent conditions with the 12–14 March 1993 cyclone (“Superstorm’93”) over eastern North America. Mon. Wea. Rev., 124, 1865–1891, https://doi.org/10.1175/1520-0493(1996)124<1865:LSACAW>2.0.CO;2.
Bosart, L. F., B. J. Moore, J. M. Cordeira, and H. M. Archambault, 2017: Interactions of North Pacific tropical, midlatitude, and polar disturbances resulting in linked extreme weather events over North America in October 2007. Mon. Wea. Rev., 145, 1245–1273, https://doi.org/10.1175/MWR-D-16-0230.1.
Castellano, C. M., and Coauthors, 2023: Development of a statistical subseasonal forecast tool to predict California atmospheric rivers and precipitation based on MJO and QBO activity. J. Geophys. Res. Atmos., 128, e2022JD037360, https://doi.org/10.1029/2022JD037360.
Cavallo, S. M., and G. J. Hakim, 2009: Potential vorticity diagnosis of a tropopause polar cyclone. Mon. Wea. Rev., 137, 1358–1371, https://doi.org/10.1175/2008MWR2670.1.
Charney, J. G., and J. G. DeVore, 1979: Multiple flow equilibria in the atmosphere and blocking. J. Atmos. Sci., 36, 1205–1216, https://doi.org/10.1175/1520-0469(1979)036<1205:MFEITA>2.0.CO;2.
Chen, S., C. A. Reynolds, J. M. Schmidt, P. P. Papin, M. A. Janiga, R. Bankert, and A. Huang, 2022: The effect of a kona low on the eastern Pacific valentine’s day (2019) atmospheric river. Mon. Wea. Rev., 150, 863–882, https://doi.org/10.1175/MWR-D-21-0182.1.
Colucci, S. J., 2001: Planetary-scale preconditioning for the onset of blocking. J. Atmos. Sci., 58, 933–942, https://doi.org/10.1175/1520-0469(2001)058<0933:PSPFTO>2.0.CO;2.
DeFlorio, M. J., D. E. Waliser, B. Guan, D. A. Lavers, F. M. Ralph, and F. Vitart, 2018: Global assessment of atmospheric river prediction skill. J. Hydrometeor., 19, 409–426, https://doi.org/10.1175/JHM-D-17-0135.1.
Dettinger, M. D., 2013: Atmospheric rivers as drought busters on the U.S. West Coast. J. Hydrometeor., 14, 1721–1732, https://doi.org/10.1175/JHM-D-13-02.1.
Dettinger, M. D., F. M. Ralph, T. Das, P. J. Neiman, and D. R. Cayan, 2011: Atmospheric rivers, floods, and the water resources of California. Water, 3, 445–478, https://doi.org/10.3390/w3020445.
Dole, R. M., 1986: Persistent anomalies of the extratropical Northern Hemisphere wintertime circulation: Structure. Mon. Wea. Rev., 114, 178–207, https://doi.org/10.1175/1520-0493(1986)114<0178:PAOTEN>2.0.CO;2.
Dole, R. M., 1989: Life cycles of persistent anomalies. Part I: Evolution of 500 mb height fields. Mon. Wea. Rev., 117, 177–211, https://doi.org/10.1175/1520-0493(1989)117<0177:LCOPAP>2.0.CO;2.
Dole, R. M., and N. D. Gordon, 1983: Persistent anomalies of the extratropical Northern Hemisphere wintertime circulation: Geographical distribution and regional persistence characteristics. Mon. Wea. Rev., 111, 1567–1586, https://doi.org/10.1175/1520-0493(1983)111<1567:PAOTEN>2.0.CO;2.
Fish, M. A., A. M. Wilson, and F. M. Ralph, 2019: Atmospheric river families: Definition and associated synoptic conditions. J. Hydrometeor., 20, 2091–2108, https://doi.org/10.1175/JHM-D-18-0217.1.
Fish, M. A., J. M. Done, D. L. Swain, A. M. Wilson, A. C. Michaelis, P. B. Gibson, and F. M. Ralph, 2022: Large-scale environments of successive atmospheric river events leading to compound precipitation extremes in California. J. Climate, 35, 1515–1536, https://doi.org/10.1175/JCLI-D-21-0168.1.
Gibson, P. B., D. E. Waliser, A. Goodman, M. J. DeFlorio, L. D. Monache, and A. Molod, 2020: Subseasonal-to-seasonal hindcast skill assessment of ridging events related to drought over the western United States. J. Geophys. Res. Atmos., 125, e2020JD033655, https://doi.org/10.1029/2020JD033655.
Hatchett, B. J., and Coauthors, 2020: Observations of an extreme atmospheric river storm with a diverse sensor network. Earth Space Sci., 7, e2020EA001129, https://doi.org/10.1029/2020EA001129.
Hecht, C. W., A. C. Michaelis, A. C. Martin, J. M. Cordeira, F. Cannon, and F. M. Ralph, 2022: Illustrating ensemble predictability across scales associated with the 13–15 February 2019 atmospheric river event. Bull. Amer. Meteor. Soc., 103, E911–E922, https://doi.org/10.1175/BAMS-D-20-0292.1.
Henderson, S. A., E. D. Maloney, and E. A. Barnes, 2016: The influence of the Madden–Julian oscillation on Northern Hemisphere winter blocking. J. Climate, 29, 4597–4616, https://doi.org/10.1175/JCLI-D-15-0502.1.
Henderson, S. A., E. D. Maloney, and S.-W. Son, 2017: Madden–Julian oscillation Pacific teleconnections: The impact of the base state and MJO representation in general circulation models. J. Climate, 30, 4567–4587, https://doi.org/10.1175/JCLI-D-16-0789.1.
Hoskins, B. J., and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38, 1179–1196, https://doi.org/10.1175/1520-0469(1981)038<1179:TSLROA>2.0.CO;2.
Hoskins, B. J., and P. J. Valdes, 1990: On the existence of storm-tracks. J. Atmos. Sci., 47, 1854–1864, https://doi.org/10.1175/1520-0469(1990)047<1854:OTEOST>2.0.CO;2.
Hoskins, B. J., and T. Ambrizzi, 1993: Rossby wave propagation on a realistic longitudinally varying flow. J. Atmos. Sci., 50, 1661–1671, https://doi.org/10.1175/1520-0469(1993)050<1661:RWPOAR>2.0.CO;2.
Hoskins, B. J., I. N. James, and G. H. White, 1983: The shape, propagation, and mean-flow interaction of large-scale weather systems. J. Atmos. Sci., 40, 1595–1612, https://doi.org/10.1175/1520-0469(1983)040<1595:TSPAMF>2.0.CO;2.
Lee, S., and I. M. Held, 1993: Baroclinic wave packets in models and observations. J. Atmos. Sci., 50, 1413–1428, https://doi.org/10.1175/1520-0469(1993)050<1413:BWPIMA>2.0.CO;2.
Lee, S. H., J. C. Furtado, and A. J. Charlton-Perez, 2019: Wintertime North American weather regimes and the Arctic stratospheric polar vortex. Geophys. Res. Lett., 46, 14 892–14 900, https://doi.org/10.1029/2019GL085592.
Lee, S. H., M. K. Tippett, and L. M. Polvani, 2023a: A new year-round weather regime classification for North America. J. Climate, 36, 7091–7108, https://doi.org/10.1175/JCLI-D-23-0214.1.
Lee, S. H., M. K. Tippett, and L. M. Polvani, 2023b: Data for “A new year-round weather regime classification for North America”. Zenodo, accessed 23 February 2024, https://doi.org/10.5281/zenodo.8165165.
Liebmann, B., and C. A. Smith, 1996: Description of a complete (interpolated) outgoing longwave radiation dataset. Bull. Amer. Meteor. Soc., 77, 1275–1277.
Lin, H., and G. Brunet, 2018: Extratropical response to the MJO: Nonlinearity and sensitivity to the initial state. J. Atmos. Sci., 75, 219–234, https://doi.org/10.1175/JAS-D-17-0189.1.
Luo, D., W. Zhang, L. Zhong, and A. Dai, 2019: A nonlinear theory of atmospheric blocking: A potential vorticity gradient view. J. Atmos. Sci., 76, 2399–2427, https://doi.org/10.1175/JAS-D-18-0324.1.
MacRitchie, K., and P. E. Roundy, 2016: The two-way relationship between the Madden–Julian oscillation and anticyclonic wave breaking. Quart. J. Roy. Meteor. Soc., 142, 2159–2167, https://doi.org/10.1002/qj.2809.
Madden, R. A., and P. R. Julian, 1971: Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702–708, https://doi.org/10.1175/1520-0469(1971)028<0702:DOADOI>2.0.CO;2.
Madden, R. A., and P. Julian, 1972: Description of global-scale circulation cells in the tropics with a 40–50 day period. J. Atmos. Sci., 29, 1109–1123, https://doi.org/10.1175/1520-0469(1972)029<1109:DOGSCC>2.0.CO;2.
Martius, O., E. Zenklusen, C. Schwierz, and H. C. Davies, 2006: Episodes of alpine heavy precipitation with an overlying elongated stratospheric intrusion: A climatology. Int. J. Climatol., 26, 1149–1164, https://doi.org/10.1002/joc.1295.
McIntyre, M. E., and T. N. Palmer, 1983: Breaking planetary waves in the stratosphere. Nature, 305, 593–600, https://doi.org/10.1038/305593a0.
McIntyre, M. E., and T. N. Palmer, 1984: The ‘surf zone’ in the stratosphere. J. Atmos. Terr. Phys., 46, 825–849, https://doi.org/10.1016/0021-9169(84)90063-1.
Michaelis, A. C., A. C. Martin, M. A. Fish, C. W. Hecht, and F. M. Ralph, 2021: Modulation of atmospheric rivers by mesoscale frontal waves and latent heating: Comparison of two U.S. West Coast events. Mon. Wea. Rev., 149, 2755–2776, https://doi.org/10.1175/MWR-D-20-0364.1.
Moore, B. J., 2023: Flow dependence of medium-range precipitation forecast skill over California. Wea. Forecasting, 38, 699–720, https://doi.org/10.1175/WAF-D-22-0081.1.
Moore, B. J., A. B. White, and D. J. Gottas, 2021: Characteristics of long-duration heavy precipitation events along the West Coast of the United States. Mon. Wea. Rev., 149, 2255–2277, https://doi.org/10.1175/MWR-D-20-0336.1.
Mullen, S. L., 1986: The local balances of vorticity and heat for blocking anticyclones in a spectral general circulation model. J. Atmos. Sci., 43, 1406–1441, https://doi.org/10.1175/1520-0469(1986)043<1406:TLBOVA>2.0.CO;2.
Mullen, S. L., 1987: Transient eddy forcing of blocking flows. J. Atmos. Sci., 44, 3–22, https://doi.org/10.1175/1520-0469(1987)044<0003:TEFOBF>2.0.CO;2.
Mundhenk, B. D., E. A. Barnes, and E. D. Maloney, 2016: All-season climatology and variability of atmospheric river frequencies over the North Pacific. J. Climate, 29, 4885–4903, https://doi.org/10.1175/JCLI-D-15-0655.1.
Mundhenk, B. D., E. A. Barnes, E. D. Maloney, and C. F. Baggett, 2018: Skillful empirical subseasonal prediction of landfalling atmospheric river activity using the Madden–Julian oscillation and quasi-biennial oscillation. npj Climate Atmos. Sci., 1, 20177, https://doi.org/10.1038/s41612-017-0008-2.
Orlanski, I., and E. K. M. Chang, 1993: Ageostrophic geopotential fluxes in downstream and upstream development of baroclinic waves. J. Atmos. Sci., 50, 212–225, https://doi.org/10.1175/1520-0469(1993)050<0212:AGFIDA>2.0.CO;2.
Orlanski, I., and J. P. Sheldon, 1995: Stages in energetics of baroclinic systems. Tellus, 47A, 605–628, https://doi.org/10.3402/tellusa.v47i5.11553.
Otkin, J. A., and J. E. Martin, 2004: A synoptic climatology of the subtropical kona storm. Mon. Wea. Rev., 132, 1502–1517, https://doi.org/10.1175/1520-0493(2004)132<1502:ASCOTS>2.0.CO;2.
Papin, P. P., L. F. Bosart, and R. D. Torn, 2020: A feature-based approach to classifying summertime potential vorticity streamers linked to Rossby wave breaking the North Atlantic basin. J. Climate, 33, 5953–5969, https://doi.org/10.1175/JCLI-D-19-0812.1.
Parker, D. J., 1998: Secondary frontal waves in the North Atlantic region: A dynamical perspective of current ideas. Quart. J. Roy. Meteor. Soc., 124, 829–856, https://doi.org/10.1002/qj.49712454709.
Payne, A. E., and G. Magnusdottir, 2014: Dynamics of landfalling atmospheric rivers over the North Pacific in 30 years of MERRA reanalysis. J. Climate, 27, 7133–7150, https://doi.org/10.1175/JCLI-D-14-00034.1.
Quinting, J. F., and F. Vitart, 2019: Representation of synoptic-scale Rossby wave packets and blocking in the S2S prediction project database. Geophys. Res. Lett., 46, 1070–1078, https://doi.org/10.1029/2018GL081381.
Ralph, F. M., M. D. Dettinger, M. M. Cairns, T. J. Galarneau, and J. Eylander, 2018: Defining “Atmospheric River”: How the Glossary of Meteorology helped resolve a debate. Bull. Amer. Meteor. Soc., 99, 837–839, https://doi.org/10.1175/BAMS-D-17-0157.1.
Ralph, F. M., J. J. Rutz, J. M. Cordeira, M. Dettinger, M. Anderson, D. Reynolds, L. J. Schick, and C. Smallcomb, 2019: A scale to characterize the strength and impacts of atmospheric rivers. Bull. Amer. Meteor. Soc., 100, 269–289, https://doi.org/10.1175/BAMS-D-18-0023.1.
Rex, D. F., 1950: Blocking action in the middle troposphere and its effect upon regional climate. Tellus, 2, 196–211, https://doi.org/10.1111/j.2153-3490.1950.tb00331.x.
Robertson, A. W., N. Viguard, J. Yuan, and M. K. Tippett, 2020: Toward identifying subseasonal forecasts of opportunity using North American weather regimes. Mon. Wea. Rev., 148, 1861–1875, https://doi.org/10.1175/MWR-D-19-0285.1.
Röthlisberger, M., O. Martius, and H. Wernli, 2018: Northern Hemisphere Rossby wave initiation events on the extratropical jet—A climatological analysis. J. Climate, 31, 743–760, https://doi.org/10.1175/JCLI-D-17-0346.1.
Saha, S., and Coauthors, 2010: The NCEP climate forecast system reanalysis. Bull. Amer. Meteor. Soc., 91, 1015–1058, https://doi.org/10.1175/2010BAMS3001.1.
Saha, S., and Coauthors, 2014: The NCEP Climate Forecast System version 2. J. Climate, 27, 2185–2208, https://doi.org/10.1175/JCLI-D-12-00823.1.
Sardeshmukh, P. D., and B. J. Hoskins, 1988: The generation of global rotational flow by steady idealized tropical divergence. J. Atmos. Sci., 45, 1228–1251, https://doi.org/10.1175/1520-0469(1988)045<1228:TGOGRF>2.0.CO;2.
Simmons, A. J., and B. J. Hoskins, 1979: The downstream and upstream development of unstable baroclinic waves. J. Atmos. Sci., 36, 1239–1254, https://doi.org/10.1175/1520-0469(1979)036<1239:TDAUDO>2.0.CO;2.
Simmons, A. J., J. M. Wallace, and G. W. Branstator, 1983: Barotropic wave propagation and instability, and atmospheric teleconnection patterns. J. Atmos. Sci., 40, 1363–1392, https://doi.org/10.1175/1520-0469(1983)040<1363:BWPAIA>2.0.CO;2.
Simpson, R. H., 1952: Evolution of the Kona storm, a subtropical cyclone. J. Meteor., 9, 24–35, https://doi.org/10.1175/1520-0469(1952)009<0024:EOTKSA>2.0.CO;2.
Sutcliffe, R. C., 1947: A contribution to the problem of development. Quart. J. Roy. Meteor. Soc., 73, 370–383, https://doi.org/10.1002/qj.49707331710.
Torn, R. D., and G. J. Hakim, 2015: Comparison of wave packets associated with extratropical transition and winter cyclones. Mon. Wea. Rev., 143, 1782–1803, https://doi.org/10.1175/MWR-D-14-00006.1.
Trenberth, K. E., 1978: On the interpretation of the diagnostic quasi-geostrophic omega equation. Mon. Wea. Rev., 106, 131–137, https://doi.org/10.1175/1520-0493(1978)106<0131:OTIOTD>2.0.CO;2.
Uccellini, L. W., and P. J. Kocin, 1987: The interaction of jet streak circulations during heavy snow events along the east coast of the United States. Wea. Forecasting, 2, 289–308, https://doi.org/10.1175/1520-0434(1987)002<0289:TIOJSC>2.0.CO;2.
Wang, J., M. J. DeFlorio, B. Guan, and C. M. Castellano, 2023: Seasonality of MJO impacts on precipitation extremes over the western United States. J. Hydrometeor., 24, 151–166, https://doi.org/10.1175/JHM-D-22-0089.1.
Wenta, M., C. M. Grams, L. Papritz, and M. Federer, 2024: Linking Gulf Stream air–sea interactions to the exceptional blocking episode in February 2019: A Lagrangian perspective. Wea. Climate Dyn., 5, 181–209, https://doi.org/10.5194/wcd-5-181-2024.
Wernli, H., and M. Sprenger, 2007: Identification and ERA-15 climatology of potential vorticity streamers and cutoffs near the extratropical tropopause. J. Atmos. Sci., 64, 1569–1586, https://doi.org/10.1175/JAS3912.1.
Wheeler, M. C., and H. H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917–1932, https://doi.org/10.1175/1520-0493(2004)132<1917:AARMMI>2.0.CO;2.
Winters, A. C., 2021a: Subseasonal prediction of the state and evolution of the North Pacific jet stream. J. Geophys. Res. Atmos., 126, e2021JD035094, https://doi.org/10.1029/2021JD035094.
Winters, A. C., 2021b: The state of the North Pacific jet and North Atlantic jet in the context of their two leading modes of variability. University at Colorado Boulder, accessed 23 February 2024, https://doi.org/10.25810/CKN0-GP39.
Winters, A. C., D. Keyser, and L. F. Bosart, 2019: The development of the North Pacific jet phase diagram as an objective tool to monitor the state and forecast skill of the upper-tropospheric flow pattern. Wea. Forecasting, 34, 199–219, https://doi.org/10.1175/WAF-D-18-0106.1.
Zhu, Y., and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725–735, https://doi.org/10.1175/1520-0493(1998)126<0725:APAFMF>2.0.CO;2.
