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Abstract
This study employs a long time series (1997–2017) of reforecasts based on a version of the ECMWF Integrated Forecast System to evaluate the dependence of medium-range (i.e., 3–15 days) precipitation forecast skill over California on the state of the large-scale atmospheric flow. As a basis for this evaluation, four recurrent large-scale flow regimes over the North Pacific and western North America associated with precipitation in a domain encompassing northern and central California were objectively identified in ECMWF ERA5 reanalysis data for November–March 1981–2017. Two of the regimes are characterized by zonal upper-level flow across the North Pacific, and the other two are characterized by wavy, blocked flow. Forecast verification statistics conditioned on regime occurrence indicate considerably lower medium-range precipitation skill over California in blocking regimes than in zonal regimes. Moreover, forecasts of blocking regimes tend to exhibit larger errors and uncertainty in the synoptic-scale flow over the eastern North Pacific and western North America compared with forecasts of zonal regimes. Composite analyses for blocking forecasts reveal a tendency for errors to develop in conjunction with the amplification of a ridge over the western and central North Pacific. The errors in the ridge tend to be communicated through the large-scale Rossby wave pattern, resulting in misforecasting of downstream trough amplification and, thereby, moisture flux and precipitation over California. The composites additionally indicate that error growth in the blocking ridge can be linked to misrepresentation of baroclinic development as well as upper-level divergent outflow associated with latent heat release.
Significance Statement
This study examines the degree to which the medium-range (out to ∼2-week lead time) precipitation forecast skill over California depends on the large-scale atmospheric flow regime over the North Pacific. An evaluation of retrospective model forecasts from ECMWF for 1997–2017 reveals that the skill tends to be considerably lower in regimes featuring a wavy, “blocked” North Pacific jet stream than in regimes featuring a west–east-oriented jet stream. This difference in skill relates to a tendency for forecasts of blocked regimes to exhibit significantly larger errors than forecasts of zonal regimes. The results could aid forecasters by increasing situational awareness and informing the interpretation and application of model forecasts for precipitation affecting California.
Abstract
This study employs a long time series (1997–2017) of reforecasts based on a version of the ECMWF Integrated Forecast System to evaluate the dependence of medium-range (i.e., 3–15 days) precipitation forecast skill over California on the state of the large-scale atmospheric flow. As a basis for this evaluation, four recurrent large-scale flow regimes over the North Pacific and western North America associated with precipitation in a domain encompassing northern and central California were objectively identified in ECMWF ERA5 reanalysis data for November–March 1981–2017. Two of the regimes are characterized by zonal upper-level flow across the North Pacific, and the other two are characterized by wavy, blocked flow. Forecast verification statistics conditioned on regime occurrence indicate considerably lower medium-range precipitation skill over California in blocking regimes than in zonal regimes. Moreover, forecasts of blocking regimes tend to exhibit larger errors and uncertainty in the synoptic-scale flow over the eastern North Pacific and western North America compared with forecasts of zonal regimes. Composite analyses for blocking forecasts reveal a tendency for errors to develop in conjunction with the amplification of a ridge over the western and central North Pacific. The errors in the ridge tend to be communicated through the large-scale Rossby wave pattern, resulting in misforecasting of downstream trough amplification and, thereby, moisture flux and precipitation over California. The composites additionally indicate that error growth in the blocking ridge can be linked to misrepresentation of baroclinic development as well as upper-level divergent outflow associated with latent heat release.
Significance Statement
This study examines the degree to which the medium-range (out to ∼2-week lead time) precipitation forecast skill over California depends on the large-scale atmospheric flow regime over the North Pacific. An evaluation of retrospective model forecasts from ECMWF for 1997–2017 reveals that the skill tends to be considerably lower in regimes featuring a wavy, “blocked” North Pacific jet stream than in regimes featuring a west–east-oriented jet stream. This difference in skill relates to a tendency for forecasts of blocked regimes to exhibit significantly larger errors than forecasts of zonal regimes. The results could aid forecasters by increasing situational awareness and informing the interpretation and application of model forecasts for precipitation affecting California.
Abstract
Prolonged periods (e.g., several days or more) of heavy precipitation can result in sustained high-impact flooding. Herein, an investigation of long-duration heavy precipitation events (HPEs), defined as periods comprising ≥3 days with precipitation exceeding the climatological 95th percentile, is conducted for 1979–2019 for the U.S. West Coast, specifically Northern California. An objective flow-based categorization method is applied to identify principal large-scale flow patterns for the events. Four categories are identified and examined through composite analyses and case studies. Two of the categories are characterized by a strong zonal jet stream over the eastern North Pacific, while the other two are characterized by atmospheric blocking over the central North Pacific and the Bering Sea–Alaska region, respectively. The composites and case studies demonstrate that the flow patterns for the HPEs tend to remain in place for several days, maintaining strong baroclinicity and promoting occurrences of multiple cyclones in rapid succession near the West Coast. The successive cyclones result in persistent water vapor flux and forcing for ascent over Northern California, sustaining heavy precipitation. For the zonal jet patterns, cyclones affecting the West Coast tend to occur in the poleward jet exit region in association with cyclonic Rossby wave breaking. For the blocking patterns, cyclones tend to occur in association with anticyclonic Rossby wave breaking on the downstream flank of the block. For Bering Sea–Alaska blocking cases, cyclones can move into this region in conjunction with cyclonically breaking waves that extend into the eastern North Pacific from the upstream flank of the block.
Abstract
Prolonged periods (e.g., several days or more) of heavy precipitation can result in sustained high-impact flooding. Herein, an investigation of long-duration heavy precipitation events (HPEs), defined as periods comprising ≥3 days with precipitation exceeding the climatological 95th percentile, is conducted for 1979–2019 for the U.S. West Coast, specifically Northern California. An objective flow-based categorization method is applied to identify principal large-scale flow patterns for the events. Four categories are identified and examined through composite analyses and case studies. Two of the categories are characterized by a strong zonal jet stream over the eastern North Pacific, while the other two are characterized by atmospheric blocking over the central North Pacific and the Bering Sea–Alaska region, respectively. The composites and case studies demonstrate that the flow patterns for the HPEs tend to remain in place for several days, maintaining strong baroclinicity and promoting occurrences of multiple cyclones in rapid succession near the West Coast. The successive cyclones result in persistent water vapor flux and forcing for ascent over Northern California, sustaining heavy precipitation. For the zonal jet patterns, cyclones affecting the West Coast tend to occur in the poleward jet exit region in association with cyclonic Rossby wave breaking. For the blocking patterns, cyclones tend to occur in association with anticyclonic Rossby wave breaking on the downstream flank of the block. For Bering Sea–Alaska blocking cases, cyclones can move into this region in conjunction with cyclonically breaking waves that extend into the eastern North Pacific from the upstream flank of the block.
Abstract
This study investigates the evolution of two zonally elongated atmospheric rivers (ARs) that produced >200 mm of rainfall over mountainous regions of Northern California in late October 2010. Synoptic-scale analysis and air parcel trajectory analysis indicate that the ARs developed within high-CAPE environments characterized by troposphere-deep ascent as water vapor was transported directly from western North Pacific tropical cyclones (TCs) toward the equatorward entrance region of an intensifying North Pacific jet stream (NPJ). The same ARs were subsequently maintained as water vapor was transported from extratropical and subtropical regions over the central and eastern North Pacific in an environment characterized by quasigeostrophic forcing for ascent and strong frontogenesis along the anticyclonic shear side of an intense and zonally extended NPJ. Although the ARs developed in conjunction with water vapor transported from regions near TCs and in the presence of troposphere-deep ascent, an atmospheric water vapor budget illustrates that decreases in integrated water vapor (IWV) via precipitation are largely offset by the horizontal aggregation of water vapor along the AR corridors via IWV flux convergence in the presence of frontogenesis. The frameworks used for investigations of predecessor rain events ahead of TCs and of interactions between recurving TCs and the NPJ are also utilized to illustrate many dynamically similar processes related to AR development and evolution. Similarities include the following: water vapor transport directly from a TC, troposphere-deep ascent in a high-CAPE environment beneath the equatorward entrance region of an intensifying upper-tropospheric jet streak, interactions between diabatic outflow and an upper-tropospheric jet streak, and strong frontogenesis.
Abstract
This study investigates the evolution of two zonally elongated atmospheric rivers (ARs) that produced >200 mm of rainfall over mountainous regions of Northern California in late October 2010. Synoptic-scale analysis and air parcel trajectory analysis indicate that the ARs developed within high-CAPE environments characterized by troposphere-deep ascent as water vapor was transported directly from western North Pacific tropical cyclones (TCs) toward the equatorward entrance region of an intensifying North Pacific jet stream (NPJ). The same ARs were subsequently maintained as water vapor was transported from extratropical and subtropical regions over the central and eastern North Pacific in an environment characterized by quasigeostrophic forcing for ascent and strong frontogenesis along the anticyclonic shear side of an intense and zonally extended NPJ. Although the ARs developed in conjunction with water vapor transported from regions near TCs and in the presence of troposphere-deep ascent, an atmospheric water vapor budget illustrates that decreases in integrated water vapor (IWV) via precipitation are largely offset by the horizontal aggregation of water vapor along the AR corridors via IWV flux convergence in the presence of frontogenesis. The frameworks used for investigations of predecessor rain events ahead of TCs and of interactions between recurving TCs and the NPJ are also utilized to illustrate many dynamically similar processes related to AR development and evolution. Similarities include the following: water vapor transport directly from a TC, troposphere-deep ascent in a high-CAPE environment beneath the equatorward entrance region of an intensifying upper-tropospheric jet streak, interactions between diabatic outflow and an upper-tropospheric jet streak, and strong frontogenesis.
Abstract
Linkages between extreme precipitation events (EPEs) in the central and eastern United States and synoptic-scale Rossby wave breaking are investigated using 1979–2015 climatologies of EPEs and upper-level potential vorticity (PV) streamers. The investigation focuses on two domains over the central and eastern United States, respectively, and emphasizes widespread EPEs, events exhibiting exceptionally large precipitation volumes. The relative frequency of PV streamers is found to be significantly enhanced relative to climatology immediately upstream of each domain during widespread EPEs. Majorities of the widespread EPEs in the central (~79%) and eastern (~56%) U.S. domains co-occur with a PV streamer positioned immediately upstream. Odds ratios of EPEs for days when a PV streamer occurs upstream of each domain indicate a strong, statistically significant association between EPEs and Rossby wave breaking. The strength of the EPE–Rossby wave breaking linkage, as measured by co-occurrence fractions and odds ratios, tends to increase with increasing EPE precipitation volume, such that the strongest linkage exists for widespread EPEs. Composite analyses reveal that Rossby wave breaking can result in widespread EPEs by establishing a persistent high-amplitude synoptic-scale wave pattern, within which strong poleward water vapor transport and ascent are forced over the EPE region immediately downstream of an elongated upper-level trough. Additional analyses demonstrate that, compared to corresponding null cases, Rossby wave breaking cases resulting in widespread EPEs exhibit a significantly higher-amplitude wave pattern that favors greater poleward transport of moist, conditionally unstable air and stronger ascent over the EPE region.
Abstract
Linkages between extreme precipitation events (EPEs) in the central and eastern United States and synoptic-scale Rossby wave breaking are investigated using 1979–2015 climatologies of EPEs and upper-level potential vorticity (PV) streamers. The investigation focuses on two domains over the central and eastern United States, respectively, and emphasizes widespread EPEs, events exhibiting exceptionally large precipitation volumes. The relative frequency of PV streamers is found to be significantly enhanced relative to climatology immediately upstream of each domain during widespread EPEs. Majorities of the widespread EPEs in the central (~79%) and eastern (~56%) U.S. domains co-occur with a PV streamer positioned immediately upstream. Odds ratios of EPEs for days when a PV streamer occurs upstream of each domain indicate a strong, statistically significant association between EPEs and Rossby wave breaking. The strength of the EPE–Rossby wave breaking linkage, as measured by co-occurrence fractions and odds ratios, tends to increase with increasing EPE precipitation volume, such that the strongest linkage exists for widespread EPEs. Composite analyses reveal that Rossby wave breaking can result in widespread EPEs by establishing a persistent high-amplitude synoptic-scale wave pattern, within which strong poleward water vapor transport and ascent are forced over the EPE region immediately downstream of an elongated upper-level trough. Additional analyses demonstrate that, compared to corresponding null cases, Rossby wave breaking cases resulting in widespread EPEs exhibit a significantly higher-amplitude wave pattern that favors greater poleward transport of moist, conditionally unstable air and stronger ascent over the EPE region.
Abstract
During winter 2016/17, California experienced numerous heavy precipitation events linked to land-falling atmospheric rivers (ARs) that filled reservoirs and ended a severe, multiyear drought. These events also caused floods, mudslides, and debris flows, resulting in major socioeconomic disruptions. During 2–11 February 2017, persistent heavy precipitation in the northern Sierra Nevada culminated in a rapid increase in the water level on Lake Oroville, necessitating the activation of an emergency spillway for the first time since the Oroville Dam was installed and forcing the evacuation of 188,000 people. The precipitation, which mostly fell as rain due to elevated freezing levels, was focused on the western slope of the Sierra Nevada in connection with orographic forcing linked to two successive ARs. Heavy rain fell on saturated soils and a snowpack produced by antecedent storms and thereby resulted in excessive runoff into Lake Oroville that led to a damaged spillway and complicated reservoir operations.
Abstract
During winter 2016/17, California experienced numerous heavy precipitation events linked to land-falling atmospheric rivers (ARs) that filled reservoirs and ended a severe, multiyear drought. These events also caused floods, mudslides, and debris flows, resulting in major socioeconomic disruptions. During 2–11 February 2017, persistent heavy precipitation in the northern Sierra Nevada culminated in a rapid increase in the water level on Lake Oroville, necessitating the activation of an emergency spillway for the first time since the Oroville Dam was installed and forcing the evacuation of 188,000 people. The precipitation, which mostly fell as rain due to elevated freezing levels, was focused on the western slope of the Sierra Nevada in connection with orographic forcing linked to two successive ARs. Heavy rain fell on saturated soils and a snowpack produced by antecedent storms and thereby resulted in excessive runoff into Lake Oroville that led to a damaged spillway and complicated reservoir operations.
Abstract
A 915-MHz wind profiler, a GPS receiver, and surface meteorological sites in and near California’s northern Central Valley (CV) provide the observational anchor for a case study on 23–25 October 2010. The study highlights key orographic influences on precipitation distributions and intensities across northern California during a landfalling atmospheric river (AR) and an associated Sierra barrier jet (SBJ). A detailed wind profiler/GPS analysis documents an intense AR overriding a shallow SBJ at ~750 m MSL, resulting in record early season precipitation. The SBJ diverts shallow, pre-cold-frontal, incoming water vapor within the AR poleward from the San Francisco Bay gap to the northern CV. The SBJ ultimately decays following the passage of the AR and trailing polar cold front aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta–Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta–Trinity region, respectively, the precipitation becomes enhanced. Vertical profiles of the linear correlation coefficient quantify the orographic linkage between hourly upslope water vapor flux profiles and hourly rain rate. The altitude of maximum correlation (i.e., orographic controlling layer) is lower for the shallow SBJ than for the deeper AR (i.e., 0.90 versus 1.15 km MSL, respectively). This case study expands the understanding of orographic precipitation enhancement from coastal California to its interior. It also quantifies the connection between dry antecedent soils and reduced flood potential.
Abstract
A 915-MHz wind profiler, a GPS receiver, and surface meteorological sites in and near California’s northern Central Valley (CV) provide the observational anchor for a case study on 23–25 October 2010. The study highlights key orographic influences on precipitation distributions and intensities across northern California during a landfalling atmospheric river (AR) and an associated Sierra barrier jet (SBJ). A detailed wind profiler/GPS analysis documents an intense AR overriding a shallow SBJ at ~750 m MSL, resulting in record early season precipitation. The SBJ diverts shallow, pre-cold-frontal, incoming water vapor within the AR poleward from the San Francisco Bay gap to the northern CV. The SBJ ultimately decays following the passage of the AR and trailing polar cold front aloft. A statistical analysis of orographic forcing reveals that both the AR and SBJ are crucial factors in determining the amount and spatial distribution of precipitation in the northern Sierra Nevada and in the Shasta–Trinity region at the northern terminus of the CV. As the AR and SBJ flow ascends the steep and tall terrain of the northern Sierra and Shasta–Trinity region, respectively, the precipitation becomes enhanced. Vertical profiles of the linear correlation coefficient quantify the orographic linkage between hourly upslope water vapor flux profiles and hourly rain rate. The altitude of maximum correlation (i.e., orographic controlling layer) is lower for the shallow SBJ than for the deeper AR (i.e., 0.90 versus 1.15 km MSL, respectively). This case study expands the understanding of orographic precipitation enhancement from coastal California to its interior. It also quantifies the connection between dry antecedent soils and reduced flood potential.
Abstract
A multiscale analysis is presented of extreme precipitation events (EPEs) in Northern California during winter 2016–17, which caused flooding and contributed substantially to highly anomalous seasonal precipitation totals. The EPEs were characterized by long durations (≥7 days) and involved persistent large-scale flow patterns. The three largest EPEs involved blocking over the Bering Sea–Alaska region. A detailed investigation of the largest EPE, occurring on 2–10 February 2017, reveals that extreme precipitation was produced as four discrete cyclones moved across the eastern North Pacific equatorward of a high-amplitude blocking ridge and impacted the U.S. West Coast in rapid succession. The latter three cyclones developed and moved in conjunction with elongated negatively tilted troughs or PV streamers resulting from repeated episodes of baroclinic development and cyclonic Rossby wave breaking on the upstream flank of the block. Each of the four cyclones interacted with a PV streamer and an associated baroclinic zone established by anticyclonic wave breaking on the downstream flank of the block and, thereby, tracked into the U.S. West Coast. The serial clustering of the cyclones during the 9-day event resulted in persistent water vapor flux and lifting that supported extreme precipitation totals in Northern California. A climatological analysis for 1979–2017 reveals a significant statistical relationship between blocking over the Bering Sea–Alaska region and EPEs in Northern California, indicating that this type of blocking pattern represents a favorable large-scale scenario for extreme precipitation in Northern California.
Abstract
A multiscale analysis is presented of extreme precipitation events (EPEs) in Northern California during winter 2016–17, which caused flooding and contributed substantially to highly anomalous seasonal precipitation totals. The EPEs were characterized by long durations (≥7 days) and involved persistent large-scale flow patterns. The three largest EPEs involved blocking over the Bering Sea–Alaska region. A detailed investigation of the largest EPE, occurring on 2–10 February 2017, reveals that extreme precipitation was produced as four discrete cyclones moved across the eastern North Pacific equatorward of a high-amplitude blocking ridge and impacted the U.S. West Coast in rapid succession. The latter three cyclones developed and moved in conjunction with elongated negatively tilted troughs or PV streamers resulting from repeated episodes of baroclinic development and cyclonic Rossby wave breaking on the upstream flank of the block. Each of the four cyclones interacted with a PV streamer and an associated baroclinic zone established by anticyclonic wave breaking on the downstream flank of the block and, thereby, tracked into the U.S. West Coast. The serial clustering of the cyclones during the 9-day event resulted in persistent water vapor flux and lifting that supported extreme precipitation totals in Northern California. A climatological analysis for 1979–2017 reveals a significant statistical relationship between blocking over the Bering Sea–Alaska region and EPEs in Northern California, indicating that this type of blocking pattern represents a favorable large-scale scenario for extreme precipitation in Northern California.
Abstract
A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supported the successive development of two quasi-stationary mesoscale convective systems (MCSs) on 1 and 2 May, respectively. These MCSs were both linearly organized and exhibited back-building and echo-training, processes that afforded the repeated movement of convective cells over the same area of western and central Tennessee and Kentucky, resulting in a narrow band of rainfall totals of 200–400 mm. Mesoscale analyses reveal that the MCSs developed on the warm side of a slow-moving cold front and that the interaction between the southerly LLJ and convectively generated outflow boundaries was fundamental for generating convection.
Abstract
A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supported the successive development of two quasi-stationary mesoscale convective systems (MCSs) on 1 and 2 May, respectively. These MCSs were both linearly organized and exhibited back-building and echo-training, processes that afforded the repeated movement of convective cells over the same area of western and central Tennessee and Kentucky, resulting in a narrow band of rainfall totals of 200–400 mm. Mesoscale analyses reveal that the MCSs developed on the warm side of a slow-moving cold front and that the interaction between the southerly LLJ and convectively generated outflow boundaries was fundamental for generating convection.
Abstract
Five 915-MHz wind profilers and GPS receivers across California's northern Central Valley (CV) and adjacent Sierra foothills and coastal zone, in tandem with a 6-km-resolution gridded reanalysis dataset generated from the Weather Research and Forecasting Model, document key spatiotemporal characteristics of Sierra barrier jets (SBJs), landfalling atmospheric rivers (ARs), and their interactions. Composite kinematic and thermodynamic analyses are based on the 13 strongest SBJ cases observed by the Sloughhouse profiler between 2009 and 2011. The analyses show shallow, cool, south-southeasterly (i.e., Sierra parallel) flow and associated water vapor transport strengthening with time early in the 24-h compositing period, culminating in an SBJ core at <1 km above ground over the eastern CV. The SBJ core increases in altitude up the Sierra's windward slope and poleward toward the north end of the CV, but it does not reach the westernmost CV. Above the developing SBJ, strengthening southwesterly flow descends temporally in response to the landfalling AR. The moistening SBJ reaches maximum intensity during the strongest AR flow aloft, at which time the core of the AR-parallel vapor transport slopes over the SBJ. The inland penetration of the AR through the San Francisco Bay gap in the coastal mountains contributes to SBJ moistening and deepening. The SBJ subsequently weakens with the initial cold-frontal period aloft, during which the shallow flow shifts to southwesterly and the heaviest precipitation falls in the Sierra foothills. An orographic precipitation analysis quantitatively links the Sierra-perpendicular (nearly AR parallel) vapor fluxes to enhanced precipitation along the Sierra's windward slope and the SBJ-parallel fluxes to heavy precipitation at the north end of the CV.
Abstract
Five 915-MHz wind profilers and GPS receivers across California's northern Central Valley (CV) and adjacent Sierra foothills and coastal zone, in tandem with a 6-km-resolution gridded reanalysis dataset generated from the Weather Research and Forecasting Model, document key spatiotemporal characteristics of Sierra barrier jets (SBJs), landfalling atmospheric rivers (ARs), and their interactions. Composite kinematic and thermodynamic analyses are based on the 13 strongest SBJ cases observed by the Sloughhouse profiler between 2009 and 2011. The analyses show shallow, cool, south-southeasterly (i.e., Sierra parallel) flow and associated water vapor transport strengthening with time early in the 24-h compositing period, culminating in an SBJ core at <1 km above ground over the eastern CV. The SBJ core increases in altitude up the Sierra's windward slope and poleward toward the north end of the CV, but it does not reach the westernmost CV. Above the developing SBJ, strengthening southwesterly flow descends temporally in response to the landfalling AR. The moistening SBJ reaches maximum intensity during the strongest AR flow aloft, at which time the core of the AR-parallel vapor transport slopes over the SBJ. The inland penetration of the AR through the San Francisco Bay gap in the coastal mountains contributes to SBJ moistening and deepening. The SBJ subsequently weakens with the initial cold-frontal period aloft, during which the shallow flow shifts to southwesterly and the heaviest precipitation falls in the Sierra foothills. An orographic precipitation analysis quantitatively links the Sierra-perpendicular (nearly AR parallel) vapor fluxes to enhanced precipitation along the Sierra's windward slope and the SBJ-parallel fluxes to heavy precipitation at the north end of the CV.
Abstract
The synoptic-scale environments of predecessor rain events (PREs) occurring to the east of the Rocky Mountains in association with Atlantic basin tropical cyclones (TCs) are examined. PREs that occurred during 1988–2010 are subjectively classified based upon the synoptic-scale upper-level flow configuration within which the PRE develops, with a focus on the following: 1) the position of the jet streak relative to the TC, 2) the position of the jet streak relative to trough and ridge axes, and 3) the positions of trough and ridge axes relative to the PRE and to the TC. Three categories were identified from this classification procedure: “jet in ridge,” “southwesterly jet,” and “downstream confluence.” PRE-relative composite analysis for each category reveals that, consistent with previous studies, PREs typically occur near a low-level baroclinic zone, beneath the equatorward entrance region of an upper-level jet streak, and in the presence of a stream of water vapor from a TC. Despite these common characteristics, key differences exist among the three PRE categories related to the phasing of a TC with the synoptic-scale flow and to the interactions between a TC and its environment. Brief case studies of PREs associated with TC Rita (2005), TC Wilma (2005), and TC Ernesto (2006) are presented as specific examples of the three PRE categories.
Abstract
The synoptic-scale environments of predecessor rain events (PREs) occurring to the east of the Rocky Mountains in association with Atlantic basin tropical cyclones (TCs) are examined. PREs that occurred during 1988–2010 are subjectively classified based upon the synoptic-scale upper-level flow configuration within which the PRE develops, with a focus on the following: 1) the position of the jet streak relative to the TC, 2) the position of the jet streak relative to trough and ridge axes, and 3) the positions of trough and ridge axes relative to the PRE and to the TC. Three categories were identified from this classification procedure: “jet in ridge,” “southwesterly jet,” and “downstream confluence.” PRE-relative composite analysis for each category reveals that, consistent with previous studies, PREs typically occur near a low-level baroclinic zone, beneath the equatorward entrance region of an upper-level jet streak, and in the presence of a stream of water vapor from a TC. Despite these common characteristics, key differences exist among the three PRE categories related to the phasing of a TC with the synoptic-scale flow and to the interactions between a TC and its environment. Brief case studies of PREs associated with TC Rita (2005), TC Wilma (2005), and TC Ernesto (2006) are presented as specific examples of the three PRE categories.
