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Jason M. Cordeira and Neil F. Laird

Abstract

It is generally understood that extensive regions of significant lake ice cover impact lake-effect (LE) snow storms by decreasing the upward heat and moisture fluxes from the lake surface; however, it is only recently that studies have been conducted to more thoroughly examine this relationship. This study provides the first examination of Great Lakes LE snow storms that developed in association with an extensively ice-covered lake. The LE snow events that occurred downwind of Lake Erie on 12–14 February 2003 and 28–31 January 2004 produced maximum snowfall totals of 43 and 64 cm in western New York state, respectively. The presence of widespread ice cover led these snows to be less anticipated than snowfalls from Lake Ontario, which had limited ice cover. For both events, a variety of ice-cover conditions and meso- and synoptic-scale factors (i) helped support LE snow storm development, (ii) lead to the transitions in LE convective type, and (iii) resulted in noteworthy snowfalls near Lake Erie. Thinner ice cover along with favorable fetch directions during the 2004 event likely aided the development of more significant snowband time periods and the resulting greater snowfall. Although Lake Erie had regions with lower ice concentration during the 2003 event, thicker ice cover was present across a greater area of the lake, fetch directions during lake-effect time periods were positioned over higher ice concentration regions, and snowbands had a shorter duration and impacted the same region to a lesser degree than the 2004 case.

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Jason M. Cordeira and Lance F. Bosart

Abstract

The “Perfect Storms” (PSs) were a series of three high-impact extratropical cyclones (ECs) that impacted North America and the North Atlantic in late October and early November 1991. The PSs included the Perfect Storm in the northwest Atlantic, a second EC over the North Atlantic that developed from the interaction of the PS with Hurricane Grace, and a third EC over North America commonly known as the “1991 Halloween Blizzard.” The PSs greatly impacted the North Atlantic and North America with large waves, coastal flooding, heavy snow, and accumulating ice, and they also provided an opportunity to investigate the physical processes that contributed to a downstream baroclinic development (DBD) episode across North America that culminated in the ECs.

Downstream baroclinic development resulted from an amplification of the large-scale flow over the North Pacific that was influenced by anomalous tropical convection, the recurvature and extratropical transition of western North Pacific Tropical Cyclones Orchid, Pat, and Ruth, and the subsequent evolution of the extratropical flow. The progression of DBD occurred following the development of a negative PNA regime and the generation of baroclinic instability over North America associated with equatorward-displaced potential vorticity anomalies and poleward-displaced corridors of high moisture content. An analysis of the eddy kinetic energy tendency equation demonstrated that the resulting baroclinic conversion of eddy available potential energy into eddy kinetic energy during the cyclogenesis process facilitated the progression of DBD across North America and the subsequent development of the ECs.

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Jason M. Cordeira and Lance F. Bosart

Abstract

This paper examines the cyclogenesis of the “Perfect Storms” of late October and early November 1991 over the North Atlantic and focuses on the influence of Hurricane Grace (HG) toward their development. The two storms considered are the “Perfect Storm” (PS) that underwent a warm seclusion process and an extratropical cyclone (EC1) with two development phases. HG, which initially formed via tropical transition (TT), influenced the first phase of EC1 via reduced atmospheric static stability and enhanced low-level baroclinicity. As a result, deep moist convection and latent heat release produced maxima in midtropospheric diabatic heating and lower-tropospheric potential vorticity (PV) that aided the development of EC1. Backward air parcel trajectories and large diabatic contributions to eddy available potential energy (APE) generation suggests that EC1 developed as a diabatic Rossby vortex (DRV)-like feature.

The second and explosively deepening phase of EC1 occurred as the cyclone coupled with an upper-tropospheric PV disturbance (PVD) over the eastern North Atlantic. Backward air parcel trajectories demonstrate the explosive deepening of EC1 involved airstreams originating from east of HG and from over the Labrador Sea. Parcel trajectories and a large baroclinic contribution to eddy APE generation further suggests that the two-phase development of EC1 may have involved a DRV-like feature.

The subsequent recurvature and extratropical transition (ET) of HG occurred in the warm sector of the PS downstream of a second upper-tropospheric PVD over the western North Atlantic. Reduced atmospheric static stability, enhanced warm air advection, and strong latent heat release during the recurvature and ET of HG contributed to the development of a strong, zonally oriented warm front and the warm seclusion of the PS. Parcel trajectory analysis demonstrates that the PS warm seclusion involved the isolation of air parcels by a bent-back warm front that were warmed via sensible heating from the underlying Gulf Stream.

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Jason M. Cordeira and F. Martin Ralph

Abstract

The ability to provide accurate forecasts and improve situational awareness of atmospheric rivers (ARs) is key to impact-based decision support services and applications such as forecast-informed reservoir operations. The purpose of this study is to quantify the cool-season water year skill for 2017–20 of the NCEP Global Ensemble Forecast System forecasts of integrated water vapor transport along the U.S. West Coast commonly observed during landfalling ARs. This skill is summarized for ensemble probability-over-threshold forecasts of integrated water vapor transport magnitudes ≥ 250 kg m−1 s−1 (referred to as P 250). The P 250 forecasts near North-Coastal California at 38°N, 123°W were reliable and successful at lead times of ~8–9 days with an average success ratio > 0.5 for P 250 forecasts ≥ 50% at lead times of 8 days and Brier skill scores > 0.1 at a lead time of 8–9 days. Skill and accuracy also varied as a function of latitude and event characteristics. The highest (lowest) success ratios and probability of detection values for P 250 forecasts ≥ 50% occurred on average across Northern California and Oregon (Southern California), whereas the average probability of detection of more intense and longer duration landfalling ARs was 0.1–0.2 higher than weaker and shorter duration events at lead times of 3–9 days. The potential for these forecasts to enhance situational awareness may also be improved, depending on individual applications, by allowing for flexibility in the location and time of verification; the success ratios increased 10%–30% at lead times of 5–10 days allowing for flexibility of ±1.0° latitude and ±6 h in verification.

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Carl J. Schreck III, Jason M. Cordeira, and David Margolin

Abstract

Tropical convection from the Madden–Julian oscillation (MJO) excites and amplifies extratropical Rossby waves around the globe. This forcing is reflected in teleconnection patterns like the Pacific–North American (PNA) pattern, and it can ultimately result in temperature anomalies over North America. Previous studies have not explored whether the extratropical response might vary from one MJO event to another. This study proposes a new index, the multivariate PNA (MVP), to identify variations in the extratropical waveguide over the North Pacific and North America that might affect the response to the MJO. The MVP is the first combined EOF of 20–100-day OLR, 850-hPa streamfunction, and 200-hPa streamfunction over the North Pacific and North America. The North American temperature patterns that follow each phase of the MJO change with the sign of the MVP. For example, real-time multivariate MJO (RMM) phase 5 usually leads to warm anomalies over eastern North America. This relationship was only found when the MVP was negative, and it was not associated with El Niño or La Niña. RMM phase 8, on the other hand, usually leads to cold anomalies. Those anomalies only occur if the MVP is positive, which happens somewhat more frequently during La Niña years. Composite analyses based on combinations of the MJO and the MVP show that variability in the Pacific jet and its associated wave breaking play a key role in determining whether and how the MJO affects North American temperatures.

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Matthew C. Sanders, Jason M. Cordeira, and Nicholas D. Metz

Abstract

Ice jams that occurred on the Pemigewasset River in central New Hampshire resulted in significant localized flooding on 26 February 2017 and 13 January 2018. Analyses of these two case studies shows that both ice jam events occurred in association with enhanced moisture transport characteristic of atmospheric rivers (ARs) that resulted in rain-on-snow, snowpack ablation, and rapid increases in streamflow across central New Hampshire. However, while the ice jams and ARs that preceded them were similar, the antecedent hydrometeorological characteristics of the region were different. The February 2017 event featured a “long melting period with low precipitation” scenario, with several days of warm (~5°–20°C) maximum surface temperatures that resulted in extensive snowmelt followed by short-duration, weak AR that produced ~10–15 mm of precipitation during a 6-h period prior to the formation of the ice jam. Alternatively, the January 2018 event featured a “short melting period with high precipitation” scenario with snowmelt that occurred primarily during a more intense and long-duration AR that produced >50 mm of rainfall during a 30-h period prior to the formation of the ice jam. Composite analysis of 20 ice jam events during 1981–2019 illustrates that 19 of 20 events were preceded by environments characterized by ARs along the U.S. East Coast and occur in association with a composite corridor of enhanced integrated water vapor > 25 mm collocated with integrated water vapor transport magnitudes > 600 kg m−1 s−1. Additional analyses suggest that most ice jams on the Pemigewasset River share many common synoptic-scale antecedent meteorological characteristics that may provide situational awareness for future events.

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Jason M. Cordeira, F. Martin Ralph, and Benjamin J. Moore

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.

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Heather M. Archambault, Daniel Keyser, Lance F. Bosart, Christopher A. Davis, and Jason M. Cordeira

Abstract

This study investigates the composite extratropical flow response to recurving western North Pacific tropical cyclones (WNP TCs), and the dependence of this response on the strength of the TC–extratropical flow interaction as defined by the negative potential vorticity advection (PV) by the irrotational wind associated with the TC. The 2.5° NCEP–NCAR reanalysis is used to construct composite analyses of all 1979–2009 recurving WNP TCs and of subsets that undergo strong and weak TC–extratropical flow interactions.

Findings indicate that recurving WNP TCs are associated with the amplification of a preexisting Rossby wave train (RWT) that disperses downstream and modifies the large-scale flow pattern over North America. This RWT affects approximately 240° of longitude and persists for approximately 10 days. Recurving TCs associated with strong TC–extratropical flow interactions are associated with a stronger extratropical flow response than those associated with weak TC–extratropical flow interactions. Compared with weak interactions, strong interactions feature a more distinct upstream trough, stronger and broader divergent outflow associated with stronger midlevel frontogenesis and forcing for ascent over and northeast of the TC, and stronger upper-level PV frontogenesis that promotes more pronounced jet streak intensification. During strong interactions, divergent outflow helps anchor and amplify a downstream ridge, thereby amplifying a preexisting RWT from Asia that disperses downstream to North America. In contrast, during weak interactions, divergent outflow weakly amplifies a downstream ridge, such that a RWT briefly amplifies in situ before dissipating over the western-central North Pacific.

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Heather M. Archambault, Lance F. Bosart, Daniel Keyser, and Jason M. Cordeira

Abstract

Although prior studies have established that the extratropical flow pattern often amplifies downstream of recurving tropical cyclones (TCs), the extratropical flow response to recurving TCs has not to the authors' knowledge been systematically examined from a climatological perspective. In this study, a climatology of the extratropical flow response to recurving western North Pacific TCs is constructed from 292 cases of TC recurvature during 1979–2009. The extratropical flow response to TC recurvature is evaluated based on a time-lagged composite time series of an index of the North Pacific meridional flow surrounding TC recurvature. Similar time series are constructed for recurving TCs stratified by characteristics of the large-scale flow pattern, the TC, and the phasing between the TC and the extratropical flow to assess factors influencing the extratropical flow response to TC recurvature. Results reveal that following TC recurvature, significantly amplified flow develops over the North Pacific and persists for ~4 days. The tendency for significantly amplified North Pacific flow to develop following TC recurvature is sensitive to the strength of the TC–extratropical flow interaction (the phasing between the TC and the extratropical flow), which is based on the negative potential vorticity advection by the divergent outflow of the TC. In contrast, the tendency for significantly amplified North Pacific flow to develop following TC recurvature is relatively insensitive to the intensity or size of the recurving TC, or whether it subsequently reintensifies after becoming extratropical.

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Lance F. Bosart, Benjamin J. Moore, Jason M. Cordeira, and Heather M. Archambault

Abstract

This study uses observations and model reanalyses to examine the multiscale processes associated with four high-impact extreme weather events (EWEs) over North America during late October 2007. The EWEs consisted of wind-driven wildfires in California, prolonged anomalous cold conditions in Mexico linked to two cold surges, heavy rainfall in the eastern United States, and severe flood-producing heavy rainfall in southern Mexico. The EWEs involved a pronounced large-scale flow reconfiguration across the North Pacific and North America in conjunction with the formation of a high-amplitude Rossby wave train. The flow reconfiguration involved perturbations to the North Pacific jet stream linked to polar, midlatitude, and tropical disturbances, including three tropopause-level polar disturbances originating over northeastern Asia, transient extratropical cyclones, a diabatic Rossby vortex, and western North Pacific Tropical Cyclone Kajiki. Eulerian and Lagrangian diagnostics indicate that ridge amplification within the wave train was enhanced in connection with latent heat release along warm conveyor belts rooted in the tropics and subtropics over the North Pacific. Two anticyclonic Rossby wave breaking events over North America established synoptic-scale conditions that supported the EWEs. The results highlight how the large- and synoptic-scale flow can evolve to facilitate multiple geographically separated but dynamically linked EWEs. Based on the results, it is posited that during autumn the North Pacific jet stream may be particularly conducive to large-scale flow amplification, possibly resulting in EWEs, in response to perturbations associated with tropical, midlatitude, and polar disturbances.

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