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1. Composite Synoptic-Scale Environments Conducive to North American Polar–Subtropical Jet Superposition Events

   https://doi.org/10.1175/MWR-D-19-0353.1  

   Winters, Andrew C. ; Keyser, Daniel ; Bosart, Lance F. ; Martin, Jonathan E. ( May 2020 , Monthly Weather Review)

   null (Ed.)

   Abstract A polar–subtropical jet superposition represents a dynamical and thermodynamic environment conducive to the production of high-impact weather. Prior work indicates that the synoptic-scale environments that support the development of North American jet superpositions vary depending on the case under consideration. This variability motivates an analysis of the range of synoptic–dynamic mechanisms that operate within a double-jet environment to produce North American jet superpositions. This study identifies North American jet superposition events during November–March 1979–2010 and subsequently classifies those events into three characteristic event types. “Polar dominant” events are those during which only the polar jet is characterized by a substantial excursion from its climatological latitude band, “subtropical dominant” events are those during which only the subtropical jet is characterized by a substantial excursion from its climatological latitude band, and “hybrid” events are those characterized by a mutual excursion of both jets from their respective climatological latitude bands. The analysis indicates that North American jet superposition events occur most often during November and December, and subtropical dominant events are the most frequent event type for all months considered. Composite analyses constructed for each event type reveal the consistent role that descent plays in restructuring the tropopause beneath the jet-entrance region prior to jet superposition. The composite analyses further show that surface cyclogenesis and widespread precipitation lead the development of subtropical dominant events and contribute to jet superposition via their associated divergent circulations and diabatic heating, whereas surface cyclogenesis and widespread precipitation tend to peak at the time of superposition and well downstream of polar dominant events. 

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2. Weather System Contributions to High Streamflow in Halele‘a, Kaua‘i

   https://doi.org/10.1175/JHM-D-23-0099.1  

   Sears, Mya_J ; Nugent, Alison_D ; Tsang, Yinphan ( September 2024 , Journal of Hydrometeorology)

   The northeasterly facing, windward side of the Island of Kaua‘i (part of the State of Hawai‘i, United States) is prone to heavy rainfall events due to its topographical features and geographical location. Persistent northeasterly trade winds, coupled with steep changes in elevation, create an ideal environment for orographic precipitation. In addition, due to Kaua‘i’s 22°N latitude, the island often experiences midlatitude weather features such as kona lows, upper-level lows, and cold fronts that frequently result in high rainfall and river discharge conditions. This work uses data from river gauges in Halele‘a to understand the seasonality and impacts of the main atmospheric disturbances on two rivers in the region. The seasonality study showed that the majority of extreme flooding events occurred during the cool season and were predominantly caused by cold fronts and upper-level troughs. The historical analysis used atmospheric disturbance cases to determine that kona lows were likely to cause high streamflow in both studied Halele‘a rivers, and upper-level lows had an approximately equal probability of causing high streamflow or not. The findings that come from this project can provide context to atmospheric disturbances in Halele‘a and help community members identify and anticipate the types of events that may contribute to flooding.

   The north shore of the Island of Kaua‘i is prone to extreme rainfall and flooding due to interactions between the typical wind patterns and the nearby mountain range. A majority of flooding events occur during the cool season (October–April) because most of the weather events that produce extreme rainfall occur during these months. Here, we examined the top 50 flooding events in two of Kaua‘i’s north shore rivers and found that cold fronts and upper-level low pressure systems are often responsible for flooding. Additionally, any kona low is likely to cause high streamflow. This study increases the understanding of flood causes and likelihood in northern Kaua‘i.

    

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3. Recurrent Synoptic-Scale Rossby Wave Patterns and Their Effect on the Persistence of Cold and Hot Spells

   https://doi.org/10.1175/JCLI-D-18-0664.1  

   Röthlisberger, Matthias ; Frossard, Linda ; Bosart, Lance F. ; Keyser, Daniel ; Martius, Olivia ( June 2019 , Journal of Climate)

   The persistence of surface weather during several recent high-impact weather events has been pivotal in generating their societal impact. Here we examine Hovmöller diagrams of the 250-hPa meridional wind during several periods with particularly persistent surface weather and find a common pattern in these Hovmöller diagrams. This pattern can be characterized as a “recurrent Rossby wave pattern” (RRWP), arising from multiple transient synoptic-scale wave packets. During such RRWP periods, individual troughs and ridges forming the wave packets repeatedly amplify in the same geographical region. We discuss the synoptic evolution of two RRWP periods, in February–March 1987 and July–August 1994, and illustrate how the recurrence of the transient wave packets led to unusually long-lasting cold and hot spells, which occurred simultaneously in several regions, each separated by roughly one synoptic wavelength. Furthermore, a simple index termed R is proposed to identify RRWPs, which is based on both a time and wavenumber filter applied to conventional Hovmöller diagrams. A Weibull regression analysis then shows that large values of R are statistically significantly linked to increased durations of winter cold and summer hot spells in large areas of the Northern Hemisphere midlatitudes. Traditionally, persistent high-impact surface weather has often been linked to the occurrence of proximate atmospheric blocking. In contrast to blocking, RRWPs affect persistent surface temperature anomalies over multiple synoptic wavelengths. We therefore argue that, in addition to blocking, RRWPs should be considered as an important flow feature leading to persistent high-impact surface weather.

    

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4. Synoptic Characteristics of 14-Day Extreme Precipitation Events across the United States

   https://doi.org/10.1175/JCLI-D-19-0563.1  

   Jennrich, Gregory C. ; Furtado, Jason C. ; Basara, Jeffrey B. ; Martin, Elinor R. ( August 2020 , Journal of Climate)

   null (Ed.)

   Abstract Although significant improvements have been made to the prediction and understanding of extreme precipitation events in recent decades, there is still much to learn about these impactful events on the subseasonal time scale. This study focuses on identifying synoptic patterns and precursors ahead of an extreme precipitation event over the contiguous United States (CONUS). First, we provide a robust definition for 14-day “extreme precipitation events” and partition the CONUS into six different geographic regions to compare and contrast the synoptic patterns associated with events in those regions. Then, several atmospheric variables from ERA-Interim (e.g., geopotential height and zonal winds) are composited to understand the evolution of the atmospheric state before and during a 14-day extreme precipitation event. Common synoptic signals seen during events include significant zonally oriented trough–ridge patterns, an energized subtropical jet stream, and enhanced moisture transport into the affected area. Also, atmospheric-river activity increases in the specific region during these events. Modes of climate variability and lagged composites are then investigated for their potential use in lead-time prediction. Key findings include synoptic-scale anomalies in the North Pacific Ocean and regional connections to modes such as the Pacific–North American pattern and the North Pacific Oscillation. Taken together, our results represent a significant step forward in understanding the evolution of 14-day extreme precipitation events for potential damage and casualty mitigation. 

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5. Synoptic Characteristics of 14-Day Extreme Precipitation Events across the United States

   https://doi.org/https://doi.org/10.1175/JCLI-D-19-0563.1.  

   Jennrich, G. C. ; Furtado, J. C. ; Basara, J. B. ; Martin, E. R. ( August 2020 , Journal of climate)

   Although significant improvements have been made to the prediction and understanding of extreme precipitation events in recent decades, there is still much to learn about these impactful events on the subseasonal time scale. This study focuses on identifying synoptic patterns and precursors ahead of an extreme precipitation event over the contiguous United States (CONUS). First, we provide a robust definition for 14-day “extreme precipitation events” and partition the CONUS into six different geographic regions to compare and contrast the synoptic patterns associated with events in those regions. Then, several atmospheric variables from ERA-Interim (e.g., geopotential height and zonal winds) are composited to understand the evolution of the atmospheric state before and during a 14-day extreme precipitation event. Common synoptic signals seen during events include significant zonally oriented trough–ridge patterns, an energized subtropical jet stream, and enhanced moisture transport into the affected area. Also, atmospheric-river activity increases in the specific region during these events. Modes of climate variability and lagged composites are then investigated for their potential use in lead-time prediction. Key findings include synoptic-scale anomalies in the North Pacific Ocean and regional connections to modes such as the Pacific–North American pattern and the North Pacific Oscillation. Taken together, our results represent a significant step forward in understanding the evolution of 14-day extreme precipitation events for potential damage and casualty mitigation. 

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