Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Andrew C. Winters x
  • Weather and Forecasting x
  • All content x
Clear All Modify Search
Andrew C. Winters and Jonathan E. Martin

Abstract

Contributions to the increased poleward moisture flux that characterized the second day of the 1–3 May Nashville, Tennessee, flood of 2010 are examined from the perspective of polar and subtropical jet superposition and its influence on the secondary ageostrophic circulation. Employing the Sawyer–Eliassen circulation equation, the analysis reveals that the poleward moisture flux attributed to the jet increased nearly 120% prior to the second day of the event in response to the superposed jet’s ageostrophic circulation, helping to further fuel the production of heavy rainfall. The full Sawyer–Eliassen circulation associated with the superposed jet is further partitioned into its geostrophic and diabatic components. The geostrophic forcing drove midtropospheric ascent that fueled the production of deep convection and the record rainfall. The diabatic component, through forcing lower-tropospheric ascent and vigorous lower-tropospheric poleward moisture flux, provided the link between the tropical moisture and the deep convective environment. Since superposed jets, by their nature, develop on the poleward edge of the tropical or subtropical air, it is suggested that such a mutually reinforcing interaction between these two component forcings of the secondary circulation may routinely characterize the involvement of superposed jet structures in high-impact weather events.

Full access
Andrew C. Winters, Daniel Keyser, and Lance F. Bosart

Abstract

Previous studies employing empirical orthogonal function (EOF) analyses of upper-tropospheric zonal wind anomalies have identified the leading modes of North Pacific jet (NPJ) variability that prevail on synoptic time scales. The first mode corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second mode corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. These NPJ regimes can strongly influence the character of the large-scale flow pattern over North America. Consequently, knowledge of the prevailing NPJ regime and the forecast skill associated with each NPJ regime can add considerable value to operational medium-range (6–10-day) forecasts over North America. This study documents the development of an NPJ phase diagram, which is constructed from the two leading EOFs of 250-hPa zonal wind anomalies during 1979–2014 excluding the summer months (June–August). The projection of 250-hPa zonal wind anomalies at one or multiple times onto the NPJ phase diagram provides an objective characterization of the state or evolution of the upper-tropospheric flow pattern over the North Pacific with respect to the two leading EOFs. A 30-yr analysis of GEFS reforecasts with respect to the NPJ phase diagram demonstrates that forecasts verified during jet retraction and equatorward shift regimes are associated with significantly larger average errors than jet extension and poleward shift regimes. An examination of the best and worst forecasts further suggests that periods characterized by rapid NPJ regime transition and the development and maintenance of North Pacific blocking events exhibit reduced forecast skill.

Full access
Andrew C. Winters, Lance F. Bosart, and Daniel Keyser

Abstract

This study considers the development of continental U.S. extreme temperature events (ETEs) during the cool season (September–May), where extreme temperatures are defined in terms of percentiles and events are defined in terms of the spatial coverage of extreme temperatures. Following their identification, ETEs are classified into geographic clusters and stratified based on the state of the North Pacific jet (NPJ) stream prior to ETE initiation using an NPJ phase diagram. The NPJ phase diagram is developed from the two leading modes of NPJ variability during the cool season. The first mode corresponds to a zonal extension or retraction of the exit region of the climatological NPJ, while the second mode corresponds to a poleward or equatorward shift of the exit region of the climatological NPJ. The projection of 250-hPa zonal wind anomalies onto the NPJ phase diagram prior to ETEs demonstrates that the preferred state and evolution of the NPJ prior to ETEs varies considerably based on the geographic location of ETE initiation and the season. Southern plains extreme warm events are an exception, however, since extreme warm events in that location most frequently initiate following a retracted NPJ during all seasons. The NPJ phase diagram is subsequently utilized to examine a synoptic-scale flow evolution highly conducive to the initiation of southern plains extreme warm events via composite analysis. The composite analysis demonstrates that a retracted NPJ supports an amplification of the upper-tropospheric flow pattern over North America, which then induces persistent lower-tropospheric warm-air advection over the southern plains prior to ETE initiation.

Full access
Bryan T. Smith, Tomas E. Castellanos, Andrew C. Winters, Corey M. Mead, Andrew R. Dean, and Richard L. Thompson

Abstract

A severe thunderstorm wind gust climatology spanning 2003–09 for the contiguous United States is developed using measured Automated Surface Observing System (ASOS) and Automated Weather Observing System (AWOS) wind gusts. Archived severe report information from the National Climatic Data Center publication Storm Data and single-site volumetric radar data are used to identify severe wind gust observations [≥50 kt (25.7 m s−1)] associated with thunderstorms and to classify the convective mode of the storms. The measured severe wind gust distribution, comprising only 2% of all severe gusts, is examined with respect to radar-based convective modes. The convective mode scheme presented herein focuses on three primary radar-based storm categories: supercell, quasi-linear convective systems (QLCSs), and disorganized. Measured severe gust frequency revealed distinct spatial patterns, where the high plains received the greatest number of gusts and occurred most often in the late spring and summer months. Severe wind gusts produced by supercells were most frequent over the plains, while those from QLCS gusts were most frequent in the plains and Midwest. Meanwhile, disorganized storms produced most of their severe gusts in the plains and Intermountain West. A reverse spatial distribution signal exists in the location between the maximum measured severe wind gust corridor located over the high plains and the maximum in all severe thunderstorm wind reports from Storm Data, located near and west of the southern Appalachians.

Full access