Synoptic-scale convective environment climatology by ENSO phase in the north central U.S

The El Niño-Southern Oscillation (ENSO) is known to affect synoptic patterns across the continental United States, particularly by its impact on the upper tropospheric jet stream position, which in turn affects mid-tropospheric ridge and trough locations and thus areas favorable for temperature and precipitation anomalies. These global circulation patterns influence synoptic weather patterns, which in turn influence regional severe weather activity. Though it is one of several factors associated with the potential for severe weather, the synoptic environment plays a key role in severe weather potential by providing favorable ingredients for the development of severe convection. While ENSO is one of many factors that influence global circulations, and by distillation may have a less distinguishable influence on the synoptic pattern, this study will investigate whether coherent signals can be uncovered in the synoptic environment, based on ENSO phase, that would influence the potential for severe convection in the north central United States. Seasonal predictions of severe weather potential are not much aid for daily operations, but they can be used by emergency managers, the media, and forecasters to increase preparedness for seasons that have the potential for above normal convective activity.

This study will investigate potential physical causes of statistical changes to tornado severe weather climatology based on ENSO phase, determining whether changes in the climatology of severe convection can be attributed to changes in the synoptic environment during the three ENSO phases. Utilizing a standard compositing methodology developed within the National Weather Service (NWS), as well as the National Oceanic and Atmospheric Administration (NOAA) standard definition of ENSO to determine phases for each season, the study will develop a climatology of tornado and severe thunderstorm activity in the central and northern Plains and middle to upper Mississippi River Valley areas based on ENSO phase and analyze the potential for a predictable shift in the climatology during the three ENSO phases. The study also addresses regional differences in ENSO signal and climatology in a nine-state area from North Dakota to Illinois, identifying how the impact of ENSO changes among NWS county warning areas within the study area. Then, utilizing reanalysis data, synoptic patterns during the convective peak season of March through June will be examined, based on ENSO phase as determined by NOAA/NWS's Climate Prediction Center, for the period 1950 to 2005.