An examination of cool-season damaging wind events in the northern mid-Atlantic region

Damaging “straight-line winds” (50 kt or greater) are the most common form of severe convective weather found in the northeastern U.S. and mid-Atlantic region, occurring about three times as often as severe hail (.75 inch diameter or greater). About 80 percent of these wind events occur during the late spring and summer, i.e., May through August. Convective wind events outside this peak severe weather season pose a special challenge to operational meteorologists, as they often occur in weather regimes that appear only marginally favorable for development of any deep convection. Moreover, they are often accompanied or followed by strong synoptic-scale gradient winds that can also reach damaging force.

To better understand these cool-season events, local severe weather records from 1999 to 2005 for the National Weather Service (NWS) Forecast Office at Mount Holly, NJ, were examined. Eight days were identified during the months September through April on which there were five or more convective wind events. Archived radar data for each day were then loaded into the NWS Weather Event Simulator, along with Rapid Update Cycle, 40 km resolution (RUC-40) model analyses for the more recent events. For events before 2003, archived surface and upper-air data from various weather archive sites on the Internet were examined.

These convective wind events were driven primarily by strong dynamical forcing, and were not limited to any particular time of day or night. A 250hPa jet streak core of 100 to 150 kt was typically present, while winds at 850hPa were usually 50 kt or greater. The air mass in which these events occurred was often just marginally unstable, with Convective Available Potential Energy (CAPE) usually less than 500 J kg-1. Radar signatures observed during these cool season events varied from nearly solid lines of 50 dBZ or greater to short bow echoes or broken-S type signatures to isolated cells. Cell movement was very rapid, usually 50 kt or greater, but storm-echo tops were relatively low, typically less than 8 km, and most cells had a maximum vertically integrated liquid (VIL) less than 30 kg m-2. Also, very little cloud-to-ground lightning was detected with these cool season storms.

This paper will present the meteorological setting and radar signatures associated with cool season convective wind events. Mean values for severe weather parameters with these events will be contrasted with those for warm season events. Examples of various radar signatures that produced severe winds will be presented, including strong linear echoes, bowing line segments, small bow-shaped cells and “broken-S” signatures. The examples will include high-resolution radar velocity data, since it often resolves structures along a convective line better than radar reflectivity. Finally, since cool season convective wind events are often embedded within larger-scale high wind regimes, the relative value of severe thunderstorm warnings vs. high wind warnings will be considered within the context of current NWS watch/warning guidelines.