Severe thunderstorms developed across the southeast US ahead of a potent shortwave trough during the day/evening of 19 April 2020. Water vapor imagery captured the strengthening shortwave as it accelerated east across the southern plains and into the southeast (Fig 1). RAP analysis overlay helps to quantify the shortwave, revealing the sharpening 500 mb trough in the height field, increasing 500 mb wind field ahead of the wave (including wind speeds over 70 knots), and the vorticity max.
By the late evening hours, the primary region of thunderstorms had shifted east into Alabama, Georgia, and the Florida Panhandle. Unfortunately, NWS TAE required emergency backup during a period covering at least 0545 – 0745 UTC, resulting in NWS Houston taking over warning issuance. To make matters worse, data from area radars was unavailable/intermittent to forecasters, primarily during the ~0600 UTC to 0700 UTC timeframe (Fig 2). Therefore, the warning forecaster was required to rely on satellite and lightning data for warning decisions.
The primary tools used following the loss of radar data was GOES-East IR imagery (Fig 3) and total lightning data from Earth Networks (not shown in examples), and later, GLM (Fig 4) as well. Cloud top trends in the IR imagery were monitored closely, including brightness temperature trends and overshooting tops. Persistence of overshooting tops and cold temperatures for storms that had previously appeared strong/severe in radar (and were warned on) aided confidence in continued warning issuance, as did rapid increases in lightning activity. Similarly, warming of storm tops, loss of OTs, and decreases in lightning density provided confidence in letting warnings expire.
Earth Networks lightning data was also used to account for parallax in the satellite imagery, helping a forecaster determine more accurately where the updraft core was located geographically. In this case, storm tops would have been displaced roughly 10-14 km to the NNW in the GOES-East imagery and GLM data.
***AWIPS display/procedures shown in this blog post were created after the event, and are similar but not exactly that used by the warning forecaster***
Bill Line (NESDIS and CIRA) and Sean Luchs (NWS HGX)