1KM SRH Tuesday Night Midnight
SB Cape Tuesday Night Midnight
Low-instability, high-shear severe weather events in the Southeast are a threat during the winter months and one that is not well advertised to the public. While most of the situations noted above were covered by a tornado watch, the threat of severe weather was not highlighted days in advance as with more "classic" severe weather outbreaks; in fact, the January 7, 2007 Day 1 severe weather outlook did not include a Slight Risk in all areas affected by tornadoes until the 1630Z outlook, the third one of the day. The silver lining is that due to the lack of instability, any tornado or severe weather event is usually not as severe as those events where instability is more plentiful, although there are exceptions; a similar setup produced an EF3 tornado in Carroll County, Georgia on February 26, 2008…while a severe thunderstorm warning was in effect, no tornado warning was issued (http://www.srh.noaa.gov/ffc/html/tor22608.shtml). While there may not be an official "protocol" to better forecast these such events on the mesoscale, looking at synoptic features that come into play a day or two in advance and treating any bit of convection that develops during the event as capable of producing severe weather, regardless of the amount of lightning or instability accompanying it, could be a better modus operandi in low-instability, high-shear situations. Some forecasting and warning techniques that are employed during tornado outbreaks in tropical systems (limited instability due to the central dense overcast arnd incredible shear values…sound familiar?) could also be utilized in cold season events with a lack of instability. All in all, improved short-term forecasting can help the area be better prepared for the abnormal severe weather setups that sometimes occur in the winter months in Alabama and Georgia.
The winter of 2006-07 was characterized by extreme cold in some parts of Alabama and Georgia, and as a result, moisture was slow to advect into the area after frontal passages. In this span of time, there were a few instances of high-shear, low-instability severe weather events in the area. The first such event occurred on November 30, 2006, in western Alabama. A low-topped line of convection approaching the border from Mississippi that night was energized by a high amount of shear in the atmosphere, as evidenced by the 00Z Birmingham sounding that yielded a 0-3 km helicity value of a whopping 273 m2/s2 and a 0-6 km shear value of 63 kt despite a CAPE value of just 162 J/kg (http://www.spc.noaa.gov/exper/archive/events/061130/index.html). Despite the lack of lightning or thunder with the line of showers, they would go on to produce four tornadoes in Hale and Marengo counties, one of which was an F0 (this was before the advent of the Enhanced Fujita scale) and three of which were F1's, and several reports of wind damage in the Tuscaloosa and Birmingham areas (http://www.srh.noaa.gov/bmx/significant_events/2006/11_30/index.php). While the Hale County storms were warned by the NWS in Birmingham, there was no tornado warning issued for Marengo County, which ironically was the heavier-populated county of the two struck by tornadoes. The unique situation was forecast on the synoptic scale with a slight risk issued by the Storm Prediction Center, but the lack of resemblance to a "classic" severe thunderstorm prevented timely warnings.
