The southeastern United States, namely the northern and central portions of the states of Alabama and Georgia, are part of a humid subtropical climate. Especially in the summer months in these areas, atmospheric moisture, and the instability that usually accompanies it, is almost always abundant. In the transition period before and after this abundance of moisture in the Southeast, the moisture and resultant instability interacts with favorable upper-level dynamics on occasion to produce severe weather across the region, sometimes in the form of notable severe thunderstorm and tornado outbreaks. However, in the cooler months, sometimes even stronger upper-level dynamics hook up with very meager amounts of instability to create severe weather. These events, characterized by a high amount of both speed and directional shear in the atmosphere but a low amount of instability as measured by CAPE, present a big forecasting challenge for the area. Although the types of severe weather in high-shear, low-instability events are usually not as intense as those of their counterparts with higher levels of instability (e.g. EF0-EF2 tornadoes as opposed to the potential for EF2-EF4 tornadoes, the usual absence of large hail), high-shear, low-instability events are just as dangerous because they can develop on the mesoscale with little warning. In the late fall and early winter months in the Southeast, cold front after cold front traverses the region, suppressing the truly marine air of the Gulf of Mexico further and further to the south. If this moisture is allowed to advect back northward over land and meets with a mid-latitude cyclone traveling over the Southeast at the same time, dangerous severe weather outbreaks can result. However, even a small amount of moisture coupled with ample shear aloft can result in severe weather over Alabama and Georgia in the winter months. These kind of events are the "sneakiest" of the bunch because they don't have the usual precursors such as sultry, humid air or extremely heavy rainfall accompanying them. In some cases, due to such lack of instability, there have been severe weather events, including tornadoes, without accompanying lightning or thunder. 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. Two other low-instability, high-shear tornado events would occur in the region later in the winter and just two days apart from each other. On January 5, 2007, an F1 tornado struck in Coweta County, Georgia, in an area with extremely modest instability but incredible shear. A sounding representative of the environment during the tornado was made with the 12Z sounding from Peachtree City, less than 20 miles from where the tornado occurred. As with the Birmingham sounding in November, CAPE values were very slim (71 J/kg at the surface) but shear values were very robust (175 m2/s2 0-3 km helicity and 67 kt of shear from 0-6 km) (http://w1.spc.woc.noaa.gov/exper/archive/events/070105/index.html). Unlike the November event in Alabama, however, a tornado watch was in effect for the area and a tornado warning was issued for Coweta County prior to the tornado touchdown (http://www.srh.noaa.gov/data/warn_archive/FFC/TOR/0105_150008.txt), preventing any injuries from occurring. Two days later, an even stronger F2 tornado struck Coweta County in almost the same type of atmospheric setup. This time, however, the storm developed in a line echo wave pattern that occurred as the squall line accelerated into western Georgia. As with the tornado two days earlier, ample warning (http://www.srh.noaa.gov/data/warn_archive/FFC/TOR/0107_235722.txt) prevented any injuries despite a four-mile long damage path (http://www.srh.noaa.gov/ffc/html/cowetator1707.shtml) through the county. The 00Z Peachtree City sounding, taken almost the same time the tornado was traversing Coweta County from the atmosphere unmodified by the squall line, yielded 243 J/kg of CAPE, 48 kt of 0-6 km shear, and a whopping 0-1 km helicity value of 287 m2/s2 (http://w1.spc.woc.noaa.gov/exper/archive/events/070107/index.html). About 100 miles to the southwest of Coweta County, an F1 tornado would touch down about an hour later in Barbour County, Alabama, in the far southeastern corner of the NWS Birmingham forecast area. Meteorologists there noted again that "lightning activity was absent and was not typical of a tornadic situation" (http://www.srh.noaa.gov/bmx/significant_events/2007/01_07/index.php). A tornado watch was in effect, but no tornado warning was issued. Fortunately, no injuries occurred. 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. |