Meteorological Synopsis
A series of strong low pressure systems formed across the southern Great Plains on February 4, contributing to record warmth on February 4 and 5 in the southern Plains and lower Mississippi River Valley. High temperatures across many areas reached the 70's°F (21–26 °C), and in Alabama temperatures reached the low 80's°F (27 °C) with dew points in the middle 60's° (17–20 °C). Dewpoints in the mid 60's°F were recorded as far north as Memphis. A strong cold front trailing the low pressure area approached the region from the west.
The high temperatures and dew points created moderate instability across the outbreak area. CAPE values, a measurement for the level of tropospheric instability, were over 1000 J/kg, and helicity levels, a measurement of the potential for rotating winds, reached over 200 m²/s². The upper level jet stream also contributed a large amount of wind shear. These factors were all indicative of a high potential for severe weather.
On February 5 at 6:39 am CST (1239 UTC), the Storm Prediction Center (SPC) issued a high risk of severe storms for most of Arkansas; the first in February since 1998. At 10:16 am CST (1616 UTC), the high risk area was extended farther northeast to include western Kentucky, northwestern Mississippi, the Missouri Bootheel, West Tennessee, and Southern Illinois. A moderate risk of severe weather was issued for northwestern Alabama, the rest of Arkansas, the rest of southern Illinois, southern Indiana, most of the rest of Kentucky, northern Louisiana, central Mississippi, parts of southeastern Missouri, southwestern Ohio, eastern Oklahoma, middle Tennessee and northeastern Texas. A total of fifteen watches were issued by the SPC during the outbreak, including thirteen tornado watches (two of which were PDS watches) and two severe thunderstorm watches.
Early on the morning of February 5, a squall line developed across eastern Texas northward into Missouri and moved east towards the high risk area. Ahead of the squall line, the cap took longer than expected to break, but when it finally did in the late afternoon hours, numerous supercell thunderstorms quickly formed across Arkansas and southern Missouri. The first tornadoes touched down around 3:30 pm CST (2130 UTC). A cap is a layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms; the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur. Some of the most powerful tornado producing supercells were situated across the Memphis and Jackson areas, as well as central Arkansas (one producing a tornado that tracked 122 miles (196 km) and lasting about two hours) between 5:00 and 7:30 pm CST (2300 to 0130 UTC) while numerous other tornadoes were reported across northern Mississippi northwest of Tupelo, Mississippi early in the evening. As the evening progressed, one of these supercells traveled across Middle Tennessee including the Nashville metropolitan area and south-central Kentucky, where several deadly tornadoes were reported during the evening hours between 7:30 pm and 11:00 pm CST (0130 to 0500 UTC).
Meanwhile, the squall line that developed from near Dallas to Kansas City moved eastward throughout the late evening and overnight hours causing numerous reports of damaging wind along with many spin-up tornadoes from Louisiana to Ohio and Indiana. It then moved through the southeast U.S. during the early morning hours. Additional strong thunderstorms developed ahead of the front across the southeast during the overnight and early morning hours. A record (at the time) of five tornado emergency declarations were issued on February 5; four for Tennessee, and one in Alabama.
On February 6, a slight risk of severe weather was issued for much of the eastern United States from Pennsylvania to the Florida Panhandle as the squall line raced eastward ahead of the cold front. After several additional tornadoes touched down in Alabama during the early morning hours just before dawn, the primary severe weather threat became damaging wind across the Mid-Atlantic States. No additional tornadoes were reported during the late morning and afternoon as the cold front exited into the Atlantic Ocean, although wind damage was reported across eastern Kentucky, eastern Pennsylvania and most of Georgia. Farther north, snow and freezing rain, at times heavy, continued from Iowa to Quebec until February 7 as the center of the system moved slowly out of the area.
Read more about this topic: 2008 Super Tuesday Tornado Outbreak