Environmental Discussion

On the evening of March 19^th and into the early morning hours of March 20^th, a strong shortwave trough moved from Arkansas and Louisiana into eastern Alabama (500mb: 00 Z, 12 Z). Initially, this area was located on the front-left quadrant of a 140-160 knot 250 mb jet and the left-rear quadrant of an 80-100 knot 250 mb jet (250 mb: 00 Z). As the stronger upper level jet moved into the northern Gulf of Mexico and the weaker jet moved off the Carolina coast, a deep, smaller-scale jet developed from 850 mb to 250 mb over Alabama and Georgia associated with the shortwave trough (250 mb: 12 Z). The front-left quadrant of this small jet was located over Northern Georgia which favored the development of severe convection in this region. Also associated with the intensifying shortwave trough was a weak surface low that deepened from 1002 mb to 998 mb as it moved from northeast Arkansas to central Tennessee and Kentucky (1000- 500 mb thickness: 00 Z, 12 Z). A surface cold front extended south from this surface low to the Gulf of Mexico. The airmass on the warm side of the front contained temperatures and dewpoints in the low 60s while temperatures and dewpoints were in the low 50s directly to the west of the frontal windshift (sfc obs: 11 Z). Although the lower troposphere was saturated from the surface up to 850 mb ahead and behind the front, signifiantly drier air existed at and above 700mb (FFC: 12 Z).

A severe thunderstorm watch (box, text) was issued at 0509 Z for eastern Alabama, eastern Tennessee, northern and western Georgia, western North Carolina, valid from 0545 Z to 1100 Z. CAPE values of 1500-2500 J/kg ahead of the squall line combined with dry midlevels and a more unidirectional windfield to suggest the primary threats were large hail and damaging winds. By 0600 Z, a solid squall line containing a few severe thunderstorms had formed on the cold front extending from northeast to south-central Alabama (obs, CAPE, and radar: 06 Z). Out ahead of the storms, the low-level winds at 850mb and the surface began to veer significantly after 0600 Z (850 mb: 06 Z, obs, CAPE, and radar: 09 Z).

By 0900 Z, the squall line began to break up and weaken significantly (obs, CAPE, and radar: 09 Z). A watch status report issued at 0948 Z reported this trend, and the weakening was expected to continue. Immediately behind the decaying squall line a small cluster of cells formed which were the origins of the Hall County tornadic storm (obs, CAPE, and radar: 10 Z). In addition to the small cluster of weak storms, an area of widespread weak radar echoes formed behind the squall line corresponding with an area of strong upward vertical motion at 700mb moving into eastern Alabama and western Georgia (700 mb VV: 10 Z). The Hall County storm emerged from the small cluster of cells as widespread upward motion developed over the northern half of Georgia. The well-defined higher (and colder) storm top with this storm, as detected by the GOES-10 IR data, further characterized an organized updraft.

In the hour before the storm moved into Hall County, the winds over eastern Georgia backed from 5 knot southwest winds to 10 knot southeast winds, thus improving the low-level shear (obs, CAPE, and radar: 11 Z). Observations are not available to unambiguously determine if the backed surface winds over northeast Georgia were associated with a significantly deep backing to adjust the unfavorable low-level shear profile (11 Z RUC raw data for Gainesville, GA: sounding, hodograph). If the 11 Z RUC data is modified with Gainesville, GA surface temperature and dew point temperature data, and the wind direction is backed in the low levels (as is suggested by the backed surface obs over a wide region), then the sounding becomes more favorable for tornadic convection (11 Z RUC modified data for Gainesville, GA: sounding, hodograph). This "optimally" modified sounding is associated with a CAPE of 1807 J/kg, 0-3 km SRH of 262 m²s², and a BRN of 27.

At 1100 Z, the severe thunderstorm watch was allowed to expire. The tornado produced its first damage around 1125 Z in western Hall County, Georgia. Twelve fatalities and one hundred seventy-one injuries occurred along the 11.5 mile damage path. Eleven of the twelve fatalities were in mobile homes. Fifty houses and forty-three mobile homes were destroyed by the tornado with maximum damage rated at F3. The tornado dissipated after moving into White County, Georgia.

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