Southeast Georgia Tornadoes
Jason Deese, Angie Enyedi, Andrew Shashy, Pete Wolf, & Al Sandrik
A band of severe thunderstorms developed over the Gulf of Mexico early Wednesday morning of December 2, 2009. The convection moved northeastward across south Georgia and north Florida during the day. This system produced tornadoes across the southeast, including two EF0-EF1 tornadoes across south-central Georgia. There was extensive structural damage, including a destroyed mobile home where those inside sustained injuries.
12Z Synopsis (7 am Wednesday Morning)
A strong upper-level low pressure system over Texas lifted quickly northeast after phasing with an upper level trough that was traversing eastward across the northern plains (Fig. 1, 12Z 300 millibar (mb) analysis). A diffluent pattern of the southern stream jet was across north Florida and a jet max near 110 knots was over southern Texas. The 850 mb pressure analysis indicated a rare and strong low level jet east of the low around 60-65 knots over Alabama and Georgia. Low level warm and moist air advection nosed northward from Florida into southern Georgia (Fig. 2, 12Z 850 mb analysis). The Hydrometeorological Prediction Center (HPC) surface analysis combined with radar reflectively indicated surface low pressure was over Louisiana and Mississippi, with a warm front extending eastward across southeast Georgia. An attendant cold front extended south of the surface low over the Gulf of Mexico. A strong pre-frontal squall line extended south of the warm front from east-central Alabama southward over the Florida panhandle (Fig. 3, 12Z HPC radar/surface analysis composite).
18Z Prediction and Synopsis (1 pm Wednesday Afternoon)
Although strong dynamical forcing was approaching the National Weather Service (NWS) Jacksonville County Warning Area (CWA), there remained uncertainty as to how much instability would develop over the area, and if there would be enough to support strong updraft formation and possibly even low level circulations. The 12Z Jacksonville modified sounding indicated sufficient surface based instability using a modified surface parcel temperature of 78 degrees and a dew point of 67 degrees (actual observed values at JAX that afternoon) (Fig. 4, 12Z JAX modified sounding). In addition, the 12Z Tallahassee unmodified sounding showed steep mid level lapse rates (Fig.5, TAE unmodified sounding). Thus, the combination of increasing surface based instability south of the northward progressing warm front, the destabilization of the mid levels and an anonymously strong low level jet all appeared to phase across southeast Georgia and the Suwannee River Valley of north Florida during the mid to late afternoon; the same time period when the squall line was expected to move across the area.
Most unstable CAPE (MUCAPE) values of around 500-1000 J/kg developed by 18Z across the JAX CWA which were sufficient for convective updrafts, given the strong vertical wind shear in place (Fig. 6, SPC MUCAPE). At 18z, the surface low pressure system was over southwest Tennessee with the trailing cold front extending over the western Florida panhandle. By this time the broken convective squall was just west of our CWA (Fig. 7, 18Z HPC radar/sfc analysis composite).
Radar data showed a broken line of low-topped showers and embedded thunderstorms across south Georgia and north Florida by 2 pm (Fig. 8). There were a total of 4 tornado warnings issued for southeast Georgia by the NWS Jacksonville between 1:39 pm and 3:45 pm local time. Figure 9 shows a composite of the three tornado warnings that were issued between 2:10 pm and 3:45 pm overlayed on the KJAX base storm-relative motion image from 2:32 pm. Tornado # 1 was moving across northeast Coffee county and western Bacon county near West Green. Tornado #2 was moving over northwest Ware county between Cogdell and Millwood.
Tornado # 1
Convective line segments, with rotational signatures of mesocyclones were common. Often, the result of the mesocyclones were the formation of S-shaped or comma-shaped reflectivity patterns. By 2:15 pm, one such comma-shaped reflectivity pattern was associated with a low-topped supercell over Coffee County (Fig 10). The rotational signature quickly tightened as the storm produced a brief tornado, rated EF0, in eastern Coffee County, about 6 miles east of Douglas. This tornado snapped trees, and caused minor structural damage. The storm later produced an EF1 tornado in Bacon county, about 8 miles northwest of Alma. This tornado damaged several chicken houses, and brought down trees. The estimated loss was nearly 3 million dollars.
Another low-topped supercell tracked across western Pierce and southern Appling counties (Fig. 11). This storm produced a high-end EF1 tornado in extreme northern Pierce County, about 4 miles NNW of Bristol. The tornado destroyed a mobile home and a few other structures (tornado track and damage from survey - 745 Kb). Miraculously, only 2 injuries were reported despite the severe damage. A strong rotational signature was observed on radar data as the storm moved across western Pierce county into southern Appling county.
By 00Z, the pre-frontal squall line had almost exited the Georgia coast (Fig. 12).