An outflow boundary approached the AltamahaRiver basin during the mid afternoon (around 3 pm local time) from the north. A quasi-stationary front was draped northwest to southeast near the basin. In addition, the low level atmosphere was unstable with CAPEs over 2500 J/kg. Lapse rates were decreasing aloft as an upper level short wave trough approached from the northwest. A few isolated severe storms developed near the Altamaha River basin and drifted southeast while producing large hail (most reports quarter to golf ball size) and wind damage (damage indicators indicated around 60 mph gusts likely). Storm total rainfall amounts neared 4 inches across portions of Jeff Davis and Appling counties as one long-lived storm tracked from Hazlehurst to south of Baxley toward Patterson. Storms interacted with the east coast sea breeze over Brantley, Pierce and Ware counties where cells began to rotate while continuing to produce large hail and damaging winds. Additional storms developed across Clinch and Echols counties in the evening where the east and west coast sea breezes combined with outflow boundaries.
Northeast Florida Synopsis
The east coast and west coast sea breezes interacted with outflows from storms over southeast Georgia. These boundaries merged across the SuwanneeRiverValley of north Florida during the evening hours as a short wave trough was passing over the region in the mid levels. This feature increased vertical instability, and surface based instability was high with pockets of CAPE over 2500 J/kg. A few severe storms erupted from LakeCity to Gainesville which produced hail and damaging winds. The largest hail reported was quarter size just south of the GainesvilleRegionalAirport, while numerous trees were blown down just north of LakeCity.Doppler radar indicated storm total rainfall amounts of 6 to 8 inches west of LakeCity and near Ichetucknee Springs, but some of the higher amounts are likely due to hail contamination.
The storms decreased in intensity and coverage after sunset.
Analysis, Products and Selected Warnings with Storm Reports
Figure 1:A composite of the Storm Prediction Center Day 1 convective outlooks and risk probabilities issued at 8:55 am on May 21st. The potential for large hail and damaging winds was indicated across southeast Georgia and the northern SuwanneeRiverValley of north Florida.
Figure 2:This is a visible satellite image taken at 1:16 pm on May 21, 2010. It shows a large stacked low pressure system impacting the mid Mississippi RiverValley, with an outflow boundary oriented west-east tracked southward toward the AltamahaRiver Basin. This boundary would soon serve as a focus for discrete supercells within a couple of hours.
Figure 3: This is a composite image of Infrared Satellite Imagery taken at 4:01 pm on May 21st that shows three storms across central and southern Georgia that formed along an outflow boundary and near a quasi-stationary surface front. The east coast sea breeze can also be seen.The next image is a surface analysis from 4 pm that indicates areas of surface based CAPECAPE values, the more surface based instability exists which support updraft formation. In addition, high CAPE indicates an airmass that could not only support storm development but also storm persistence. Areas of surface convergence are also noted per the wind field, generally near the AltamahaRiver Basin and across our western zones. (convective available potential energy) based on the surface temperature and dew point value (extrapolated between observation sites). The higher the
Figure 4: This is a composite radar images of the discrete cells forming along the outflow boundary while north of the quasi-stationary front that was across the region.
Figure 5: The 5 pm surface analysis shows a very stable airmass over Jeff Davis, Appling and Pierce counties in the wake of the severe storm cold pool. This stable air contributed to weakening of storms later in the evening that approached from the west, but weakened as they moved across western Jeff Davis due to the stable airmass in place. The visible image was taken at 5:15 pm and shows three large cells over central and southeastern Georgia, and it also shows the west coast sea breeze near the Interstate 75 corridor and the east coast sea breeze west of Interstate 95 corridor.
Figure 6:This is a composite image of a long-lived supercell storm that tracked across southeast Georgia. It produced large hail, damaging winds and storm total rainfall amounts of 3 to 4 inches as it persisted for about 1 hour and 45 minutes in the Jacksonville County Warning Area.
Figure 7: This is a storm total precipitation graphic from 8 am on May 21st through 8 am on May 22nd. It captures the 3-4 inches across southeast Georgia from Hazlehurst to Patterson. Areas of the Suwannee River Valley had widespread amounts of 3 to 5 inches near Suwannee Springs and west of Lake City, with isolated amounts as high as 6 to 8 inches. It is likely that some of the larger amounts include hail contamination.