An unseasonable upper level closed low positioned over South Georgia and a surface low over the Florida panhandle combined to bring two distinct storm environments capable of supporting supercells to the Jacksonville County Warning area through the day on October 9, 2008. The first round of convection affected mainly southeast Georgia where widespread discrete supercells produced large hail. During the afternoon, low level instability over north Florida supported stronger surface based storms in advance of the approaching surface low and its attendant cold front, and several of these storms produced significant rainfall amounts and downburst winds.
Regional 12Z (8 am EDT) soundings and water vapor imagery positioned an unseasonably cold upper level closed low over South Georgia (figure 1). The surface low was over the Florida panhandle, with a surface warm front extending from the surface low eastward across Southeast Georgia (figure 2). A trailing surface cold front extended into the Gulf of Mexico. The 12Z Jacksonville sounding revealed stable and dry low level conditions just above the surface fog inversion (figure 3, orange circle). Satellite imagery and surface observations indicated widespread fog and low stratus blanketed much of north Florida during the early morning. Other regional soundings and model analysis alluded to 500 mb temps from -10 to -12 C across much of the Jacksonville County Waning Area, just under the upper level low. The sounding also revealed plenty of elevated CAPE to support hail growth and wind shear aloft to support the hailstones reaching the surface as large hail (figure 3, red and yellow lines; blue box). Thus, we anticipated that our Georgia zones would be impacted first by severe hail storms during the morning, while the low stratus over most of North Florida keeping the airmass capped and stable.
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By 18Z (2 pm EDT), the upper level low and attendant surface low shifted slightly eastward (figures 4 (note from 12Z 10/10/08) and 5). The strongest convection over Georgia was focusing toward the Atlantic coast, while a cluster of severe storms developed over North Central Florida in advance of the surface low and cold front. Once morning low stratus lifted, diurnal heating raised surface based temperatures into the low to mid 80s across North Florida. In addition, a southeast flow off of the Atlantic raised dew points into the lower 70s. After modifying the 12Z Jacksonville sounding for a maximum temperature of 84 F and surface dew point of 70, surface based instability across northeast Florida rose significantly with CAPEs now over 2500 J/kg with lifted indices just above -8 (figure 6). Our concern was now shifting from severe hail across Georgia, to heavy rainfall and downburst wind potential across Northeast Florida during the late afternoon and evening.
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Evolution of Convection
Our first Severe Thunderstorm Warning was issued just before 9 am for Clinch and Atkinson counties in Southeast Georgia. Through noon, 6 more Severe Thunderstorm Warnings for large hail were issued for southeast Georgia and 2 Tornado Warnings were issued. Widespread reports of penny to golfball size hail were received across Southeast Georgia with these morning storms, and one confirmed tornado touched down briefly about 7 miles North of Fargo causing some tree damage. We believe that the tornado was spawned where a strong inflow notch developed and wrapped a tight micro-scale low pressure center in the storm. Storms continued to fire across Southeast Georgia during the afternoon and early evening, mainly along and east of U.S. Highway 1, as the upper level low slowly tracked offshore of the Atlantic coast.
At noon, the first Severe Thunderstorm Warning for the Suwannee River Valley of north Florida was issued. This cluster of storms progressed slowly eastward through mid afternoon. There was broad rotation in these first cells, but not strong enough to indicate a possible tornado. The main affect from this first round of afternoon storms, especially in Gilchrist County, was very heavy rainfall in a short period of time. One NWS Skywarn Storm Spotter measured 4.57 inches East-Northeast of Trenton within 2 hours. Widespread trees were blown down across Alachua County as the storms progressed eastward, with most reports concentrating around Gainesville between 2 and 3 pm.
At 2 pm, the first Northeast Florida Tornado Warning was issued for Alachua, Bradford and Union Counties. This circulation persisted and tightened as the cell neared Putnam county, and around 1:50 pm, the Tornado Warning was reissued. As these cells tracked farther east, the storms continued to produce severe size hail (penny to quarter size) as well as down several trees and power lines. Just after 4 pm, the first Severe Thunderstorm Warning was issued for Flagler County. As this storm tracked toward the coast, it tapped into plenty of low level instability, which combined with relatively cool temperatures aloft, prompted strong updraft formation, and, consequently strong downdraft formation. At 4:38 pm, a significant downburst wind leveled a large warehouse in Daytona North (figure 7), which is about 10 miles west of Bunnell. The storms continued to track east and moved offshore of the Atlantic coast during the evening. In the wake of the storms, a NWS Storm Spotter in Palm Coast, Linda Kremkau, took some gorgeous pictures of mammatus clouds (figure 8).
|Figure 7, photo by Flagler County Emergency Management||Figure 8, photo by Linda Kremkau|