Severe Weather Event April 5, 2008
Discrete Supercells and Alachua County Bow Echo
Angie Enyedi, Meteorologist
NWS Jacksonville, FL
An approaching upper level trough and pre-frontal squall line combined with strong low level instability and upper level divergence to produce scattered severe thunderstorms across Southeast Georgia and Northeast Florida throughout the day on April 5th. By Saturday evening, a persistent and severe bow echo embedded in the pre-frontal squall line blasted through the eastern Florida panhandle and over Gilchrist and Alachua counties where winds were measured at over 70 mph which caused areas of extensive damage.
A broad quasi-stationary trough extended across the Jacksonville forecast area during the early morning hours with a weak upper level trough near the lower Mississippi River Valley (figure 1). A surface cold front extended from the western Carolinas southwestward into the central Gulf of Mexico near Mobile Bay. A pre-frontal squall line was advancing east of the cold front across the western portion of the Florida panhandle (figure 2).
|Figure 1||Figure 2|
The 12Z sounding from Jacksonville, Florida, (fig. 3) indicated high mean layer moisture and instability over the area in advance of the approaching squall line. A modified sounding using a forecast maximum temperature of 87 degrees yielded an extremely unstable airmass with convective available potential energy (CAPE) of over 3400 J/kg and surface based lifted indices of -8. Nearly unidirectional shear was not supportive of long-lived rotating updrafts, however 0-6km cumulative shear magnitudes of 35-40 kts did support supercell formation. The presence of dry air above 650 mb, strong mid level lapse rates as well as a 950 mb low level of 35 kts supported the potential for damaging wind gusts in downdrafts. High CAPE above -10 degrees Celsius in the modified convective updraft but weak unidirectional shear supported the potential for small hail. Finally, high precipitable water content of 1.55 inches (nearly 183% of normal), nearly unidirectional flow and strong upper level divergence supported heavy rainfall and possibly training cells.
Evolution of Convection
Only one isolated severe thunderstorm developed during the pre-dawn hours over Southeast Georgia which affected Northern Clinch and Ware counties. This storm produced estimated wind gusts of 55-60 mph and produced power outages in Homerville. Through the remainder of the morning, most convection affected southeast Georgia with storms remaining below severe criteria. The main concern with this convection was training of cells over Echols, Clinch and Ware counties. Doppler radar estimated 3 hour rainfall amounts around 3 to 4 inches, however no flooding problems were reported.
As the morning progressed, debris clouds blanketed much of Southeast Georgia which helped to stabilize the area. However, skies were mostly to partly sunny over Northeast Florida where temperatures were rising into the mid to upper 80s and dew points were rising into the upper 60s to lower 70s. A severe thunderstorm watch was issued around 11:30 am local time for Southeast Georgia and most of Northeast Florida. By early afternoon, convection began to ignite over the Suwannee Valley of inland North Florida. These initial pulse storms were strong and yielded excessive lightning and non-severe downbursts. However, when numerous outflows began to converge across the highly unstable airmass of Northeast Florida, scattered severe storms began to develop with elevated cores of 50-60 dbz extending above 21 kft (the -10 deg C level) and 40 dbz above 31 kft. Scattered reports of wind damage were received across northeast Florida from these pre-squall line cells.
By late afternoon, the organized pre-frontal squall line raced through the Florida panhandle and was encroaching on the Suwannee River Valley of North Florida. At 3:30 pm, a 73 mph wind gust was measured along the leading edge of a bow echo at St. Teresa in Franklin county Florida. By 3:55 pm, estimated wind gusts to 70 mph occurred at Wakulla Gardens in Wakulla county Florida. At 4 pm, as the bow echo raced through Shell Point, five porches were damaged by severe strength thunderstorm winds.
The bow echo continued to race toward the Jacksonville County warning area. A cold outflow boundary from convection over Georgia stretched from Hamilton county eastward through Northern Duval. This boundary helped to stabilize the airmass over much of our area, mostly north of Interstate 10; thus the imminent threat area as the bow echo neared was mainly south of Interstate 10 in Suwannee county. As the bow echo neared, a severe thunderstorm warning was issued at 5 pm local time which included Suwannee county. A report of trees blown down in Wellborn was received at 5:45 pm local time. The southern extent of the bow echo continued to race toward Gilchrist and Alachua counties, and a severe thunderstorm warning was issued at 5:41 local time. By 6:15 pm, numerous reports of extensive wind damage were reported first in Trenton with widespread tree damage and powerlines down (figure 4). From 6:30 to 6:40 pm, reports of widespread tree damage were received from Alachua, Newberry to Archer as the bow echo proceeded through the county.
As the bow approached eastern Alachua, a severe thunderstorm warning was issued for portions of Bradford and Clay counties. The strength of the downdraft winds weakened significantly as the bow moved over our eastern Florida counties which had experienced convection earlier in the day.