by Angie Enyedi
A weak surface trough positioned across southeast Georgia near the Altamaha River basin during the afternoon of June 13, 2011. Northwest flow aloft around the periphery of an upper Gulf of Mexico ridge steered a few lobes of weak differential positive vorticity advection across the area during the afternoon and evening hours. In the low levels, a very hot and unstable airmass was present with afternoon highs climbing into the upper 90s to near 100 degrees as the east coast sea breeze front progressed inland during the mid afternoon hours. All of these convective ingredients combined which initiated widespread pulse strong to severe storms across the Jacksonville County Warning Area. The 12Z JAX Sounding (image 1) indicated a highly unstable environment was present with CAPES over 4300 J/kg. There was abundant CAPE in the hail growth zone (updraft -10 to -20 deg C range), however weak shear and weak steering flow indicated that hail would likely melt significantly upon descent toward the ground. Despite melting, high MPDI values of over 1 unit and high differential potential temperature values (updraft potential temperature minus the environmental potential temperature of the mid level wet buld curve) supported a high potential of wet downbursts with mostly small hail…and that’s exactly what happened…especially across eastern Duval county (image 2)!