A PRELIMINARY INVESTIGATION AND DIAGNOSIS OF WEAK SHEAR SUMMERTIME CONVECTIVE INITIATION FOR EXTREME SOUTHWEST ALABAMA

The initiation and location of summer season convective activity, occu rring between 1500 and 1800 UTC within a 100-km radius of the Mobile, Alabama, National Weather Service WSR-88D radar site, was examined on days when weak vertical wind shear environments existed. Data were col lected during June and July of 1996 based on 1200 UTC analyses of uppe r-air charts, radar, and GOES-8 satellite imagery. Thirteen case days were selected for detailed analysis. The precise location of initiatio n was found to be a function of both the 0-1-km base-slate flow and th e extent of the flow's interaction with physiographic features and loc al sea- and bay-breeze circulations. The modified mean 1800 UTC soundi ng for case days was characterized by a well-mixed boundary layer, a d eep moist-adiabatic lapse rate, and an overall decrease in relative hu midity with height, For all case days, the surface parcel was found to be the most unstable air parcel within the mixed layer. A distinct bi modal distribution was found to exist in the 0-1-km base-state wind fl ow pattern with flow predominantly east-southeast or west-southwest. T hunderstorms were observed to initiate in a ''mirror image'' pattern o n either side of Mobile Bay and were focused near elevation peaks, alo ng the sea breeze, and at or near the orthogonal intersection of the s ea- and bay-breeze circulations on either side of the bay.