The initiation and location of summer season convective
activity, occurring between 1500 and 1800 UTC within a 100 kilometer radius
of the Mobile, AL National Weather Service WSR-88D radar site, was examined
on days when weak vertical wind shear environments existed. Data were
collected during June and July of 1996 based on 1200 UTC analyses of upper air
charts, radar and GOES-8 satellite imagery. Thirteen case days were selected
for detailed analysis and compositing revealed three distinct synoptic patterns
according to the location of mid-tropospheric ridging.
The precise location of initiation was found to be a function
of both the 0-1 km base state flow and the extent of the flow's interaction
with physiographic features and local sea- and bay-breeze circulations.
The modified mean 1800 UTC sounding for case days was characterized by a well-mixed
boundary layer, a deep moist adiabatic lapse rate and an overall decrease
in relative humidity 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 flow pattern with flow predominantly east-southeast or west-southwest.
Thunderstorms were observed to initiate in a 'mirror-image' pattern
on either side of Mobile Bay and was focused near elevation peaks, along the
sea-breeze and at or near the orthogonal intersection of the sea- and bay
breeze circulations on either side of the Bay.
National Weather Service
Mobile/Pensacola Weather Forecast Office
(251) 633-6443
sr-mob.webmaster@noaa.gov
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