P3.4
Two slow-moving hurricanes produce vastly different rainfall patterns over the Alabama coastal area
Keith G. Blackwell, University of South Alabama, Mobile, AL; and S. K. Kimball
The Alabama coast experienced two slow-moving hurricanes in just as many years: Hurricane Danny in July 1997 and Hurricane Georges in September 1998. Each storm produced extreme rainfall amounts in Mobile and Baldwin Counties, but the rainfall patterns varied greatly between these two storms.
Hurricane Georges made landfall along the Mississippi/Alabama coast on 28 September 1998. The slow motion of Georges produced heavy rainfall over a broad area of coastal Mississippi, Alabama, and Northwest Florida. Rainfall amounts up to 15-20 inches were observed in the core of the storm near the Alabama-Mississippi line; however, much larger rainfall amounts of 25-30 inches occurred in persistent bands east of the center near the Alabama-Florida line.
Doppler radar from Mobile indicates no fewer than 6 rainbands moved sequentially over eastern Baldwin County AL and extreme western sections of the Florida Panhandle during Georges' landfall. Initially, these bands exhibited rapid propagation onshore as the storm neared landfall; however, shortly thereafter, a quasi-stationary principle band developed along the Alabama-Florida line and persisted for many hours. This band was oriented meridionally along the coastline and contained heavy convective elements. Doppler-estimated rainfall totals of 16-24 inches extended in a meridional swath over eastern Baldwin County AL; however these estimates may be low as a CO-OP observer's gauge recorded 29.66 inches at Bay Minette AL near the northern terminous of this swath and represented the greatest rainfall amount recorded in Georges. Additional gauges in northwest Florida and south Alabama received 24 to 27 inches of rainfall.
The rainfall pattern with Hurricane Georges was much different from the pattern which accompanied Hurricane Danny in this region a year earlier. Danny's maximum rainfall was associated with eyewall convection on the west side of the storm. Yet, Georges' maximum rainfall was associated with east-side rainbands well removed from the storm center. Unlike Danny, which displayed a Doppler-observed rainfall maximum >43" over the warm waters of Mobile Bay, a distinct rainfall minimum of <8" occurred over Mobile Bay during Georges. Also, the east side of Danny was nearly void of convection, completely opposite to Georges' pattern. Different vertical wind shear profiles in the storm's respective environments may have played a role in the drastically different precipitation patterns.
Poster Session 3, Extreme precipitation associated with Tropical Cyclones
Thursday, 18 January 2001, 1:30 PM-9:30 PM
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