Danny, a minimal hurricane, produced devastating rains over southwest Alabama on 19-20 July 1997. Hurricane Danny probably is the first tropical cyclone to produce over 1000 mm of rainfall over a flat coastal area while continuously observed by a nearby Doppler radar. Thus, unlike earlier record-breaking rain producers, such as 1979’s Tropical Storm Claudette in Texas, Hurricane Danny provides a unique opportunity for uninterrupted Doppler observation of a monumental precipitation event.
Danny’s eye remained within 50-80 km of Mobile’s NWS Doppler radar for more than 24 h, thus providing a unique, multi-hour close-up view of Danny’s structure while it approached the Alabama Coast, entered and stalled in Mobile Bay, then made landfall on the Bay’s eastern shore. Danny experienced an eyewall replacement cycle as it approached and entered Mobile Bay. One day earlier, while over open water, a very symmetric Danny was just completing an earlier eyewall contraction which resulted in a single closed eyewall, a 16 km eye diameter, and minimum pressure reading of 984 mb. As Danny edged closer to land, a partially-closed outer eyewall developed near the coast, similar to friction-induced outer eyewalls described by Willoughby. Reconnaissance observed a rising pressure and concentric eyewalls. Doppler radar showed the heaviest rains and strongest winds shifting from the inner to the new outer eyewall (i.e. an increase in the radius of maximum wind (RMW) corresponding with a weakening inner eyewall). This new eyewall replacement cycle continued as the storm moved northward toward the Alabama coast, culminating in strengthening winds, a shrinking of the RMW and contraction of the radar eye to 10 km, and a lowering of pressure to 984 mb for a second time.
Although pressure was falling and the storm was intensifying as the outer eyewall contracted, eyewall convection and precipitation rates and coverage decreased substantially. The area bounded by the the Doppler-derived 25-50 mm h-1 isohyet diminished to only 100 km2 just prior to the storm's lowest pressure, smallest RMW, and best symmetry. Upon completing this eyewall contraction, the storm rapidly lost east-side precipitation as the eye opened and the wind maximum expanded, weakened, and became elevated to the east. In contrast, a huge multi-hour increase in precipitation intensity and coverage occurred in the western eyewall as the storm’s pressure slowly rose. Maximum Doppler winds were within this highly convective western eyewall, and (unlike to the east) were surface-based and at a shorter RMW. The area bounded by the 25-50 mm h-1 isohyet drastically increased over an 11 h period during landfall and ultimately reached 1400 km2.
Nearly all of Danny's precipitation at landfall favored the western semicircle of the storm (Blackwell and Medlin, 1998). Landfalling Hurricane Frederic also displayed this same type of "west-side" asymmetry near Mobile Bay (Parrish et al., 1982; Powell, 1982). Interestingly, enhanced frictional convergence of onshore winds east of the landfall point did not produce enhanced precipitation over the eastern sector of either storm.