A shallow supercell resided for over 6 h in Mobile Bay within west and northwest sections of Hurricane Danny’s eyewall on 19 July 1997. For more than 24 h, Danny’s center remained within 50-80 km of the Mobile NWS Doppler Radar. During this time, the storm’s inner structure changed dramatically as nearly all the precipitation shifted to the west side of the storm’s center (Blackwell and Medlin, 1998). Excessive rainfall exceeding 700 mm in 12 h began shortly after this structural transition with amounts exceeding 1000 mm in 48 h at some locations in Mobile Bay. Much of this precipitation appears to be associated with the development and persistence of this quasi-stationary supercell.
McCaul and Weisman (1996) discuss the characteristics of shallow supercells within a hurricane circulation. Similar to their findings, Danny’s supercell displayed peak reflectivities (and presumed peak updraft velocities) at rather low levels. Elevated reflectivity cores exceeding 55 dBZ persist at elevations below 4.5 km within a 7-11 km long "upwind-oriented" reflectivity overhang defining an extensive weak echo region (WER) over northern sections of Mobile Bay (immediately north northwest of Danny’s eye). Base reflectivity and velocity loops show a succession of weak reflectivity "inflow notch" features pulsing within this WER associated with storm-relative boundary layer flow exceeding 25 m s-1. Strong reflectivity gradients are observed immediately downwind of these inflow notches. At times, the WER becomes bounded (BWER). Radar loops also indicate numerous streamwise horizontal rolls within the boundary layer which are embedded within the tight cyclonic circulation on Danny’s northwest side. Streamwise vorticity associated with numerous horizontal rolls is being ingested into the supercell’s inflow notch and WER.
This supercell developed only after Danny’s symmetric precipitation pattern transitioned to one of large asymmetry, favoring the west-side of the storm, and coincident with a multi-hour period of extreme precipitation rates.