National Weather Service United States Department of Commerce
November 5, 2007
The Mini-Supercell / Marginal Hail Event of November 5, 2007

Overview: A quick-moving cold front moved across the ILN CWA during the afternoon of 11/5/07.  A cool late-autumn airmass was in place ahead of the front (temperatures in the upper 50s and lower 60s), which inhibited any surface-based instability.  However, very weak elevated instability existed above the stable boundary layer (MUCAPE <500 J/Kg lifted from 800 hPa).  In addition, shear ahead of the front was high, with 0-6 km shear around 60 kt, and 0-3 km helicities ranging from 250-500 m2/s2.
As the front entered ILN's CWA, numerous thunderstorms developed and quickly took the form of elevated mini-supercells.  The cores of the storms were small and were generally confined to below 15,000 feet, and none of them extended above the -20 °C level.  However, many of the storms exhibited weak midlevel rotation, and almost all storms with rotation ended up producing severe hail. The high 0-3 km helicity favored midlevel mesocyclones, which led to significantly stronger dynamically-induced updrafts.  The air mass in place was cool and freezing levels were low, so these meso-induced updrafts grew within the hail-growth zone of -10 to -30 °C, and large hail was the result.  Because of the low freezing levels, hail was able to make it to the surface without much melting.