Snow Squalls May Impact New Year's Eve Travel from the Great Lakes to the Mid-Atlantic; Heavy Rain & Isolated Severe Thunderstorms in Southern California

Snow squalls are likely to cross the Lower Great Lakes this evening and the Ohio Valley through the interior Northeast/northern Mid-Atlantic overnight, before potentially reaching the I-95 corridor from D.C. to New York City early New Year’s Day morning. Heavy rain and isolated severe thunderstorms will impact coastal southern California including Los Angeles tonight into early on New Year's Day. Read More >

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.