Overnight Severe Weather Outlook for Saturday, July 25, 2009...

Discussion...
Bulk of the area should experience rain-free conditions for the better part of the day, with rain/thunder chances decreasing with eastward extent. A 10% chance or better for thunder will be found along/west of a line from approximately Richmond VA- Philadelphia P - Poughkeepsie NY- Keene NH- Portland ME- Bangor ME. A Slight Risk includes parts of Western NY/PA southwest into parts of OH/WV/KY.

Across the Slight Risk area, current thinking is that bulk of significant severe will be across OH and Ontario. SBCAPE, LI, and low level shear is shown to be maximized in this area during peak heating hours of the Afternoon/Early Evening. Overall storm mode should be more linear nature with damaging winds the biggest threat. However, helicity values are also shown to be favorable for a few rotating supercells in this corridor, which suggests an enhanced severe hail threat will also exist, as well as the chance for a brief tornado or two. SPC currently has this area bookmarked with an enhanced 30% Probability on their D2 SWO. For now, have probabilities capped at 15/15 for severe hail/damaging wind, but an increase to 30%, and perhaps 5% for tornadoes given projected helicity values, may be in the cards later this Morning for a portion of the area once the RUC can be analyzed. Most likely area for an upgrade will be across Ohio.

Fallout from any organized convection and possible MCS could reach central NY and PA and into remaining parts of WV and Eastern KY by late Evening towards 00-03z timeframe, therefore a 5% severe probability will be posted from the I-81 corridor in NY southwest along the Appalachians to account for this possibility. To the east of the Slight Risk, until possible MCS remnants reach part of the area outlined by the 5% probability area, storm coverage is expected to very isolated throughout the day given overall lack of synoptic lift/forcing. Any storms that do develop should be more of “garden variety” with heavy rain the primary threat.

..EGAN.. 07/25/2009