12AM Day 2 Convective Outlook for Thursday, July 17. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN MISSOURI INTO PARTS OF THE OHIO VALLEY MID-ATLANTIC AND NORTHEAST
SUMMARY
Isolated damaging gusts are possible from southern Missouri into the Ohio Valley, Mid-Atlantic, and Northeast on Thursday.
Northeast
A shortwave trough is forecast to extend from southern Quebec to near Long Island early Thursday. At the surface, low pressure will track from Lake Huron northeast along the St. Lawrence Valley. A trailing cold front will develop east across the Upper Ohio Valley and Northeast during the afternoon and evening. Ahead of the front, a very moist airmass will be in place, supporting MLCAPE values from around 1000-2000 J/kg.
While 30-40 kt of westerly flow will overspread portions of the Northeast, effective shear will remain modest, but sufficient for isolated organized cells (20-30 kt). The surface low is likely to remain north of the international border, but the cold front should provide some focus for afternoon/evening thunderstorm activity. However, subsidence behind the morning shortwave and possibly some early day showers and cloud cover could hinder thunderstorm development initially. As a result storms may develop a bit further south and east than previously forecast, and the Marginal risk (level 1 of 5) has been removed for parts of the Upper OH Valley/Lower Great Lakes vicinity. Storms that do develop across the Northeast still may pose a risk for locally strong gusts, and closer to the surface low, a tornado or two.
Southern MO/OH Valley to the Mid-Atlantic
A seasonally very moist airmass will reside to the south of the southward sagging cold front extending from the MO Ozarks into southern IL/IN/OH. A corridor of strong instability is expected ahead of the front, aided by 70s dewpoints and heating into the mid 80s to mid 90s. Vertical shear will be much weaker with southward extent, but any MCVs from prior day's convection coupled with the sagging cold front, should provide focused for scattered thunderstorm development. Thunderstorm clusters will mainly pose a risk for strong gusts from water laden downdrafts.
Additional storms are expected to develop over the higher terrain of WV/VA along a surface trough and ahead of the surface cold front. A similar environment to that further east will exist (high CAPE, weak shear). Thunderstorm clusters may produce isolated strong gusts.