The main severe threat today will be expected over the southern Great Lakes, Ohio Valley, Mid Mississippi Valley, and central Plains.  A cold front and strengthening trough will move through these locations ahead of a warm and unstable air mass.  Winds aloft will veer to the west, which will produce moderate wind shear over these locations, which will further enhance thunderstorm development and produce the environment favorable for tornadoes this afternoon and evening.  Further, the advancing dry, cool air aloft will support the production of large hail and wind gusts over 65 mph in the strongest severe thunderstorms.  The high moisture content at the surface will provide a significant source for heavy downpours over these locations and localize small stream and urban flooding along with flash flooding will be possible.

Meanwhile, an upper low over the Mid Atlantic will support the potential for isolated severe thunderstorms from Connecticut down through South Carolina including New York City, Philadelphia, Baltimore, and Washington, D.C.  The upper level low will help destabilize a very moist and warm atmosphere at the surface by advecting dry, cool air at 500 MB.  This dry, cool air mass will support isolated severe thunderstorms with hail, wind gusts to 60 mph, and very heavy downpours.

Tags: central Plains, Great Lakes, hail, heavy downpours, Mid Atlantic, Mississippi Valley, Ohio Valley, severe weather, tornado