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Heat index tops 100; severe storm watch issued

Oppressive humidity and 90-degree temperatures had heat index values pushing 100 degrees Wednesday afternoon and posed risks of severe thunderstorms with damaging winds and large hail in the afternoon and evening.

The heat index hit 105 degrees by 2 p.m. at Baltimore-Washington International Thurgood Marshall Airport, but fell to 101 degrees at 3 p.m. It was 90 degrees there, but a dew point of 76 degrees, a level of humidity only seen on the most brutal of summer days, made it feel much hotter.

Baltimore City health officials declared a "code red" heat advisory, opening senior centers for cooling with free water and air conditioning. The National Weather Service placed Baltimore City, southern Baltimore County and Howard and Anne Arundel counties under a heat advisory from noon to 7 p.m., with heat index values expected to hit 105 degrees.

Wednesday started off with a balmy morning, with temperatures dropping to only 74 degrees at BWI.

The heat and moisture are forecast to fuel thunderstorms in the afternoon and/or evening. A severe thunderstorm watch was issued across most of the state through 8 p.m., with a risk of storms delivering damaging winds of up to 70 mph and hail as large as an inch in diameter, according to the Storm Prediction Center.

Storms moving through western Maryland into Frederick County prompted severe thunderstorm warnings Wednesday afternoon.

Severe thunderstorm watches were also issued in Virginia and across Pennsylvania, upstate New York and parts of New England for Wednesday afternoon and evening.

Storms are also likely throughout Thursday, when a cold front is expected to advance into the region. Heat index values are forecast up to the mid-90s Thursday.

Heavy downpours are likely as the cold front interacts not just with the humid air mass over the region but also with moisture Tropical Storm Arthur is expected to carry up from the south. At least 2 inches of rain could fall Thursday, with more possible in isolated areas, raising concerns of flash flooding.

Once the cold front passes through, storm and rain chances are expected to diminish Friday morning, with skies clearing and temperatures and humidity falling by the afternoon, in time for Fourth of July festivities.

Dramatically drier air and clear, sunny skies are forecast for the rest of the holiday weekend, with highs only in the lower and mid-80s.

Copyright © 2014, The Baltimore Sun
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