One of the keys to the generation of strong-to-severe thunderstorms later today in the I-95 corridor has to do with how much sunshine can poke through the current rather widespread band of low clouds and patchy fog. If some brightening skies were to occur – and prospects are quite good – then this would help to generate some low-level heating which, in turn, would cause instability in the atmosphere. In fact, the GOES visible satellite images from early morning-to-late morning show clearing skies are slowly advancing eastward across Pennsylvania and skies should brighten during the next few hours in areas to the north and west of the big cities along I-95. As a result, odds do favor the formation of showers and thunderstorms later today in upstream interior locations of the Mid-Atlantic and some of the storms that develop can become quite strong with damaging wind gusts, heavy downpours, and even hail. The most likely timetable for showers and thunderstorms to reach the DC-to-Philly-to-NYC corridor is 4PM to as late as midnight as activity will tend to be slow to arrive and slow to leave.
An impressive upper-level wave of energy (circled area on forecast map) will progress slowly eastward this afternoon from the Lower Great Lakes to western New York State. This feature will add to the upper motion in this area and help to destabilize the atmosphere to the point where strong-to-severe thunderstorms can form and survive for awhile. At the same time, a surface cold front will drift eastward into the Mid-Atlantic region and some low-level heating should take place after being suppressed all morning by dense cloud cover. The air mass in place is very humid with dew points as high as 74 degrees at DCA at mid-day and 71 degrees at PHL. This abundant available moisture will support locally heavy downpours later today and early tonight as the cold front and upper-level wave inch through the region. The cold front will grudgingly drift off the east coast on Wednesday and the overall humidity should noticeably lower during the day. Thursday looks quite nice throughout the Mid-Atlantic region and will probably be the nicest day of the week.
Meteorologist Paul Dorian