A shortwave trough of low pressure will crash into the Bermuda High Wednesday, ushering in more moisture with its arrival. The corridor of tropical moisture will narrow as the shortwave treks northeastward through the end of the week. Like a running hose being squeezed at its end, the axis of tropical moisture will narrow, intensifying its transport to the Northeast. With lift provided from the trough, two rounds of heavy downpours will deluge parts of the inland Northeast. Some of the storms producing the downpours could become severe, with strong wind gusts and even a tornado in the realm of possibility.
Early Wednesday morning, a jet streak will slide across the Mid-Atlantic coast behind a warm front. Dew points were already uncomfortable Tuesday, with readings in the upper 60s to low 70s. The arrival of the jet streak will force the humidity over the top, pushing dew points into the mid and upper 70s throughout much of the Northeast. With all this moisture in place, it wont take much forcing to generate torrential downpours.
Heavy rain will develop in the left exit region of the jet streak, where air is forced to rise. Eastern Pennsylvania, northern New Jersey, and New York’s Southern Tier will all wake up to rain. A few isolated storms will be possible from New York City southward to the Delaware Bay. The downpours will not clear these areas until late morning, and could linger in the Hudson Valley, Capital District, and western New England through early afternoon.
Threats extend beyond just heavy rain through the first half of Wednesday. Believe it or not, a brief tornado or two are possible during the morning and early afternoon downpours in North Jersey, northeastern Pennsylvania, and southeastern New York. This area will reside where the left exit region of the incoming jet streak overlaps intense low-level shear and an instability gradient along the warm front. The warm front will already provide lift for downpours to develop in the instability, but the jet exit region will enhance updrafts and the low-level shear could spin up a tornado.
More widespread woes will result from the severe weather potential Wednesday afternoon over northern and western Virginia and Maryland, eastern Pennsylvania, northern New Jersey, and southeastern New York State. An abundance of wind shear will develop over this region and intersect large quantities of instability that will develop during the afternoon. The materialization of this instability is absolutely contingent on the cloud debris remaining from the morning rain. If rain showers and overcast skies hang on too late into the afternoon, the sun may not be able to destabilize the atmosphere to generate severe storms.
If the sun does peek through the clouds, watch out. Deep layer shear will be strong enough in much of the aforementioned region for the development of a few supercell thunderstorms before quickly organizing into linear systems in the evening. The overall coverage of the storms is uncertain, but an adjacent jet streak over western Pennsylvania and New York is expected to encourage thunderstorm development that will eventually spread east.
Wind is the most probable threat with these storms, mainly over the inland areas. Low level shear could be strong enough to support weak tornado development again, especially in northeastern Pennsylvania and Southeastern New York State. Charlottesville, Frederick, Harrisburg, State College, Scranton, and Binghamton are all likely targets for wind damage, with the latter two cities also at risk for a brief tornado. Hail is not much of a concern with Wednesday’s severe threat, though small hail cannot be ruled out.
Most of the evening activity will stay west of the Philadelphia to Boston segment of the I-95 corridor. Flooding rains and damaging straight line winds will be the main threats. The storms will die out by the time they reach the Jersey Shore, Long Island, and eastern New England later in the evening. The marine layer and loss of day time heating will curb any severe threat in these areas.
Rain chances return Thursday, Friday, and Saturday to the Northeast as tropical moisture continues to usher in. Several inches of additional rain could fall over localized areas where storms continuously redevelop, especially in the eastern half of Pennsylvania and the Southern Tier of New York. These areas were already the bullseye of torrential rains last week, so the risk of localized flooding of rivers, creeks, and low-lying areas will be especially high here.
Completely dry conditions still appear like a dream for the inland Northeast. Slight thunderstorm chances will remain Sunday into early next week, especially for the areas most affected by this week’s and last week’s rain. Heat in the lower 90s will build along the coastal plain, with the lower rain chances Sunday into early next week. Hefty uncertainty clouds the details, but it appears that another trough of low pressure could spawn more rounds of widespread downpours by the middle of next week.