Analysis: Heavy rain, storm potential Friday morning

A mid level atmospheric disturbance will approach the area from the southwest later Thursday Night into Friday morning, bringing with it plenty of tropical moisture and lift. As the disturbance shifts toward the Mid Atlantic states and eventually off the coast, a surface low pressure area will develop in the same adjacent areas. This will enhance lift in the atmosphere — allowing heavy rain to develop just to the north of the low pressure area.

Within an airmass characterized by precipitable water values over 2.50″, this lift will support the development of heavy, torrential rains from Thursday Night into Friday. Movement from southwest to northeast of heavy rain, and embedded thunderstorms, is anticipated. Exactly where the heaviest bands fall will depend on a few things; namely convection, and placement of the best mid level lift and vertical velocity.

Models have shifted back and forth a bit with the placement of the best vertical velocities — upward vertical motion, or lift — throughout the area late Thursday into Friday. A general southwest to northeast orientation is anticipated, similar to many of the cold-season storms, even Nor’Easters, that our area experiences. A surface front is expected to form, with a notable thermal gradient enhancing heavy rains as well.

The NAM is farthest northwest with this boundary, bringing an enhanced southerly flow into our area and keeping the swath of heavy rains near or just north of New York City. The frontal boundary stalling right near our area, and the enhanced lift and tremendous advecting of tropical moisture, would suggest rainfall amounts in excess of 3″ are possible from Southeast Pennsylvania through New Jersey, the NYC Metro and Long Island.

Other models are both farther north and south with this boundary, owing to the placement of mid level lows and according highest vertical velocities. Exactly where this strongest lift sets up will determine where the heaviest rain falls. But still, even south of those areas, training thunderstorms could produce prolific rainfall as they move toward the main, steady band.

The thunderstorm threat will also be of interest as this event unfolds. Forecast models suggest that tropical moisture may enhance instability, along with the presence of very strong low level shear, suggesting the potential for a weak tornado or two in any stronger thunderstorms. Tropical-like moisture, when juxtaposed with a low pressure system, has historically produced weak tornadoes, and this setup is no different.

The main point remains the same, however, that pining down the track of mid level features will be critical. Where the best vertical velocities set up will have a major impact on the entire evolution throughout our area. The convective nature of the system suggests we may not truly know until later Thursday afternoon or even the early evening.

Stay tuned to all of our Premium Products and hang tight for a new video update around 2:00pm Thursday afternoon as well.