A strengthening storm system just west of southern New England is slowly allowing a warm front to push northward. Currently, the warm front is draped across the mid-Atlantic region, however, that front will continue sliding north towards southern New England during the overnight and early morning hours. As the warm front approaches moisture will begin to rapidly increase and this will be accompanied by periods of extremely heavy rainfall overnight tonight and through the morning hours tomorrow. Pretty much everyone across southern New England will see at least one period of heavy rainfall between tonight and tomorrow afternoon. In fact, rainfall amounts on average should be between 1-2'' with isolated rainfall amounts as high as 3-4'' when all said and done. This will lead to pockets of flash flooding. What's the cause for the heavy rainfall?
A very strong an anomalous low-level jet and anomalously high precipitable water values will be the drivers behind very heavy rainfall:
18z GFS model showing a 40+ knot low-level jet working into southern New England by 5:00 AM Wednesday morning:
18z GFS model showing precipitable water values approaching and even slightly exceeding 2'' by 5:00 AM across portions of southern New England:
While the focus for tomorrow will mainly be on the periods of heavy rainfall, another focus, albeit perhaps to a lesser extend will be the potential for a few strong to perhaps severe t'storms. This potential all hinges upon how much sunshine occurs and how much instability develops once the back end of the heavier rainfall moves through. If just enough instability can develop we will have to pay close attention to the development of t'storms.
Winds aloft will be quite strong thanks to the strengthening storm system off to the west and with the warm front in the vicinity of the region this will lead to winds turning with height, especially in the lowest 3km of the atmosphere. If t'storms are able to develop and become strong enough to utilize these winds aloft, they will have the potential to produce strong to damaging wind gusts and perhaps the potential for a brief tornado.
While these setups don't always materialize and generate strong to severe t'storms or tornadoes, these type of setups have produced strong to severe t'storms and tornadoes in the past, including 3 weeks ago. It's all dependent on how much instability can be generated and this typically is not known the night before...it's known more the next morning.
All in all we are looking at widespread heavy rainfall totals (although it will not be raining the entire time) with the potential for isolated areas of flash flooding. Rainfall totals on average should be 1-2'' with isolated amounts as high as 3-4''. While heavy rains are the main threat we can't rule out a few strong to severe t'storms capable of strong to damaging wind gusts or even a brief tornado.
