The next 24-48 hours will remain very active across the Mid-Atlantic region followed by New England as we continue to track numerous heavy showers and thunderstorms, which will threaten millions with flash flooding through Thursday. The combination of a large area of high pressure over the Atlantic Ocean and a deep trough, or dip in the jet stream, over the eastern US is making for the perfect setup of a constant stream of rich, tropical moisture from the Caribbean Sea midweek.
One tool that we find very useful when trying to determine where the heaviest rain will be located is by taking at look the the precipitate water values. This basically indicates the amount of available moisture in the atmosphere. Oftentimes when the setup is conducive to produce rain and predictable water values are in excess of 2 inches, that hints at the possibility to the likelihood for heavy precipitation.
This Wednesday into the overnight time period, there will be an abundance of moisture of that magnitude spanning from the Outer Banks of North Carolina up through parts of the Mid-Atlantic — mainly east of the Appalachian Mountains — and into southern New England. This aligns with where we think the risk for heavy rainfall will be at its highest. As to no surprise, NOAA’s Weather Prediction Center has highlighted a large area (as shown below) in a slight risk (yellow) for excessive rainfall and even a rare high risk for central Pennsylvania. Some towns have already received over a foot of rain since Saturday, and additional rain will make it only worse.
On Wednesday, that axis of the deepest tropical moisture, or the highest precipitable water values, will become centered over the Hudson River Valley of New York. That will put western New England in play for the more widespread showers and thunderstorms and the greater flash flood risk. Meanwhile to the west, it will remain soggy across the eastern half of the Mid-Atlantic as the precipitation slowly out. The I-95 corridor will remain quite wet through the duration of the day Wednesday with numerous heavy downpours present, especially north of the Mason-Dixon Line.
It’s not until Wednesday night when the cold front will begin to speed up, kicking much of the moisture out of the Mid-Atlantic by the time you wake up Thursday morning. Conditions will improve across this region overnight, especially after midnight as the storms become more scattered and the atmosphere gradually drys out. The main action will be focused in New England Wednesday night. The highest risk for flooding will be inland but everyone will get their fair share of the rain and thunderstorms. The combination of the heavy precipitation and gusty winds may make it relaxing for some at sleep while others may have a difficult time, especially if there’s thunder.
We’ll also be watching the potential of a weak low pressure that may develop off the Mid-Atlantic coast and track into Long Island then southern New England Wednesday night. This would bring enhanced wind gusts of up to 45 mph and rainfall, especially on the northwestern portion of the center of low pressure. Numerous high-resolution models, like the HRRR and NAM, do show a trough of low pressure or even a closed-low forming off the coast of the DelMarVa or New Jersey this evening, then making “landfall” on Long Island and the Connecticut coastline shortly after midnight. This may lead to some power outages as well as localized flash flooding. Rainfall rates may exceed 1 inch per hour near the low pressure, and may dump over 3 inches to some localizes Wednesday night along. It is then expected to track int southeastern New England Thursday morning, keeping the risk for gusty winds and heavy rain elevated.
On Thursday. a few lingering, brief downpours may still me possible near the Mid-Atlantic and into southern New England. Otherwise, these areas can expect a dry day. Across Upstate New York and northern New England, the rain will wait until midday to clear out followed by a few scattered showers during the latter half of Thursday.
Some towns, especially in the Mid-Atlantic, are still yet to receive over 4 inches of rain while a widespread 1-3 inches comes down in most neighborhoods between Wednesday and Thursday.