The second half of this work week will pose the greatest risk for flash flooding as the atmosphere becomes the most conducive to produce copious amounts of rain. Areas affected will be the Southeast up through the Mid-Atlantic and into parts of New England.
Similar to last week’s set up, a large subtropical ridge of high pressure is located just off the East Coast of the United States, while an unusually large dip in the jet stream associated with an upper-level low pressure remains almost stationary over the interior East work. These two will work in conjunction with each other to funnel rich, tropical moisture from the Gulf of Mexico to the north.
This funnel results in the constant flow of an ample amount of atmospheric moisture across the Eastern Seaboard, while surface features (such as low pressure and fronts) work to take that moisture and translate it into heavy rain.
Precipitable water is how meteorologists typically determine where there best risk for heavy rain is. Precipitable water exceeding 2 inches is a big indicator for heavy rainfall. This will be the case Wednesday for parts of the Hudson River Valley and New England, but especially in parts of the Southeast and Mid-Atlantic.
On Thursday, precipitable water will decrease overall across the East Coast of the US as values drop from 1.5-2.5 inches to 1-1.8 inches. The highest of moisture will drift into Georgia and South Carolina. Nonetheless, there will still be the risk for some flash flooding as the cold front stalls out, from the eastern Great Lakes down through the central Gulf Coast.
Given how messy this weather pattern is, it’s impossible to predict exactly when the rain will hit and how these storms will behave, but we can at least provide you with a general overview. We anticipate for there to be an area of training rain spanning from the central Appalachian Mountains down through the western Florida Panhandle — again — during the morning hours. Then in the afternoon, widespread, scattered thunderstorms will develop to the east, affecting much of the southeastern US and Mid-Atlantic. A couple storms may even sneak into western New England late in the day but the main action will be to the south and west. The close proximity of the high pressure offshore will keep the day mostly dry for the New England.
Now as we get into Friday, the weather pattern will begin to brake, thus easing the concern for flash flooding as the trough of low pressure lifts off to the north and east, allowing for a more zonal flow to develop thanks to the formation of a coast-to-coast ridge of upper-level high pressure.
Before then, however, we’ll still be tracking the rain along the East Coast as numerous showers and thunderstorms remain in the cards across much of the Northeast on Friday. The Southeast will definitely dry out relative to Wednesday and Thursday. Hit or miss storms will still be in the forecast for many, but the chance for rain will be between 30 and 50 percent opposed to the 100 percent for some locations.
By Saturday, heavy showers and storms will remain possible mainly in the New England and especially in the morning before much of the eastern US dries out, thanks to the westward progression of the Bermuda High offshore.
Rainfall will be heavy in some areas between now and the end of this week. The below rainfall graphic and more of a general forecast but understand that some locations will receive over half a foot of rainfall. The best chance for rain of this magnitude will be in the western Florida Panhandle and up the spine of the Appalachian Mountains—hence the Flash Flood Watches issues by the National Weather Service for Alabama through Pennsylvania.