National Weather Service offices issued a Flood Watch for six states, including portions of West Virginia, Pennsylvania, Maryland, Virginia, and New Jersey as well as Washington, DC on Wednesday. This is due to the increased risk for flooding through this weekend as several rounds of heavy rain are forecast to move in. Thanks to a stationary boundary draped across the Mid-Atlantic paired with deep tropical moisture flowing in from the Caribbean Sea, this will keep storms nearly-stalled over the the region while allowing for heavier rainfall rates.
On Thursday, much of the day will actually be dry in the Mid-Atlantic with occasional light rain showers passing through, especially in the morning. A few, widely scattered thunderstorms are then expected to develop in the afternoon, mainly south of the Mason-Dixon Line ahead of a large batch of moderate to heavy rain associated with a surface low incoming from the southern Appalachian Mountains.
This surface low will move farther north into the Mid-Atlantic Thursday night, bringing the heavy rain into cities like Washington, DC, Baltimore, Philadelphia, and Pittsburgh. This also includes Fredrick, Maryland, a town that dealt with over half a foot of rain, prompting water recuses, Tuesday night. Therefore, there is a higher-than-normal risk for flash flooding across the eastern Ohio River Valley through the Mason-Dixon Line. Rainfall rates may exceed half an inch per hour in some of the heavier bands that move through. Notice how where the best chance for heavy rain corresponds with the highest precipitable water values, as shown in the European model image below.
Then on Friday, that large, widespread area of light to moderate rain with pockets of heavier rain will move to the north and east into the northern Mid-Atlantic region and southern New England. The best chance for heavy rain will be in Pennsylvania and New Jersey during the day, then southern New England overnight. It’s these areas where there’s the greatest risk for flash flooding to occur. To the south, scattered showers and thunderstorms will be around throughout the day across the Carolinas up through Maryland and the DelMarVa.
The Potomac River at Washington, DC will likely reach moderate and possibly even major flooding beginning on Friday and continuing into the weekend as 3-6+ inches of rain come down. pic.twitter.com/OPTlFwT8BB
— Jackson Dill (@Jackson_Dill) May 17, 2018
Then on Saturday, while much of New England experiences a moderate rain, lighter rain will impact the Mid-Atlantic. On both Saturday and Sunday, scattered showers and thunderstorms will be around. There is still the risk for a few heavier tropical downpours, but the main risk for flooding rains in this region will be on Thursday into Friday.
The model guidance is in somewhat good agreement in terms of where the heaviest of rain will fall and how much there will be. The EURO (European) and GFS (American) models are in better agreement while the NAM (North American Model), which is a more high-resolution model, produces heavier rain of a similar magnitude but to the north of the Mason-Dixon Line.
With that said, here’s our official rainfall forecast through this weekend: