The month of May was very wet across much of the Eastern US. Numerous cities, including Asheville, Wilmington, Fort Myers, Charleston, Richmond, and Chicago all recorded their wettest May on record. Some of these cities have even measured over a foot of rainfall in just the 31-day time frame. Not only was it the wettest May in recorded history in Asheville, it was also the wettest month ever. The total of 14.68 inches is nearly an inch more than the previous record of 13.75 set in August of 1940.
All of this rain has made for some very saturated soil with above-average soil moisture across much of the Eastern US. With more rain in the forecast, there is the concern for flash flooding, especially in the Mid-Atlantic region. Ellicott City, Maryland, a city that experienced catastrophic flooding during the Memorial Day Weekend, will be at risk for another round of flooding this weekend. Even Thursday afternoon, the city had a scare as heavy thunderstorms moved through the area, prompting a Flash Flood Warning for the city.
On Saturday, there will be two areas at risk for flooding: the Mid-Atlantic and from the Upper Midwest through the Mid-Mississippi River Valley. In the Mid-Atlantic, a weak coastal low pressure will form right near the coastline. This coastal storm will bring in rounds of showers and thunderstorms to the region, especially from Saturday afternoon into Saturday night. Some of the model guidance suggests this moderate to heavy rain trains near the Washington, DC area and toward the DelMarVa. If this is the case, some areas will encounter flash flooding. The combination of this coastal storm and high precipitable water values of around two inches will make the atmosphere prime to produce heavy rainfall. These precipitable water values tell forecasters how moist the atmosphere is and signifies the risk for heavy rain. Once values near the two inch threshold, that warns that some areas will indeed experience heavy rain.
Back toward the west, there will also be moderate to high moisture content that will stream in from the Gulf of Mexico, aiding in the risk for heavy rain across the Midwest and Mid-Mississippi River Valley as thunderstorms form ahead of a cold front. There will be multiple lines of thunderstorms that we will be tracking. The one over the Upper Midwest seems the most concerning given its slow forward movement, so that may bring flooding to some locations.
By Sunday, the risk for flash flooding will drop to near zero across the Central US as the cold front clears out for a brief time period. Meanwhile across the Mid-Atlantic, that threat will continue. That coastal low won’t really budge as it sticks around near the coast on Sunday. This will continue to feed tropical moisture into the Mid-Atlantic region, keeping rain and thunderstorms around, especially across the DelMarVa and near the Mason-Dixon Line. Once again, some of this precipitation may be training. Some areas may experience heavy rain on a day-by-day basis, which definitely concerns us when discussing the risk for flooding.
Rounds of showers and storms will persist into next week, keeping the flash flood threat present for some areas at times. In terms of rainfall through Sunday night, portions of the Mid-Atlantic will very likely receive over two to three inches.