The wet weekend continues Sunday into early Monday for the Ohio River Valley and Mid-Atlantic as widespread rounds of heavy downpours deluge the already-waterlogged regions. Several inches of rain may accumulate in time spans of less than a few hours, as torrential downpours present localized flash-flooding risks. To make matters worse, some of the storms in the Ohio Valley and areas south and west of the Chesapeake Bay in the Mid-Atlantic may become severe.
This weekend’s rain is brought by yet another stalled frontal boundary over the Mid-Atlantic. A parade of upper-level low pressure systems have been providing ample sources of lift in addition to generating several waves of surface low pressure systems along the frontal boundary. Moist and moderately unstable air to the south of this boundary serves as a ripe environment for the development of storms capable of producing torrential rain and strong wind gusts.
Two separate low pressure systems have been bringing a wide swath of heavy rain in the Ohio River Valley and in Pennsylvania Sunday morning. The flash flooding risk in the Ohio River Valley and western Pennsylvania will be greatest through early afternoon as these rain shields pass through.
Heavy rain should hold off in the eastern Mid-Atlantic until sources of lift from the upper-level low catch up in the afternoon. Low-level dryness and stability, courtesy of wind from the Atlantic Ocean, will limit the eastward development of heavy rain until the disturbance approaches, though light showers may progress eastward throughout the day.
The focus of heavy rain will then shift to the Mid-Atlantic late Sunday afternoon into Sunday evening when the highest flash-flood risk will be felt in central Pennsylvania, West Virginia and the Washington, DC metropolitan area. The transport of warm, moist air aloft will increase as the upper-level disturbance approaches late in the afternoon. This air will ride over the cooler, drier air at the surface in a process called “overrunning” to generate a large swath of showers. Along and south of the boundary in Maryland and Virginia, downpours will be torrential but more discrete. Rainfall of up to 2 inches is possible with storms here, though due to their scattered nature, some areas may not receive much rain at all. Additionally, lightning and damaging wind gusts are possible with these storms.
Overnight, the frontal boundary will shift south as the best source of upward motion, and therefore greatest flash flood risk will approach the eastern Mid-Atlantic. Discrete thunderstorms will evolve into a larger shield of heavy rain south of the Mason-Dixon line, but the heaviest downpours will develop south of I-78 in Pennsylvania, New Jersey, eastern Maryland and Delaware. Heavy rain will persist for several hours overnight, producing narrow swaths of 1-3″ of rain within this region, with locally higher amounts where storms continuously redevelop. Drivers are recommended to keep off the roads as it will be difficult to see flooded roadways at night.
The forecast is sensitive where a narrow northern cutoff is forecast to develop between areas that receive excessive amounts of rain and zones that glimpse only passing showers. This cutoff is expected to occur across central New Jersey and Pennsylvania between I-195 and I-78. This cutoff is the result of a boundary between very dry and very moist air, so further south such a cutoff is not expected.
Rain will clear north of the Mason-Dixon line Monday morning, with the sun emerging by afternoon. Further south, chances will persist into Monday afternoon. Maryland, Virginia, and the southern Ohio River Valley will be closer to the frontal boundary, which may not clear until Tuesday.