Saturday marks a fourth consecutive day for the chance of rain for parts of the Mid-Atlantic as the barrage of cold fronts continues into early next week for parts of the region. The highest flood risk will be Saturday but it may be until Wednesday when things finally dry out.
A lull in precipitation Saturday morning will end in the early afternoon with the approach of an upper-level system of low pressure that will stall over Virginia, bringing widespread downpours northward into southern New Jersey. This area resides south of a slow-moving cold front that will help initiate and maintain widespread downpours in conjunction with a warm, moist airmass and lift associated with the upper-level low. Heavy showers will spread over the region from west to east as the day progresses.
Downpours may repeatedly form over the same areas when the upper-level low comes to a halt and “cuts off” from the jet stream, resulting in a high flash flood risk. A general 2-3″ of rainfall is expected over this area, which includes Washington, D.C. However, heavier rainfall is possible in the surrounding areas that see downpours repeatedly forming over them. Much of the region already has saturated soils due to previous deluges, with Ellicott City, Maryland’s catastrophic flooding all over the news. It will not take much more rain for these communities to flood again. The flood risk is lower from today’s showers over New Jersey and Pennsylvania, where the rain will be less widespread and precipitation has been less persistent.
By Sunday, the cold front will have made it offshore, allowing a weak cyclone to develop alongside it over the Chesapeake Bay and later the Atlantic. Winds will shift to the north, reducing instability. With the cut-off low still stalled over Virginia, there will still be showers present over generally the same areas as Saturday, but they will be lighter in intensity. Temperatures will be unseasonably cold, with the high only reaching the low to mid 60s. North of roughly I-78, temperatures will be slightly warmer as the sun breaks through the clouds.
By Sunday night, another incoming upper-level low will finally kick-out the cut-off low. The weak cyclone off the Delmarva coast will travel with it, sending rain northward into New Jersey, eastern Pennsylvania, eastern New York and Southern New England. This time, the heaviest rain will be focused further northward, closer to the axis of the incoming upper-level low.
Monday, the heaviest rain will be focused over Long Island and Southern New England, giving the areas deluged by rain over the weekend a chance to dry out. Unfortunately, the chance for rain returns ahead of several more cold fronts later next week between Tuesday and Thursday.
Stay safe this weekend where flooding is possible. Remember to turn around if a road appears flooded since it is not possible to estimate the depth of a flooded roadway. We will continue to update you as the rainfall events unfold.