28 July 2017
Gale warnings in July are rare along the mid-Atlantic coast, but if a tropical cyclone is nearby, they do happen. But those are rare, and a gale warning for a nor’easter in late July?
That does not happen. No way.
A strong trough in the troposphere is indeed going to bring a real fall-like nor’easter to the Mid-Atlantic this weekend with winds gusting nearly 50 mph on the coast. The bigger threat is flooding rains, however, and model guidance is predicting some rather alarming rainfall totals. At least 50-100 mm (2-4 inches) of rain seems likely in the next 36 hours and much of it may fall tonight. This much rain will cause problems anywhere, but if it falls in an urban area like DC or Baltimore, severe flash flooding will be the result.
The reason for the heavy rain is not just the strong low-pressure system. The atmosphere is soaking wet and it’s not just warm and muggy near the ground. A deep layer of moist air covers the region and one way we measure this is by a parameter called precipitable water. We take a column of air and using physics equations we squeeze all of that water out into rainfall. When you see values of over 2 inches, watch out, and I’m seeing values of 2.5 inches and higher, so there’s real cause for concern.
The heaviest rain will likely fall from DC/Baltimore across Central Delmarva, where some areas may see over 6 inches (150mm!).
Flash floods can cause great damage and loss of life, and the threat in urban areas is even higher since heavy rainfall runs down streets instead of soaking into the ground. This event is happening almost a year to the day from the historic flood in Ellicott City, Maryland last July 30.
The only thing left to say is TURN AROUND, DON’T DROWN