A vigorous atmospheric disturbance will drop into the D.C. area Thursday night and has the potential to unleash some very heavy rainfall.
The models that are predicting heavy rain suggest that a mesoscale convective system (MCS) will develop, which is a complex of heavy thunderstorms. These can produce flooding rain. Sometimes the storms embedded within them are severe and also produce damaging winds.
In this morning’s forecast discussion, the National Weather Service forecast office in Sterling discussed the MCS scenario:
Cluster of storms … perhaps an MCS … are expected to develop over the Ohio Valley, then dive SE coincident with the forcing. Mid-level winds increase ahead of the shortwave, so CAPE [convective available potential energy]/shear will be supportive of severe storms, with damaging winds being the most likely threat . . . rain could linger well into the night, and with a deep warm cloud layer and high PWAT [precipitable water, a measure of atmospheric moisture], heavy rain could result. Flooding may be a bit more of a concern …
The National Weather Service Weather Prediction Center has placed the D.C. area under a slight risk of excessive rainfall late Thursday into early Friday. This means there is at least a 5-10 percent chance of flooding rainfall.
The threat of heavy rain appears to be somewhat higher than the wind threat because the storms may arrive after dark, when instability from daytime heating will have waned some.
The National Weather Service Storm Prediction Center suggests the risk of severe thunderstorms will diminish by the time the storm cluster or MCS reaches our area. It predicts an elevated risk of severe storms mainly from the Appalachians northwest into the Ohio Valley.
The specific timing of this storm event, the intensity of the storms, as well their exact track still need to be pinned down. We’ll attempt to provide more details in future updates.
Below, you can see rainfall predictions from the National Weather Service and several computer models:
National Weather Service