DISCUSSION: Over the past 24 hours, a storm similar to a Nor’easter dropped an immense amount of rain in the Mid-Atlantic region. Highly unusual for this time of year, this coastal storm (with a track that is typical of a winter Nor’easter) carried lots of tropical moisture which allowed it to drop nearly 6-7 inches of rain near Washington, DC. Rain gauges in parts of southwestern Delaware measured more than 10 inches of precipitation! While the rain is still continuing, a map detailing 24-hour precipitation totals for the region showcases the effects of this coastal storm (see above, courtesy of NWS WPC).
Early this morning the NWS Weather Prediction Center released a Mesoscale Precipitation Discussion (MPD #:0592) warning the Delmarva area of the threat of flash flooding from this coastal storm. The graphic they released for the MPD (below, courtesy of NWS WPC) shows the conditions that were set up for the storm. A low pressure center with counterclockwise winds around it was riding along the coast. This allowed for warm, moist air from the south to mix into the stationary front, providing it a source to draw more energy from, and cooler air from the northeast to drive strong winds into the area. The storm had a history of heavy rains, and with convection that continued to pop up along and in front of the stationary front, the storm had another source to draw energy and moisture from that permitted it to slow down and downpour.
In addition, the coastal storm had the help of the unusually sharp contrast of sea surface temperatures in the Atlantic for this time of year, (see below, courtesy of NWS Eastern Region HQ) which is typically a driving force for Nor’easters. This contrast also shows the Gulf Stream pushing further north than we usually see it. With the combination of all of these conditions, rain poured at large rates over a long duration, allowing for incredible amounts of precipitation for the area.
To learn more about other flooding events from around the world, be sure to click here!
©2017 Meteorologist Katie McCracken