* Flash flood watch late Wednesday to Friday morning for 3 to 5 inches of rain and locally higher amounts *
Starting Wednesday, a long duration, heavy rain event is likely in the D.C. area and much of the Mid-Atlantic.
Several inches of rain and areas of flooding are possible in the hardest-hit areas, although amounts are likely to vary widely throughout the region.
The period of steadiest and heaviest rain is expected between Wednesday night and Thursday night. Intermittent and more widely scattered showers may start as early as Wednesday morning and linger into the weekend.
Several ingredients are in place for a significant event, but specific amounts will depend on exactly where the heaviest rain bands set up. Model forecasts range from one to eight inches of rain between Wednesday and Friday, but most concentrate around three inches.
Heavy rainfall possible Wednesday night through Thursday night. See image for more details. pic.twitter.com/Mqg72imV2l
— NWS DC/Baltimore (@NWS_BaltWash) September 27, 2016
As the D.C. region has been abnormally dry since August, a three-inch soaking over 24 to 48 hours would be beneficial. It would mostly erase the rainfall deficit that has built up.
However, if the heaviest model simulations are correct, and more than four inches of rain fall, then flooding would become a concern.
Some areas could get hit repeatedly by waves of torrential rainfall and embedded thunderstorms, in a process known as “training.” This would cause streams to rise and overflow their banks, and poor drainage areas to be overcome. Remember to never attempt to drive through flooded roads.
The National Weather Service indicates there is a slight risk of “excessive rainfall” Wednesday night and a slight to moderate risk of excessive rainfall Thursday. Excessive rainfall is defined as the quantity of water that would trigger flash flooding.
The setup for this event shares the characteristics of those that have produced the most extreme rainfall at this time of year in historical records.
A strong area of low pressure at high altitudes is forecast to dive south from the Great Lakes on Wednesday into the Tennessee Valley by Thursday.
This area of low pressure will be cut off from atmospheric steering currents, meaning it will sit and spin for up to several days. The low pressure area’s counterclockwise circulation will continuously draw very moist air from the Atlantic Ocean into the Mid-Atlantic.
“The set-up for this event basically is consistent with that of an East Coast atmospheric river given the long-fetch Atlantic trajectory that will be in place,” writes the National Weather Service. “Atmospheric rivers,” common along the West Coast, act like fire hoses, channeling streams of moisture in narrow swaths and then unloading a deluge.
Capital Weather Gang’s Wes Junker, who has specialized knowledge of Mid-Atlantic flash flooding, thinks areas of flooding will be difficult to avoid somewhere between central Virginia and southern Pennsylvania.
“Upper-level low [pressure areas] west of the mountains that stall providing a prolonged period of low-level easterly flow tend to be big rainmakers, especially along the east slopes of the mountains,” Junker said. “Usually the heaviest rainfall occurs west of the city, although it’s hard to say exactly where the bulls-eye will set up.”
Over the next day, determining how much rain falls, and where, emerges as the biggest forecast challenge. Some areas may receive only moderate amounts of rain, while record-challenging amounts could occur in others, which would lead to flooding.
We should have a better idea of how much rain will fall and where on Wednesday, although — even then — upper-level low-pressure systems and the distribution of rainfall they generate are notoriously difficult to forecast.
Stay tuned to our posts for the latest, and find rainfall simulations from several models below:
European model: 2.5-7 inches
GFS model: 2-6 inches
NAM model: 1-4 inches
Wes Junker, Ian Livingston and Jeff Halverson contributed to this post. This post, originally posted early Tuesday afternoon, was updated Tuesday evening.