* Flood watch for D.C. area, except southern suburbs, through Friday evening *
Waterlogged air from both the Gulf of Mexico and Atlantic Ocean are converging over the Washington area through early Saturday, setting the stage for torrential rainfall and possible flooding. A wet weather pattern will persist for the next week, at least, although the coverage and intensity of rainfall should ease over the weekend.
Through Friday evening alone, the National Weather Service is predicting 3 to 5 inches of rain in the region. It has issued a flood watch within a one county radius of the District and to the north. The “primary concern is flooding of small streams and low-lying areas,” the Weather Service said. “Streams are already elevated and soils saturated from earlier rainfall, increasing the flood threat.”
Flood Watch: While more rain is expected today, the heaviest is expected Thu PM – Fri eve as tropical moisture is swept north into our stationary front overhead. That will bring the potential for #flooding. pic.twitter.com/PeodWZXCE7
— NWS DC/Baltimore (@NWS_BaltWash) May 16, 2018
Additional rainfall is likely Friday night into Saturday morning.
Washington has now received measurable rain for five straight days, totaling 2.59 inches as of Wednesday afternoon. Some areas to the northwest, like Frederick, have received more than half a foot.
Three features are primarily responsible for the heavy rain threat into Saturday.
First, a low-pressure system from the eastern Gulf of Mexico is lifting north through the Southeast toward the Mid-Atlantic.
Second, high pressure off the East Coast is acting like a pump, drawing the gulf moisture northward while also pulling in moisture from the Atlantic Ocean.
Finally, a stalled front draped over the region, which has triggered severe thunderstorms in recent days, will help focus heavy rainfall over our area.
Through Friday, the National Weather Service has placed the region in its slight risk zone for “excessive rainfall,” which means there is a 10 to 20 percent chance of flash flooding within 25 miles of a particular location.
It is impossible to pinpoint which areas will be hardest hit, but the concerns are the corridors of heavy showers and thunderstorms where individual cells track over the same areas repeatedly.
The potential for heavy rainfall is highest in the afternoon and evening Thursday and Friday, although waves of rain are possible at any time extending into Saturday morning. It won’t rain nonstop, but more often than not, you’ll need an umbrella.
Thursday into Friday, a measure of atmospheric moisture from the ground up to cloud level, known as total precipitable water, is forecast to be near record levels (possibly near or exceeding two inches).
Here’s how much rain the models are predicting through Friday night in Washington:
- GFS: 3.1 inches
- Canadian: 4.6 inches
- European: 4.7 inches
- NAM: 6.3 inches
The weekend and beyond
For those with plans on Saturday, it may well start off soggy, but there are hints the worst of the rain will lift to the northeast in the afternoon. Sunday is the more promising of the weekend days, although a few showers cannot be ruled out.
The high-pressure system off the East Coast that has been pumping moisture into the region for days is expected to remain in place next week, keeping the daily risk of showers and storms in the picture.