Flood watch Thursday afternoon through Friday evening, except far southern suburbs

3:50 p.m. update: A flood warning has been issued for much of the mainstem Potomac River from Shepherdstown, W. Va. downstream into Washington starting Friday evening “until further notice.” It includes the river at Little Falls which, by midafternoon Sunday, could crest around 13.7 feet at which point “significant portions of the C&O Canal towpath are flooded.”

Original post from 12:05 p.m.

The Washington region is soaked and will only become more sopping through Friday and even into Saturday. Flood watches and warnings are in effect as streams are rising and, in some cases, overflowing their banks.

The flood watch, which takes effect this afternoon and continues into Friday night, calls for three to five inches of rain in addition to what has already fallen. The saturated ground will make it easy for water to collect, especially in low-lying areas.

It will not rain continuously. Downpours will tend to come in waves, which should increase in frequency Thursday night and Friday. Strong winds are generally not expected.


Areas under flood watches and warnings as of Thursday midday. (National Weather Service)

How much rain has fallen and where flooding is occurring

Measurable rain has now fallen in Washington on six straight days. Through Thursday morning, more than 3.5 inches had accumulated since last week. Generally, across the region, about 2 to 5 inches were most common, although our far northwestern areas between roughly Winchester and Frederick had received 5 to 10 inches.


7-day rainfall estimated by National Weather Service.

As of late morning Thursday, the National Weather Service Mid-Atlantic River Forecast Center reported moderate flooding at 12 river and stream gauges and minor flooding at 33 in the region. Many others were near flood stage.

Flood warnings mostly focused in western Loudoun and Frederick counties, where numerous streams have overflowed, resulting in some road closures. In Loudoun County, some of the areas affected include Purcellville, Lovettsville and Hillsboro.

In the city of Frederick, the Monocacy River was nearing flood stage Thursday morning and expected rise above it during the afternoon. The river is forecast to crest Saturday afternoon at 17.7 feet. Once the levels hits 17 feet, which is possible Friday or Saturday, the National Weather Service said the following will unfold:

Significant lowland flooding … along the river, with backwater flooding also occurring. Numerous roads are closed. Water is approaching the parking lot at Gambrill Mill on the Monocacy National Battlefield. Backwater flooding from Carroll Creek is probably approaching the underside of the bridge leading to the Frederick city wastewater treatment plant.

When additional rain is expected and how much

Timing

On and off waves of rain, sometimes torrential and containing thunder, are likely at least through Friday night. Determining exactly where it will be raining and how heavily at a given time is next to impossible, but we can make the following generalizations:

  • Thursday afternoon and evening: It will be showery, but some areas may see the rain pause at times, and the heaviest activity may focus more in Central Virginia
  • Thursday night into Friday morning: Widespread rain is likely in the D.C. area, and it could be heavy at times.
  • Friday and Friday night: Waves of rain, heavy at times
  • Saturday morning: Rain tapers off.
  • Saturday afternoon through Sunday afternoon: Mostly dry, but slight chance of hit or miss showers.

Next week, the wet pattern will continue through Tuesday or Wednesday, but rains should not be quite as frequent and widespread. We will return to a set up that supports more afternoon and evening storms, kind of like we experienced earlier this week. There are some indications we will have a dry stretch for a few days starting either Wednesday or Thursday next week.

Amounts


National Weather Service rainfall forecast through Saturday afternoon. (WeatherBell.com)

The National Weather Service is calling for about 4 inches of additional rainfall in D.C. through Saturday afternoon. This is on the high end of different computer model forecasts, which predict the following amounts:

  • NAM: 4.2 inches
  • High-resolution NAM: 2.4 inches
  • GFS: 2.5 inches
  • Canadian: 3.5 inches
  • European: 3.4 inches

For the most part, models predict the highest totals (over 4 inches) just south and southeast of the District, but a few show some of the heavier pockets to the north and northwest.

This is the sort of rainfall set up where there will be a lot of variability in amounts depending on the track of the heaviest showers and storms that often come in narrow bands and small cells.

Will the Potomac River flood?

The Mid-Atlantic River Forecast Center said to expect minor to moderate flooding at many locations in the Potomac basin through Sunday.

In the District and Alexandria, a coastal flood watch is in effect Friday through the weekend. “Freshwater inundation expected beginning Friday morning, with potential to significantly raise tide levels above normal by several feet,” the National Weather Service said.

The River Center forecasts the level at Little Falls to pass the flood stage of 10 feet Friday and to crest between Saturday and Sunday at 13.6 feet which is a moderate flood level.


(National Weather Service)

In Georgetown, moderate flooding is also expected this weekend, especially at high tide. The forecast is for minor flooding at high tide for both the District’s Southwest Waterfront and in Alexandria.

What is going on?

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. Through 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).

Finally, a stalled front draped over the region — which triggered severe thunderstorms early this week — will help focus heavy rainfall over our area, even as it weakens and becomes more diffuse. It is not terribly uncommon for fronts to get stuck in our region in May and June as the cold air that typically drives them south moderates. Usually our heavy rain events occur in the presence of these washed out fronts at this time of year.

Flooding, high water photos