Capital Weather Gang's Jason Samenow explains how much rain has fallen in the Washington region, and how much more the area will get. (Victoria Walker,Breanna Muir/The Washington Post)

* Flood watch for entire region through Saturday morning | Flood warning for the main-stem Potomac River and much of Southern Maryland *

(This post, originally posted at noon, was updated at 1:45 p.m. based on the latest rainfall totals.)

Rain has lashed the Washington region every day for the past week, and is not done yet. Persistent downpours likely will continue into Saturday, and more intermittent showers and storms are in the forecast through midweek.

A flood watch covers the entire region through Saturday morning; warnings are in place for a host of rivers and streams that have already overflowed their banks or could over the weekend.

According to the National Weather Service Middle Atlantic River Forecast Center, 37 stream and river gauges in the region indicate moderate to minor flooding, and 26 more are near flood stage. Water levels along the Potomac River are forecast to crest at their highest levels in four years between Friday and Monday.

Heavy rain over the past day has resulted in flooding and multiple road closures in Prince George’s (around Upper Marlboro), Charles and St. Mary’s counties, the Weather Service reports.

The seemingly never-ending rain has been annoying and disruptive, cancelling a multitude of outdoor events. Saturday’s Preakness in Baltimore is sure to be a muddy mess. But in most of the metro region, the rain has fallen at a slow and steady enough pace to avoid flash flooding which occurs when there is extreme rainfall over a short time period.

“Most of the intensity [Thursday into Friday] stayed south in Charles and St. Mary’s counties,” said Jonathan Dillow, a hydrologist with the U.S. Geological Survey. “The [heaviest] rain didn’t run over the urban areas,” which would have caused more problems.

The wettest month gets wetter

This rainy stretch, now in its 7th day, has been remarkable – mostly for its duration.

More than 5 inches of rain has fallen in the District in the past week, which is more than it receives on average for the entirety of May (3.99 inches), the wettest month of the year.

It has received at least 0.4 of an inch of rain on six consecutive days, breaking the longest daily streak with so much rain on record.

Across the region, amounts have mostly ranged from three to six inches, but totals from six to 10 inches have accumulated between Winchester and Frederick.


Seven-day estimated rainfall totals. (National Weather Service)

Over the past 24 hours, areas of east and southeast of Washington toward Annapolis and Southern Maryland have witnessed the most rain, with widespread one-to-three-inch amounts, pushing their seven-day totals into the four-to-eight-inch range.

When additional rain is expected and how much


New rainfall forecast through Saturday night from National Weather Service. (WeatherBell.com)

The region has another 18 to 24 hours of frequent bands of rain showers, some heavy, before they become more scattered and intermittent.

Timing

Determining exactly where it will be raining and how heavily at a given time is next to impossible, but we can make the following predictions:

  • Friday and Friday night: Frequent showers, some moderate to heavy.
  • Saturday morning: Frequent showers linger, especially from the District to the northeast, but taper south.
  • Saturday afternoon: Showers taper and become much more widely scattered, southwest to northeast.
  • Saturday night and Sunday: Mostly dry, but chance of widely scattered showers or maybe a thunderstorm, especially Sunday afternoon.

From Monday through Wednesday next week, we’ll have daily chances for mostly afternoon showers and storms before a couple of dry days Thursday and Friday.

Accumulation

The National Weather Service is calling for up to one to three inches of rain, and up to three to five inches in Southern Maryland. This is in line with model forecasts, which predict the following for the District through Saturday:

  • GFS model: 1.4 inches
  • NAM: 1.6 inches
  • High-resolution NAM: 1.2 inches
  • Canadian: 1.7 inches
  • High-resolution Canadian: 2.1 inches
  • European: 3.2 inches

In general, new rainfall is predicted to be lowest well northwest of Washington and highest to the southeast.

What’s the status of the Potomac River?


(National Weather Service Mid-Atlantic River Forecast Center)

Minor-to-moderate flooding of the Potomac River is expected Friday through the weekend. It is expected to begin first upstream near Shepherdstown, W.Va., on Friday afternoon, reach Point of Rocks, Md., on Saturday morning, and Little Falls on Sunday morning.

Minor-to-moderate flooding is also forecast for the Tidal Potomac from Georgetown to the Southwest Waterfront in the District, where a coastal flooding warning is in effect from 6 a.m. Saturday to 8 a.m. Sunday. Water levels may rise two to three feet above normal at high tide.

“We’re forecasting the Potomac to crest at 7.4 feet at Wisconsin Avenue” in Georgetown, said Jason Elliott, a hydrologist at the National Weather Service office serving the D.C. area. “Last time something comparable was in May of 2014.”


River level forecast in Georgetown. (National Weather Service)

Elliott added: “I think the key message even if it doesn’t rain any more, there’s a lot of water in the system that has to get down here. Everything that fell up in West Virginia still has to come down and we’ll be dealing with this through the weekend. We aren’t going to back to Agnes [in 1972] or [January] 1996 [which were two of the river’s biggest flooding events], but it’s still something we haven’t seen for a while.”

Flood safety

With all of the water bombarding our region, it’s important to remember never to try to drive across a flooded road. The water level is difficult to judge and you could get swept away.

Importantly, stay away from the banks of streams and rivers — and don’t let your children play there. The water can rise suddenly and unexpectedly in these heavy rain situations.

What is causing this?

Three features are primarily responsible for the heavy rain threat into Saturday.

First, a low-pressure system from the eastern Gulf of Mexico has lifted north from 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.

More simply, think of the Gulf of Mexico and Atlantic Ocean as hoses, and the front over our region acting like a wall. Essentially, we are being sprayed at from two directions and the water has nowhere else to go.

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.