Arlington’s Four Mile Run, a tributary of the Potomac River, struggled to drain all the rain Sunday. (Tom Mockler via Flickr)

* Flood warning for Potomac River at Little Falls | Coastal flood warning for tidal Potomac River *

On Sunday, the latest in a long series of deluges drenched the Washington region, as the sky dispensed widespread rainfall amounts of 1 to 3 inches — with localized totals up to 3 to 5 inches. The torrents sent streams and creeks over their banks flooding many area roads and even washing out bridges.

The massive volume of water flowing into the Potomac is now pushing it well above flood stage. Gauges along the river reported moderate to major flooding early Monday, and levels are forecast to rise further — to their highest levels since March 2010.

In the last 23 days, Washington has received 10.43 inches of rain. The rainfall output over the past three weeks ranks second most on record for the time of year. We have seen five separate storms unload at least an inch of rain.

30-day rainfall over the Washington region shows widespread amounts between 8 and 15 inches. A few spots have seen 15 to 20 inches. (National Weather Service)

Forecast river levels along the Potomac

All of that water that flooded creeks and streams Sunday is now flowing into area rivers, already at high levels due to rain over the last three weeks.

Flood warnings are up all along the Potomac, including both the main stem (above the District) and the tidal portion (including the District).

At Little Falls, the river is forecast to crest at 13.0 feet Monday night, which is three feet above flood stage. The National Weather Service says at this level “significant portions of the C and O Canal towpath are flooded.” It added that water approaches the retaining wall at Overlook 1 at Great Falls National Park but “does not overtop it.”

Water level forecast at Little Falls (National Weather Service)

The river is expected to settle below flood stage by Wednesday morning.

Farther downstream, in Georgetown, a coastal flood warning is in effect from noon Monday to Wednesday morning. Moderate flooding is forecast due to levels 2 to 3.5 feet above normal. Impacts will be most notable at high tide when flooding is expected at the D.C. seafood market, along the boardwalk at Washington Harbor and in portions of East Potomac Park.

Water level forecast at Georgetown (National Weather Service)

More minor flooding is expected in Old Town, Alexandria at high tides through Tuesday, where a coastal flood advisory is in effect.

Flooding is also occurring along portions of the Shenandoah and Rappahannock rivers.

Sunday’s rainfall in detail

The rain fell almost nonstop between 8 a.m. and 8 p.m. Sunday and, at times, was torrential.

Reagan National Airport, Washington’s official weather observing site, picked up 1.65 inches of rain Sunday, which was fairly indicative of amounts inside the Beltway.

Sunday’s estimated rainfall. (National Weather Service)

However, small pockets of 2 to 4 inches fell inside the Beltway and, just to the north, 3 to 5 inches came down. Some of the heaviest totals concentrated around Laurel and Sykesville, where bridges were washed out by floodwaters.

Here are some select totals:

  • Greenbelt, Md.: 4.64 inches
  • Sykesville, Md.: 4.62 inches
  • Beltsville, Md.: 4.6 inches
  • Fredericksburg, Va.: 3.45 inches
  • Laurel, Md.: 3.2 inches
  • Ellicott City, Md.: 3.0 inches
  • Silver Spring, Md.: 2.89 inches
  • Bowie, Md.: 2.57 inches
  • National Arboretum, D.C.: 2.51 inches
  • Alexandria, Va.: 2.04 inches
  • Manassas, Va.: 1.89 inches
  • McLean, Va.: 1.86 inches
  • Annapolis, Md.: 1.83 inches
  • Baltimore-Washington International Marshall Airport: 1.7 inches
  • Waldorf, Md.: 1.68 inches
  • Falls Church, Va.: 1.67 inches
  • Vienna, Va.: 1.37 inches
  • Gaithersburg, Md.: 1.08 inches
  • Potomac, Md.: 0.97 inches
  • Reston, Va.: 0.79 inches

Link: Full list of totals from National Weather Service

Forecasting fits: Sunday was — surprisingly — rainier than Saturday

Before the weekend, forecasts had predicted Saturday would have the highest flood potential due to heavy thunderstorms while Sunday would have steadier though lighter rains. It turned out Saturday was mostly dry (National Airport picked up just 0.05 inches), while the torrents came Sunday.

So what happened?

First, model forecasts for Saturday were flawed, and they simulated more rain than actually fell. Saturday’s rainfall was convective — meaning highly dependent on where thunderstorms developed. Numerous thunderstorms did actually form as forecast, but they managed to spread all around Washington instead of in Washington itself. Notice the rain hole around the Beltway.

Estimated rainfall on Saturday. (National Weather Service)

Models tend to smooth out thunderstorm rainfall and cannot always pinpoint which areas will get hammered and which passed over. We should have done a better job explaining there was a chance storms could end up missing certain areas, which would enable outdoor plans to proceed.

While the rains fizzled on Saturday, they overachieved some on Sunday. In fairness, we had said Sunday would be a damp, rainy day, but the intensity of the rainfall surprised us and was much higher than model forecasts.

Greg Porter offered a nice explanation of what happened in our PM Update on Sunday:

The low pressure (denoted by the red L below) took its time reaching the North Carolina/Virginia coast [Saturday] night and kept the D.C. area locked in a relatively dry pocket of air, which resulted in little rain falling. That all changed today, as the low pressure hit the coast. Winds rotate counterclockwise around low pressure, keeping us locked in a moisture-rich easterly flow. However, that flow was amplified by a tiny wedge of high pressure (you can just barely see it in the top right of the image below). Winds flow clockwise around high pressures, so we had two pressure systems pumping moisture in from the Atlantic into an environment that was already fully loaded with moisture. Hence, the drenching rains.

Easterly flow from the Atlantic (black arrows) is amplified by a tiny wedge of high pressure (blue arrows also blowing from the east. The result has been a nearly stationary blob of heavy rain this afternoon.)

Why does it keep raining so much?

Since the second week of May, we have seen a fairly persistent flow of tropical moisture from the Gulf of Mexico. We can blame clockwise flow from high pressure off the East Coast that has acted like a pump, drawing the moisture over the area.

At times, weak fronts have set up over the Mid-Atlantic, focusing extreme rain storms in places like Ellicott City, Fredericksburg and outside Charlottesville.

The overall flow regime is also related to the position of the jet stream, the current of upper level winds that divides cold air from warm air. It lifted north into Canada at the beginning of May that allowed this very tropical weather pattern to take over.

(NOAA, adapted by Capital Weather Gang)

The jet stream will try to take some more dips into the United States in the coming week or two, breaking up this stuck, muggy weather pattern. Storminess along it will bring occasional rain chances, but we will not have the nonstop humidity and heavy rain threats.

Aside from a few showers late Tuesday, mostly dry weather is predicted for the next several days in Washington. But some storminess could return this weekend.