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Extremely heavy rain tonight into Friday may trigger ‘severe flooding,’ challenge records

We do have a flash flood watch in effect through Friday. We are expecting the heaviest rain, today, tonight and into Friday morning. (Video: Jason Samenow/The Washington Post)

* Flash flood watch 6 tonight through Friday morning for three to six inches of rain, and locally higher amounts | Severe thunderstorm watch for Southern Maryland through 10 p.m. *

9:10 p.m. update: Rain has blossomed across the broader region late this evening, and that should continue to be the case into the night. The Weather Prediction Center is highlighting the D.C. area and to the south and east for flash flooding risk in the short term. Rainfall rates of one to two inches per hour or more are possible in the heaviest activity.

Original post, from 3:00 p.m.

The D.C. area should prepare for the possibility of the heaviest rain event in at least five years and possibly longer. It could trigger areas of “severe flooding,” the National Weather Service warned.

Forecast rain amounts through Friday could exceed five inches. The region hasn’t experienced this much rain over a three-day period since September 2011, when the remnants of Tropical Storm Lee swept through and unloaded half a foot in the District. Upward of a foot fell in a few areas, which caused severe flooding.

How does the early September 2011 East Coast flood event rank in the Washington D.C. area?

Some forecasts suggest similar amounts to that 2011 event could occur through Saturday in the D.C. region. The National Weather Service predicts eight to 10 inches in the immediate metro area. This is a somewhat aggressive forecast, and we think three to six inches is more likely, but we cannot rule out such amounts in pockets of the region.

The period of greatest concern for heavy rainfall is tonight through Thursday night. Our short-term models suggest conditions could deteriorate quickly this evening as today’s intermittent showers evolve into a more sustained, driving rain. Thunderstorms capable of locally strong winds may be embedded within the cycling waves of rainfall. An isolated tornado is even possible, mainly south of Washington, where a severe thunderstorm watch is in effect through 10 p.m.

We can’t pinpoint exactly where the heaviest rain will occur. But some models favor the Interstate 95 corridor, where multiple inches of rain could fall tonight. Also, areas near and along the eastern slopes of the mountains in northwest Virginia and Maryland are likely to experience enhanced rainfall as the terrain helps lift the super-moist air streaming in from the ocean.

In areas where more than three to four inches of rain falls in six hours, flash flooding becomes a possibility. If you live in an area which is prone to flooding during heavy rain events, be prepared to move to higher ground and heed any warnings issued by the National Weather Service. Small streams and poor drainage areas are especially vulnerable.

This is how a flash flood occurs and what you need to do to stay safe. (Video: Claritza Jimenez/The Washington Post)

Bands of heavy rain may continue to sweep through the area during Thursday morning’s commute, which may be challenging in areas due to heavy rain and/or flooded roads.

Although there are likely to be occasional breaks in the rainfall, the overall setup supports regenerating waves of rain even through Thursday night and into Friday, when the flood risk will continue.

This is why the D.C. region could see torrential, flooding rain this week

After the rain ends, the flood threat does not as rainwater causes river levels to rise. The Middle Atlantic River Forecast Center notes significant river flooding is possible both during and after the storm:

It’s worth pointing out many areas will not experience flooding and, as the region is abnormally dry from little rain since August, the rain will largely be beneficial. Amounts are likely to vary widely and just as some areas are likely to receive more than three to six inches of rain, some areas will receive less.

Record rainfall totals in jeopardy

As this storm is expected to produce at least three to six inches of rain, the following daily rainfall records in the District could be challenged:

Sept. 28 (Wed.): 2.46 inches (2004)
Sept. 29 (Thurs.): 2.68 inches (1924)
Sept. 30 (Fri.): 4.66 inches (2010)

Friday’s record will be most difficult to break as the coverage and intensity of rainfall should be diminishing by then.

The monthly record for the wettest September day of 5.16 inches from Sept. 2, 1922, will be difficult though not impossible to beat in this situation.

For reference, the top three-day and one-day rainfall amounts from any months are provided below:

It’s not out of the question that this event could enter the above rankings.

Flood preparation and safety

Flooding is one of the top weather-related killers in the United States and causes hundreds of millions of dollars in damage every year.

If you haven’t already, clear storm drains (without endangering yourself) and make sure your sump pumps are operating. The graphic from the National Weather Service, below, has some other useful pointers:

Once the rain has begun in earnest, it is vitally important to remember to never drive through flooded roads. Your car may be swept away putting you, your passengers and potential rescuers in grave danger. Turn around, don’t drown.

Also, do not let your children play near streams during this rain event. Water rises can occur suddenly, putting people nearby in danger.

Model rainfall forecasts

European model: 3-10 inches, heaviest Interstate 95 corridor

NAM model: 2-6 inches, locally higher in the mountains

GFS model: 2.5-6 inches