Build yourself an ark: 3-5 inches of rain possible for D.C. area through Thursday

* Flash flood watch Tuesday evening through Wednesday night for entire region *

Rain is coming to the D.C. metro area, and lots of it.   The National Weather Service has declared a flash flood watch for the entire region Tuesday evening (10 p.m.) through late Wednesday night (6 a.m. Thursday morning).

National Weather Service rainfall forecast through Friday for Mid-Atlantic

National Weather Service rainfall forecast through Thursday for Mid-Atlantic indicates 4 to 5 inches will fall.

The powerhouse, pinwheeling weather system in the Plains will act like a pump, drawing a river of moisture from the Gulf of Mexico up the East Coast as it slowly wobbles east and then northeast over the next 72 hours.

NAM model shows deep, tropical moisture (yellow shades indicate high amounts) being drawn out of the Gulf of Mexico up the East Coast Wednesday (WeatherBell.com)

NAM model shows deep, tropical moisture (yellow shades indicate high amounts) being drawn out of the Gulf of Mexico up the East Coast Wednesday (WeatherBell.com)

Moist flow off the Atlantic ocean will also enhance the rain.

NAM model shows surface wind flow off the Atlantic Tuesday night-Wednesday morning (WeatherBell.com)

NAM model shows surface wind flow off the Atlantic Tuesday night-Wednesday morning (WeatherBell.com)

Computer model forecasts generally simulate 3-5 inches for the D.C. area between late this afternoon and Thursday afternoon, with the heaviest rain favored Tuesday late afternoon and night, and Wednesday late afternoon and evening.

“We are concerned about the potential rainfall amounts tonight through Wednesday,” writes the National Weather Service office in Sterling, Va.

The rain is unlikely to fall continuously over this coming three-day period, but will come in waves. Nailing down the exact timing of these rainfall surges is close to impossible, but we tend to favor the afternoon and evening hours for the highest chance of heavy rainfall. Note there may well be windows when no rain falls or just widely scattered, intermittent showers.

As with any “warm season” (April-October) rain event, significant variability in rainfall totals is likely. Whereas the average amount of rain may be in the 3 to 5 inch range, some areas could easily get more or less.  Because of the storm’s slow motion, there’s some potential for training – or heavy rain cells tracking over the same areas continuously – elevating totals.  On the other hand, the hit-or-miss nature of rain bands – which may be narrow and/or splotchy – could leave some areas with underwhelming totals.

Simulated radar, 4 p.m. Tuesday, high-res NAM model (WeatherBell.com)

Simulated radar, 4 p.m. Tuesday, high-res NAM model (WeatherBell.com)

The National Weather Service’s Weather Prediction Center indicates the region has a slight risk (5-10 percent chance) of “excessive rainfall” or enough to cause flash flooding both Tuesday and Wednesday.

Excessive rainfall outlook Tuesday  (National Weather Service)

Excessive rainfall outlook Tuesday (National Weather Service)

Excessive rainfall outlook Wednesday (National Weather Service)

Excessive rainfall outlook Wednesday (National Weather Service)

Precipitable water, a fancy way to describe the depth of rain if all humidity in the air column were to condense, will exceed 1.5 inches during that time, and may come close to 2 inches. This is well above normal for this time of year.

With the heaviest rain probably holding off until Tuesday, there is still time to clear storm drains and gutters – which will lower your risk from flooding due to clogs.

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