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Posted at 02:45 PM ET, 08/11/2011

Washington, D.C. is in drought. Will weekend rains bust us out?

It’s official. Our region is in a drought. Neither severe nor expectional like in vast portions of Texas and Oklahoma, but a drought nonetheless. The good news is rain - perhaps substantial rain - is possible this weekend.

(U.S. Drought Monitor)
The drought

Technically, we are in “moderate drought” - the lowest designation of drought. The impacts of moderate drought include: Some damage to crops, pastures; streams, reservoirs, or wells low, some water shortages developing or imminent; voluntary water-use restrictions requested.

Our rainfall deficit since January is about 5 inches, and about 3 inches since June. Fortunately, a moderate drought is not difficult to overcome. An inch or so of rain would drop us back into the “abnormally dry” category as long as rain kept coming thereafter. A tropical dousing of several inches would put us out of drought danger altogether - at least in the short term (although damage to lawns/plants is already done).

Thank your lucky (lone)stars, that we’re not Texas which needs 20”+ to get out of the rain hole in some areas.

Weekend rain - the set up

Precipitable water at 10 a.m. Sunday from SREF model. (NOAA)
A fairly potent frontal system for early August over the center part of the country scoots towards the East Coast this weekend. The strong southerly flow ahead of it will pump in moist, tropical air - priming the atmosphere for heavy rain.

Precipitable water amounts - the amount of moisture that can be squeezed out of the atmosphere - exceed 2 inches on Sunday.

When will it rain?

* Widely scattered showers are possible starting Saturday afternoon, as a warm front pushes through. But chances of Saturday afternoon activities being disrupted are just 20-30%.

NAM model shows very heavy rain haven fallen between 2 a.m. and 8 a.m. Sunday morning just to the west of Washington.
* On Saturday night , evening activities may well be dry but rain chances increase as the night wears on. The far western suburbs (especially Loudoun and Frederick counties) could get into some very heavy rain, especially late at night and towards Sunday morning as the cold front gradually arrives. I wouldn’t be surprised to see some flash flooding from this activity. The D.C. metro region may be on the southeast fringe of this first shield of rain and only see a few showers - especially towards morning.

* Sunday brings the greatest promise for heavy rains in the immediate metro region as the cold front drags its heels through the region. But the rain is unlikely to persist the entire day and the timing is a bit difficult to pin down at this point. The way I see it now, we may see some early morning showers followed by a break, with the coverage and intensity of rain increasing sometime between midday and the afternoon. Any activity would probably wind down during the evening.

* The front may hang around close enough to the region to bring a chance (40%) of showers/storms again Monday - particularly in the afternoon.

How much will it rain?

NOAA’s rainfall forecast encompassing Saturday through Monday
NOAA’s Hydrometeorological Prediction Center projects between 1 and 2 inches over the metro region between Saturday and Monday. Even heavier rain is predicted north of the region. I like the idea of the heaviest rain falling to our north and northwest due to the track of the low pressure system just to our north. However, we *should* have enough moisture over the area and enough lift from the incoming cold front to at least produce 0.5” on the low end.

Of course, we’ve seen situations before this summer where rainfall totals have underperformed even the most modest expectations, so there are no guarantees. Having said that, Sunday’s weather system offers the best potential for meaningful rain in weeks.

Here’s my earlier video discussion of the drought and rain.

By  |  02:45 PM ET, 08/11/2011

Categories:  Local Climate, Droughts, Latest

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