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Posted at 11:00 AM ET, 03/12/2010

Significant flood risk from rising rivers & streams

By Dan Stillman

* Flood Watch through Saturday evening: CWG's Full Forecast *
* Flooding forces Frederick road closures | CWG T-Shirts! *
* Outside now? Radar, temps & more: Weather Wall | Traffic *

Flood threat: Outlined areas indicate risk of excessive rainfall capable of causing small streams to flood through Saturday morning. Most of the D.C. area is categorized as "High" risk (red), which corresponds to a greater than 15% chance of excessive rainfall. Courtesy the Hydrometeorological Prediction Center. See full image.

Late-winter warmth has pretty much wiped out our snow cover except for some dirty snow piles still scattered around town. However, that is not the case in Western Md. and northeastern W.V., where the Potomac River begins and where some locations still have more than a foot of snow on the ground.

Therein lies just one of several reasons that rains today through the weekend could cause local waterways to flood low-lying areas into early next week. The National Weather Service nicely summarized the forces at work in a discussion yesterday:


In other words, the 2-4 inches of rain we're expecting across the mid-Atlantic today through Sunday (heaviest should be tonight and tomorrow) could be enough for moderate flooding issues near rivers, streams and creeks, starting mainly tonight and into early next week.

Keep reading for more on the flood threat...

Points along and near the Potomac River are at the highest risk for flooding starting late tomorrow, again not helped by that added snow melt upstream. The National Weather Service predicts:

*Point of Rocks in Frederick County, Md., to reach flood stage (16 ft.) Saturday evening with moderate flooding possible there by Sunday morning.

*Little Falls in Montgomery County, Md., to see minor flooding as early as Saturday night as waters likely rise past the flood stage of 10 feet. Moderate flooding is possible by Sunday morning with water levels potentially reaching what's considered "major" flood stage (over 14 feet) by Sunday evening.

*Paw Paw, which is well to our northwest in northeastern W.V. along the border with Western Md., could experience major flooding Saturday night and Sunday as water levels threaten to surpass 32 feet (flood stage there is 25 feet).

See forecasts for more locations along the Potomac and Rappahannock rivers. Flooding problems may continue into Tuesday even as rivers recede.

6-hour flash flood guidance (see adjacent text for further explanation). Courtesy the Middle Atlantic River Forecast Center. See larger image.

While rivers are a bit slower to respond (they're already rising but have a ways to go before approaching or reaching flood stage), the prime time for any flooding associated with swelling creeks and streams and/or poor drainage would be during the heaviest rains late tonight through tomorrow. Computer model flash flood guidance for how much rain in a 6-hour period would be needed to cause small streams to overflow (based on soil moisture and stream flow conditions) is around 1-2 inches across the area.

Heavy rain tonight and tomorrow could approach or reach that threshold.

By Dan Stillman  | March 12, 2010; 11:00 AM ET
Categories:  Floods  
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Next: PM Update: Periods of heavy rain into Saturday


2-4" is quite a spread. Are some areas going to get more than others?

Posted by: steske | March 12, 2010 12:09 PM | Report abuse

Once again, thanks so much for this thoughtful and thorough piece on flooding as well as the links to the previous stories. Obviously, I am relatively new to CWG and missed these earlier stories. While I will not hold you to it, do you think the small stream flooding for this storm is similar to the May 2008 storm? (Obviously the snowpack outside the DC area will have a greater impact on the major rivers.)

Posted by: erbele | March 12, 2010 12:37 PM | Report abuse

CWG folks: The link to the Little Falls river stage prediction from NWS goes instead to the Point of Rocks page. The predicted level at Little Falls Monday morning is 16 feet, which would put in in the top 10 floods on record.

Posted by: jmbethesda | March 12, 2010 1:06 PM | Report abuse

How about the situation along the Shenandoah [Cootes Store, Broadway & other locales in VA and WV]? Well over fifty percent of the Potomac's water arises in the Shenandoah watershed; the rest is from the Monocacy and other tributaries. Snowmelt in the Shenandoah Valley is a big factor in Potomac runoff. Much of this area is SW of Washington, but at higher elevations.

Posted by: Bombo47jea | March 12, 2010 1:15 PM | Report abuse


Flooding probably won't reach the levels of May 2008 (more than 10" of rain fell that month).

Posted by: Jason-CapitalWeatherGang | March 12, 2010 1:16 PM | Report abuse


Don't think there is much snowpack remaining in the Shenandoah.

Posted by: Jason-CapitalWeatherGang | March 12, 2010 1:17 PM | Report abuse


Little Falls link has been fixed. Thx.

Posted by: CapitalWeatherGang | March 12, 2010 1:22 PM | Report abuse

wouldn't think we've had much more than a quarter inch or so since last night. Does that mean 2.5"+ possible between this evening and tomorrow evening? That'd be a pretty good rain storm. What does the RLCB say for our chances at 2.5" of rain?:)

Posted by: BH99 | March 12, 2010 2:01 PM | Report abuse

2-4" of rain? Imagine if the temperatures were 30 degrees colder!

Posted by: Dougmacintyre | March 12, 2010 2:59 PM | Report abuse

The only snow that's still out here in the valley is from the biggest piles that were plowed, in places like parking lots. But all that melting has left the ground pretty saturated, so the more rain we get in the next 48 hours, the more I'd been concerned...

Posted by: ValleyCaps | March 12, 2010 3:19 PM | Report abuse

Even better, thanks.

Posted by: erbele | March 12, 2010 3:31 PM | Report abuse

What fun can one have in all of this rain?

Posted by: irish031 | March 12, 2010 8:27 PM | Report abuse

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