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:
THERE ARE MULTIPLE FACTORS INVOLVED - 1) FRESHWATER FROM POTENTIALLY HEAVY RAINS FRI NGT AND SAT FLOWING DOWNSTREAM IN THE PTMC 2) INCREASING ASTRONOMICAL/LUNAR INFLUENCE AS WE APPROACH THE NEW MOON ON MONDAY AND 3) LOW LVL SERLY FLOW FRI-SAT WILL HINDER DRAINAGE IN THE PTMC/CHES BAY.
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.
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.