Although moderate and benign rains have fallen in the immediate Washington, D.C. area the last two days, locations to the west and southwest have endured more persistent downpours, leading to multiple instances of flooding. Additional flooding is possible into this evening, even into some of the District’s western suburbs as new storms pop-up.
In parts of western and central Virginia, more than half a foot of rain has already fallen, and flood warnings are in effect. Flooding has resulted in numerous road closures in Shenandoah, Orange, Madison, Green, Albemarle, Nelson and Augusta counties due to streams and rivers overflowing their banks.
Link: Rainfall totals
Flood watches are active in fairly close proximity to the immediate region, including the nearby counties of Loudoun, Fauquier, and Stafford. Frederick County in Md. is also included in the watch. Writes the National Weather Service:
SCATTERED SHOWERS WITH MODERATE TO HEAVY RAIN ARE EXPECTED TO DEVELOP AGAIN LATE THIS MORNING AND INTO THE EARLY AFTERNOON HOURS. ANOTHER ONE TO TWO INCHES OF RAINFALL IS POSSIBLE TODAY…WITH LOCALLY HIGHER AMOUNTS. SINCE THE GROUND IS ALREADY SATURATED OVER MANY AREAS AND RIVERS ARE HIGH DUE TO YESTERDAYS RAINFALL…THERE IS AN ENHANCED FLOODING POTENTIAL THROUGH THIS EVENING.
The culprit for the heavy rains is a pinwheeling cut-off low pressure system currently centered in southern West Virginia. The counterclockwise circulation around it is tapping a feed of moist air from the ocean (easterly direction). As that feed runs into the mountains to our west, the air is forced up, enhancing the rainfall.
For the immediate Washington, D.C. metro region, showers and thunderstorms are likely to develop this afternoon and evening, some of which may be heavy and even produce some gusty winds and small hail. But, as rainfall totals have generally been 1 inch or less from the recent rain, the ground isn’t yet saturated and widespread flooding is not anticipated.
The coverage of showers and storms will be scattered, meaning not everyone will see them. Activity that develops should begin to decrease in coverage and intensity between 7 and 10 p.m.
To the right, a simulated radar effective 5 p.m. generated by a high resolution model offers you a sense of what might be coming. But please note this kind of simulation is only an approximation and will inevitably err in the exact details of the timing/placement of rain.