The National Weather Service (NWS) places the region from Richmond to Boston, including the Washington, D.C. area, in its slight risk zone for excessive rainfall Saturday and Saturday night. This means there is a small possibility (5-10 percent chance) rain will be heavy enough to cause flash flooding.
The NWS’ Weather Prediction Center generally predicts around 2 inches of rain for the D.C. area, approaching 3 inches in our northeast suburbs.
I tend to think 1-2 inches will be more common in the D.C. area than 2-3 inches. However, locations to our west (towards the mountains) and north (towards the Pa. line) have a good chance of seeing totals exceed 2 inches.
Here are rainfall total predictions from four computer models (note: these totals include rain predicted today, in addition to over the weekend):
The cause of the rain is a strengthening low pressure system, forming along a front extending from the Southeast to southern New England. The NWS forecast office in Sterling, Va. offers this nice explainer, even noting the potential for some thunderstorms in the region:
WARM AND MOIST AIR FROM THE GULF OF MEXICO WILL OVERRUN THE SURFACE COLD AIR IN PLACE AHEAD OF THE STATIONARY BOUNDARY…CAUSING RAIN TO CONTINUE OVERSPREADING THE AREA SATURDAY MORNING. RAINFALL RATES WILL INCREASE SATURDAY AFTERNOON INTO SATURDAY NIGHT AS FRONTOGENETICAL FORCING STRENGTHENS DUE TO THE STRENGTHENING LOW. THE COMBINATION OF RELATIVELY HIGH PRECIPITABLE WATER AND STRONG FORCING WILL CAUSE LOCALLY HEAVY RAINFALL DURING THIS TIME. THE STATIONARY BOUNDARY WILL LIFT NORTH INTO MARYLAND AND THERE MAY BE ENOUGH INSTABILITY FOR A FEW THUNDERSTORMS IN THE WARM SECTOR ACROSS CENTRAL VIRGINIA TOWARD THE I-95 CORRIDOR. . . .
The NWS adds there is some limited potential for flooding:
THERE IS ALSO A THREAT FOR FLOODING WITH RAINFALL AMOUNTS AROUND TWO TO THREE INCHES POSSIBLE. HOWEVER…THERE IS STILL A HIGH DEGREE OF UNCERTAINTY FOR WHERE THE HEAVIEST RAIN WILL FALL AND EXACTLY HOW MUCH WILL FALL. ALSO…THE RAIN IS EXPECTED TO FALL OVER A LONG PERIOD OF ABOUT 36-HOURS FROM SATURDAY INTO SUNDAY.
The way it looks right now, the most likely timing of the heaviest rainfall is Saturday afternoon. However, some downpours could occur overnight Saturday and even on Sunday. However, some models show the rain being much more widely scattered Sunday with extended dry periods possible.
Neither Saturday nor Sunday looks great for outdoor plans – but keep an eye on radar and forecast updates as it’s unlikely to rain the entirety of both days. Sunday, especially, may offer some windows to sneak outside.