9:50 p.m. update: The flash flood watch has been canceled a little early as the risk of widespread heavy rain has dwindled. Showers will remain possible through the night.
9:15 p.m. update: The flash flood warning noted below has been canceled as storms have dissipated. Additional showers are dotting the region, including D.C., at this time and that may continue into the night. For the most part, all rain in the area is light to moderate, though some heavier activity is still possible.
7:55 p.m. update: Some showers and thunderstorms with heavy rain have been impacting areas to the southwest of the city. A FLASH FLOOD WARNING is in effect until 10:45 p.m. for northwest Prince William County, south central Loudoun County, northeastern Fauquier County and west central Fairfax County. This area of rain is moving quite slowly. As we head into night, caution is advised in areas of heavy rain as flooding will be harder to gauge. Additional showers and storms may begin to move across parts of the area later this evening.
From 4:04 p.m: Afternoon temperatures today were as cool as they’ve been in three weeks. We’ve shed the heat, but rain chances are tough to shake away due to the painfully slow movement of a front sinking south through region. Showers and storms chances are moderate-high tonight (especially late), low-moderate Saturday, and down but not out Sunday.
Through Tonight: The area of scattered, mainly light, showers moves out of eastern areas by 5 p.m Outside of that, most locations experience a break in the rain through mid-evening - though isolated showers or thunderstorms could still pop up anywhere through that time (30 percent chance). Between 9 p.m. and 3 a.m. (from west to east), the chance of showers and storms increases (to 50-60 percent chance) as a line of showers and storms may move in from the west which could produce locally heavy rain. Skies are mostly cloudy, with lows in the upper 60s to low 70s (downtown).
Saturday: The front makes some progress south, but not far enough to quell rain chances. Skies are variably cloudy (some partly sunny intervals are possible), with highs in the upper 70s to low 80s. The best chance of rain showers (30 to 50 percent chance, north to south) and maybe an isolated thunderstorm are during the afternoon and south of the District. Just a slight chance of evening showers Saturday night, then partly to mostly cloudy and pleasant, with lows 64-70 (suburbs-city).
Sunday: The better of the two weekend days with slightly lower humidity and partly sunny skies. Highs are below average for a third straight day, reaching the mid-to-upper 80s with a light breeze. I can’t rule out a shower (20 percent chance), but don’t see a lot of potential.
Difficult rain/flooding forecast: A flood watch remains in effect, but whether substantial rain actually materializes is a difficult question. The National Weather Service has a good explanation on why rain/flooding chances are qualified:
SO FAR TODAY HAS BEEN VERY BENIGN HYDROLOGICALLY...BUT WE ARE NOT YET OUT OF THE WOODS. WATCHING A DEVELOPING LINE OF CONVECTION ALONG A BOUNDARY IN OH/WRN PA.
GUT FEELING IS THAT A LOT OF THAT EXISTING LINE WILL MISS OUR AREA TO THE WEST...BUT THE EASTERN END OF THAT LINE /AND ANY OTHER DEVELOPMENT NEARBY OR WITHIN/ STILL NEED TO BE WATCHED AND WOULD STILL BE CAPABLE OF PRODUCING HEAVY RAIN.
WHILE WIDESPREAD FLASH FLOOD THREAT SEEMS TO BE DECREASING...FCST MODELS ARE NOT HANDLING ANY OF THIS WELL...SO THAT LEADS TO MORE UNCERTAINTY THAN USUAL...AND THAT IS A BIG PART OF THE REASON WHY THE FLASH FLOOD WATCH HAS BEEN MAINTAINED. TRENDS WILL NEED TO BE MONITORED THROUGH THE EVENING JUST IN CASE.
OVERALL...WE COULD ABSOLUTELY USE MORE RAIN...JUST NOT TOO MUCH AT ONCE.