Flash Flood Watch in effect through Sunday for good reason as PWATS increase above 2.5″ this afternoon into this evening.
- Strong frontogenetic forcing (strong upward lift at the surface), along a warm front aided by strong divergence aloft (lift) with the approach of a potent upper-level low pressure system will all aid in repeated rounds of strong to possibly severe thunderstorms containing torrential rainfall rates of 2″ per hour at times.
- I can’t stress enough how dangerous conditions will become later this afternoon through the overnight hours as rain gauges across parts of our area will have already picked upwards of 3-5″ of rain with an additional 1 to 3″ of rain possible in spots through Sunday afternoon as the steady rain associated with the back side of the exiting upper-level low pressure system only exacerbates swollen creeks and streams.
- If you live in a flood prone area, you need to be prepared to move to higher ground this afternoon through Sunday.
Latest Rainfall Estimate from the High Resolution Rapid Refresh Model through midnight tonight (doesn’t include the rain that will continue falling overnight through Saturday:
Darker purple colors indicate areas where 4 to 6″ of rain is possible
Here is the latest North American Model which unfortunately showing unrelenting torrential rain from widespread thunderstorms converging right across the Washington Region:
- Notice the steady rain that continues to fall on the back side of the upper-level low pressure system during the day on Saturday
Severe Thunderstorm Threat: Isolated Tornadoes are still possible
Overcast skies will help to prevent what could have become a widespread severe thunderstorm outbreak.
With that said, wind shear values will still be moderate as we head through the evening hours and with a warm frontal boundary draped across our region and an area of low pressure moving along it, wind shear at the surface along the boundary and deep moist convection aided by the approaching upper-level low pressure system could still produce a few storms that rotate. While flooding is obviously the main threat, a few isolated tornadoes can’t be ruled out (the EF-2 tornado that hit Stevensville Maryland occurred while no Tornado Watches had been issued). Severe thunderstorms can produce tornadoes will little to no warning.
Latest Forecast Sounding for National Airport (DCA) valid at 8PM this evening shows low-level backing winds (easterly flow at the lower-levels) with southwesterly to westerly winds aloft. Notice the bottom right “Psbl Haz. Type”: TOR (Tornado). Bottom line, even though we currently do not have a Severe Thunderstorm Watch or a Tornado Watch, any thunderstorms that develop along the warm-frontal boundary could spawn a tornado.