* Flash flood watch from noon Friday to 5 a.m. Saturday *
Another vigorous, springtime cyclone is poised to cross our region Friday afternoon through Saturday morning. In the immediate D.C. region, flooding rains are the biggest concern, but scattered damaging wind gusts from strong to severe thunderstorms remain on the table.
For three days, we have been forecasting the potential effects of a strong low-pressure system that will track north up the spine of the Appalachians, placing much of the Mid-Atlantic on the storm’s warm side. That will allow mild and very humid air to surge northward tonight, Friday and Friday night.
That air will be sourced from the western Caribbean, meaning there is potential for heavy rainfall. The deep, rich moisture will be lifted and condensed by an extremely powerful jet stream that will align parallel to the low-level flow. Air spreading apart at high altitude along the axis of the jet will draw up air from below.
Rainfall of 1.5 to two inches is predicted in the Washington region (see top image), although amounts may vary, depending on where the heaviest storm cells develop and track.
The rains will be of moderate intensity and fairly constant, with waves of heavier rainfall. Isolated amounts in the three- to four-inch range are possible because of the effects of “training,” whereby heavy storm cells move over the same region repeatedly from south to north. If and where this occurs, flooding of poor-drainage areas and creeks and streams is possible.
It will also be breezy Friday afternoon and overnight. As for widespread, strong to severe winds in thunderstorms, that possibility is greater to the south and east of the District. The National Weather Service Storm Prediction Center edges a zone of slight risk (Level 2 out of 5) for severe storms into Washington proper, but this is the very northern edge of the envelope. The more significant concern lies from Richmond south and, particularly, over the Carolinas.
There, extremely strong winds, perhaps record-setting for that location in the spring, will overspread lower and middle levels of the atmosphere — as part of the jet stream. Such strong wind shear (increase in winds with altitude) is likely to trigger one or more damaging “bow echoes,” or fast-moving arcs of thunderstorms.
Over eastern North Carolina, winds near 5,000 feet may approach 80 mph to 90 mph. Across our region, the winds will be weaker (50 mph to 60 mph) and the atmosphere stabler — because of the effects of thick cloud cover. Enough instability will be present, however, for a few thunderstorms.
Even though thunderstorms in our region may not erupt to great heights (and lightning may be scant), downdrafts in those storms could periodically stir some of that wind momentum to the ground. This is why we must remain vigilant for strong to severe gusts in a few spots.
The higher-resolution forecast models suggest that a line of showers and storms crossing the District is most likely to develop between 7 p.m. and 1 a.m. But waves of showers containing heavy downpours are possible earlier.
Should clouds thin for a time, it’s also possible that some locations may destabilize enough to permit deeper, more intense storms. We will need to carefully assess trends Friday afternoon; we cannot completely rule out the elevated risk area for severe storms advancing farther north, putting us closer to the zone of most likely severe weather.