* Numerous flood warnings for rising rivers/streams in area | Flooding leads to water rescues across Washington area *
A mad rush of warm, humid air from the sub-tropics triggered an unusual January episode of wind-driven rains over the Washington, D.C. metro region Wednesday night. West of the beltway, 2 to over 4 inches of rain fell, with widespread reports of flooding in northern Fauquier, Loudoun, northern Montgomery, and Frederick counties.
The setup for this event was much more characteristic of the warm season. Very strong flow from the south pushed record challenging warmth into the Mid-Atlantic, with temperatures surging into the 70s ahead of a powerful cold front.
This super-charged flow carried deep moisture from the subtropical Pacific and Gulf of Mexico up the East Coast at high speeds, up to 90 mph just a mile above the surface. We call this feature a “low level jet” and it enables the efficient transport of vast quantities of moisture over large distances.
Weather Underground meteorologist Jeff Masters, in his Wednesday morning blog post, noted the storm system contained record levels of January moisture. Precipitable water - a measure of the available moisture in the atmosphere for producing rain - reached record or near record levels for January in numerous cities, Masters said. The precipitable water in Sterling, Va. Wednesday was 4th highest on record for January.
Band after band of heavy rain traveled along this high-speed river of air, causing rivers, creeks, and streams to rise, prompting road closures and multiple water rescues in the region’s western counties.
There was concern, due to some turning of the winds of height in the atmosphere, of tornado activity, prompting a “watch” from the National Weather Service (NWS). Although there was some weak rotation - which apparently produced a funnel cloud near Ft. Belvoir - no confirmed tornadoes touched down.
The NWS issued a number of severe thunderstorm warnings associated with the convective bands of rain coming through. The motivation was a concern the very strong winds at higher altitudes might crash down to the ground. But a very shallow layer of more stable air near the surface helped to attenuate the force of any downward transport of these raging winds. To be sure, winds were gusty at times, as high as 45-50 mph, but below damaging levels.
By far, rainfall resulted in the greatest impacts. The highest totals exceeded 4 inches in Loudoun, Frederick and northern Montgomery county. An observer in Damascus, Md. reported an astonishing 6.15 inches of rain.
Here are totals from our local airports:
Reagan National: 1.14 inches
Dulles: 2.32 inches (2” that fell Wed before midnight a daily record)
BWI: 2.00 inches
Flood warnings remain in effect for several area rivers and streams, which have yet to crest.