

Satellite view of powerful weather system that will sweep through Mid-Atlantic Wednesday (NASA)

A powerful Great Plains low swinging across the U.S. – with a history of severe weather across the Mid-South – will transit the Mid-Atlantic Wednesday through Wednesday night. While our chance of severe thunderstorms is low, expect a period of moderate to heavy rain, with embedded strong wind gusts. Localized flooding of streams and low lying areas is possible.

Heavy rain could begin streaming up the I-81 corridor between late morning and noon, and move into D.C.’s far western suburbs (Loudoun and Frederick counties) by early afternoon. The onset of rain in the immediate metro region is likely to between around 2 and 5 p.m. Wednesday’s rush hour could be a mess, with downpours, reduced visibility, and ponding of water on roadways.

This story explains how the storm will likely unfold.

Fall’s first powerful storm system crosses the U.S.

A large-scale storm system, called an extratropical cyclone, is approaching our region from the west. On Monday, this storm spawned an outbreak of tornadoes and severe thunderstorms across a swath along the Mississippi (Figures 1 and 2).



Figure 1. Radar snapshot from Monday afternoon showing a violent squall line and numerous tornado watches (red boxes) across the Mid-South. (Unisys Weather)



Figure 2. Severe thunderstorm damage reports from Monday. (NWS SPC)

The system is particularly intense because it is connected to, and driven by, a vigorous wave in the jet stream. Today, we break into the warm sector of this cyclone. Winds will intensify from the south, ushering in warmer air and Gulf moisture. By Wednesday afternoon, the cold front approaches the Appalachians. This sequence is illustrated in the following two figures, showing the cold front and stripe of heavy rain (dark green shading) within the warm sector and along the front.



Figure 3. Surface forecast chart for tomorrow morning, showing cold front and moisture plume with heavy rain (dark green shades). (NOAA)



Figure 4. Surface forecast chart for tomorrow evening, showing cold front and moisture plume with heavy rain (dark green shades). (NOAA)

In terms of severe weather potential, there is good news: the primary low center, over the Ohio Valley, will be in the process of occluding. This means that the cold front, wrapping around the low pressure center, catches up to the warm front. In the process, the warm sector of southerly flow gets pinched off. Once enshrouded in cooler air, the cyclone loses energy. The central pressure rises, and the storm begins to slow down.

However, we are not completely off the hook. Heavy precipitation is likely for up to 12 hours, and this is related to the system’s moisture supply, which is impressive.

A conveyor of moisture

Sometimes in these strong autumn cyclones, a conveyor of rich tropical moisture gets pulled northward, streaming through a deep layer of the warm sector. This level-conduit takes the form of a jet-like, humid air current, about 5,000 feet above the surface. The two figures below, from the prediction models, illustrate how fast southerly winds and high moisture content (total precipitable water – a measure of water vapor – will exceed two inches) will combine over the Mid Atlantic.



Figure 5. Forecast for a narrow ribbon of high precipitable water (2 inches, blue tones) valid 5 p.m. Wednesday. (WeatherBell.com)



Figure 6. Forecast for a low level, southerly jet 3,000 feet above the surface (45-50 knots or around 55 mph, red color) valid 2 p.m. Wednesday. (WeatherBell.com)

The low-level ribbon of fast winds and moisture develops in earnest Wednesday afternoon. Waves and bands of showers may develop, reforming to the south (along and ahead of the cold front), passing repeatedly over the same locations. The slowly advancing moisture plume, with winds parallel to the cold front, is a classic setup for what we call “echo training” (repeated surges of rain over the same area).

Figure 7 illustrates the expected rain accumulation from this storm system. Overall, a widespread inch seems reasonable; localized amounts may push 2”-3”. Rain accumulation plumes from this morning’s model guidance are suggesting around an inch. NWS Sterling is considering a flood watch area-wide.

I cannot completely outrule a severe thunderstorm watch for tomorrow evening (there is even some talk of an isolated, weak tornado, given a bit of directional wind shear that might be present), but this seems like a low probability.



Figure 7. 3-day rain accumulation map, through Thursday morning. (NOAA)

Thunder (and possibly more) in the rainbands?

With warm, humid air streaming north ahead of the cold front, and cold air aloft, some degree of air mass instability will be present Wednesday afternoon and evening. The models do not suggest significant destabilization but enough such that crops of thunderstorms could erupt within the larger plume of moisture.

Again, overnight is not the optimum time of day for severe weather. However, with strong winds, up to over 50 mph (40-50 knots) just a few thousand feet off the deck, there is some concern that downdrafts could transport some of this fast-moving air to the surface, in the form of strong wind gusts.

The Storm Prediction Center (SPC) puts the severe gust (greater than 58 mph) risk area-wide across the Mid Atlantic at about 5 percent, which is marginal. But saturated soils in regions of water ponding, trees heavy with wet foliage, and strong gusts over 40-50 mph, could create some spotty treefall problems.

Starting Wednesday afternoon, and continuing into the early morning hours, expect one or more convective bands generating locally heavy rain with embedded strong wind gusts.