* Flash flood watch through 2 a.m. Friday *

Numerous showers and storms are likely to break out late Thursday afternoon and evening and could unload heavy rain. It’s the same story Friday afternoon and evening and again Saturday. The ground is nearly saturated and any location that gets dumped on could experience flooding.

A flash flood watch is in effect for the region through 2 a.m. Friday (and may well be extended). The National Weather Service says storms could quickly produce two to four inches of rain in localized areas.

“Rapid rises of creeks and streams will be possible due to torrential rainfall, potentially spilling beyond their banks,” the Weather Service cautioned. “In addition, low points along roadways may become impassible due to accumulating runoff. Flash flooding may develop quickly.”

A point we’d like to emphasize is that, especially Thursday and Friday, storms are likely to be scattered (hit-or-miss), so not everyone will see heavy rain. And especially during the overnight and morning hours, there may be extended dry intervals.

We think the best chance for widespread showers and storms may come during the second part of Saturday, so that’s a forecast we suggest monitoring, especially if you have outdoor plans. Saturday is also the day we think the threat of flooding may be highest.

By Saturday night and Sunday, we expect more rain — which could fall moderately — but we do not expect flash flooding (from torrential rain over a short duration) as the atmosphere cools, and the risk of thunder should diminish. However, by that point, some larger streams and rivers may be starting to overflow.

The Mid-Atlantic has been a magnet for tropical moisture in recent weeks. Persistent flow off the Gulf of Mexico and the Atlantic Ocean have pushed moisture levels, on multiple occasions, to near-record levels, feeding slow-moving thunderstorms with torrential rainfall.

First it was Frederick, next came Ellicott City, and now Charlottesville is the latest location to drown in an exceptional heavy rain event. Western parts of that Central Virginia town received 7 to 9 inches in just a few hours Wednesday night.

While it’s impossible to say who might next be hit and exactly when, the environment is favorable for more excessive rainfall to occur somewhere from Central Virginia to the Mason Dixon line through Saturday (when the risk may be highest), including the Washington and Baltimore areas.

Through Sunday, the National Weather Service predicts a widespread two to three inches of rain, but local amounts could be significantly higher in a few spots.

Discussion

As of Thursday afternoon, a stationary front draped across our region has lifted north. With this boundary retreating, the atmosphere is destabilizing and turning more warm and humid, which is fuel for storms.  With weak southwesterly flow aloft, thunderstorm cells will move slowly toward the northeast.  These cells will probably generate heavy rain, and some locations could experience more than one storm. Flash flooding remains a possibility. We don’t expect a widespread severe storm threat, but a wet microburst or two is possible given the heavy weight of rainwater, which can accelerate cool masses of downdraft air toward the surface, bringing strong winds.

On Friday, the frontal boundary is expected to be over New England, while we remain in a very warm, humid and unstable air mass. Without the clouds hanging around a frontal boundary, the atmosphere will be more unstable and thus more likely to produce vigorous storms in the afternoon and evening.  However, in the absence of a front, the trigger (source of uplift) for storms will be lacking.  But given the continued weak flow aloft, storms that develop (perhaps over the terrain, and along a trough of low pressure cutting through the D.C. region) will again move slowly, increasing the chance of flash flooding in affected areas.  The risk of severe storms will again be hit or miss, limited to intense lightning and perhaps a few wet microbursts.

We are becomingly increasingly concerned about the period from late Friday night through Saturday, in terms of flash flooding potential. We will be in a very warm and humid air mass, with ample deep moisture available for efficient rain generation. A frontal boundary will be slowly encroaching on the area, with a stationary front along the Mason-Dixon Line, and an advancing cold front.  A wave of low pressure will pass southeast through the region, causing strong convergence and uplift of air.

Additionally, a dynamic feature called a “cutoff low” in the upper atmosphere will develop and track southward across the Mid-Atlantic on Saturday. This will further add to the sustained, widespread uplift of very moist air. The result of all these processes may be a 12-18 hour period of widespread, heavy rain generation, from several rounds of convective storms.  Given the orientation of the front, and airflow aloft, one or more regions may be primed for echo training (repeated thunderstorm passage over the same locations).

Once the front sinks to our south Saturday night and Sunday, the thunderstorm risk will diminish, the cutoff low will continue to generate moderate rain showers and much cooler air will move over the region. Highs Sunday may only reach around 70 degrees.