WEATHER NEWS: Storms with damaging winds possible in D.C. area late Thursday
The Storm Prediction Center (SPC) has placed much of the eastern U.S. in a slight risk zone for severe thunderstorms. This is Level 2 out of 5 on the severity threat scale.
The main threat with any storms will be damaging winds, although a short-lived tornado or two isn’t out of the question. It’s possible that the showers and storm pass through parts of the area without much fanfare. Although severe storms aren’t a sure thing, it would be wise to remain weather aware through late tonight.
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Interstate 81: Between 4 p.m. and 8 p.m.
Germantown/Dulles/Warrenton: 6 p.m. to 10 p.m.
Interstate 95 and Beltway area: 8 p.m. and midnight
Southern Maryland to Annapolis: 9 p.m. and 1 a.m.
Note that brief, gusty showers are possible before the mainline of showers and storms late this evening.
All clear: Midnight west of the Beltway, around 1 a.m. around the Beltway, and 2 a.m. near the Chesapeake Bay
High (2-in-3) chance of: Gusty winds (30 to 50 mph), brief downpours
Medium (1-in-3) chance of: Damaging winds (50-65 mph)
Small (1-in-10 or less) chance of: brief tornado, small hail, destructive winds (over 65 mph), lightning, flooding
Rainfall potential: Average 0.25 to 0.5 inches; locally amounts up to 1 inch or so possible.
A look at the day’s forecast map reveals that a warm front pushed through our region last night, ushering in wind from the south and a milder and humid air mass. This front is connected to a deepening low pressure system north of the Great Lakes. Our region will remain in this storm’s warm sector throughout the day and evening, as a cold front approaches from the Ohio Valley.
In the upper levels, a potent trough of low pressure is approaching the Eastern Seaboard. The uplift of air is expected to intensify across the D.C. area as the trough amplifies. With the influx of southern moisture and rising air, waves of showers will develop and transit our region through the afternoon and evening.
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Our concern shifts to the early evening, as the front nears and a pocket of unstable air develops just along it. The early morning weather balloon at Dulles Airport shows that the atmosphere — for the moment — is quite stable and not conducive to deep thunderstorms. However, with arrival of milder air from the south (and cooling of the mid levels by the approaching trough), some measure of instability is expected to develop, at least through the middle atmosphere, by the early evening.
Extensive cloud cover through the day will likely prevent the strong, late March sun from destabilizing the atmosphere to any large degree.
While the instability may be somewhat lacking through a deep layer, the low-level wind fields are exceptionally strong and ideally configured such that any deeper cells may achieve rotation. A look at these very strong winds about a mile above the surface is shown below; note that they are in the range of 70+ mph.
Even with shallower convective clouds (lacking significant lightning and thunder), we are concerned that downdrafts within the clouds may bring down blasts of strong wind in locally damaging gusts. The strong wind shear (change in speed and direction with altitude) may also promote transient, rotating storm cells with the possibility of a short-lived tornado or two.
The two images below present radar snapshots as simulated by various forecast models. In the first, the high resolution NAM model suggests a squall line, with embedded strong to severe cells, will pass through the area between 10 p.m. and 1 a.m.
The second simulation (the HRRR model) presents an earlier scenario, with a line of storms traversing our region between 8 p.m. and 11 p.m.
The Capital Weather Gang will stay on top of this scenario through the day and post updates if a severe thunderstorm watch is issued and any local warnings are generated.