Social media has been abuzz with the possibility of severe thunderstorms this Friday. We explain why there remains a low probability of this, with greatest likelihood of heavy rain showers accompanied by gusty – and possibly damaging – winds.

The Storm Prediction Center’s take

On Monday, NOAA’s Storm Prediction Center (SPC) portrayed a general risk of severe thunderstorms for the Mid Atlantic and Southeast U.S.. On Tuesday, SPC moved the threat region further east, placing the D.C. – Baltimore region on the knife edge of the outlook area.

Today, SPC continues this general outlook, with a 15% probability of severe storms along a line from D.C. – Baltimore and to the south and east, with up to 30% probability across southern Maryland and the Virginia Tidewater region. This is shown in the graphic below:

Figure 1. SPC severe thunderstorm threat for Friday, February 21. Note the very tight gradient in severe thunderstorm risk trending northwest to southeast across the D.C. metro region.
Figure 1. SPC severe thunderstorm threat for Friday, February 21. Note the very tight gradient in severe thunderstorm risk trending northwest to southeast across the D.C. metro region.

The gradient in severe threat across the metro region, with highest probability well to the south and east, reflects the timing of the frontal system generating severe weather. A strong cold front will sweep through our region Friday morning. Ahead of this front lies an unseasonably warm and humid air mass. The front will approach regions to our southeast during the afternoon, when surface heating is strongest, destabilizing the air mass.

The surface weather forecast chart for Friday morning is shown below. The map depicts a deep, occluding low pressure system passing through the Great Lakes, during its most intense phase. The D.C. region remains in the warm sector, ahead of the cold front.

Figure 2. Surface weather map for Friday morning depicting fronts and regions of weather. Green shaded area denotes heavy showers.

The band of heavy showers may drop anywhere from 0.5″ to 0.75″ of rain, in the space of just two to three hours. While the NAM implies this range, the mean of the SREF plumes suggests lower (0.4″) while the GFS comes in higher (1.0″).

We should all be aware of ponding water as this heavy rain interacts with piles of snow clogging street culverts and gutters. The normal path to sewers is impeded area-wide by significant snow. Additionally, heavy rain falling on thick snowpack will mobilize further runoff. Thus, we raise the issue of some transient, localized street flooding during the morning rush through mid- to late-morning.

What type of severe weather are we anticipating?

Strong wind shear (winds increasing speed with height) and a very unstable air mass are two factors that push ordinary thunderstorms into the severe category. On Friday, air mass instability is not expected to be significant, but the shear in low levels of the atmosphere will be sufficient to generate storms bearing intense, straight-line winds.

The concern is for a type of narrow, convective line termed a High Shear Low Cape (HSLC) system. These systems are fairly common during the winter in the southeast U.S., and less frequent in the Mid-Atlantic. CAPE (convective available potential energy) is a measure of thermal instability in the atmosphere. When instability is weak but the dynamic forcing and shear are strong, intense storms can still develop.

However, these storms tend to be shallow (cloud tops perhaps 25,000 to 30,000 feet) and produce only small amounts of lightning, if any. Thus, the term “severe thunderstorm” tends to be a bit of a misnomer when applied to this setting.

Figure 3. NAM simulated reflectivity shows a robust convective line containing small bowing segments 7 AM Friday morning. The bowing segments could produce locally damaging winds.

HSLC storms and the conditions that spawn them have been the subject of intense research among weather service offices in our region for a number of years, particularly the Research Triangle region of North Carolina. New parameters are being developed to better predict HSLC occurrence. HSLC storms can occasionally produce weak to moderate intensity tornadoes, particularly in the Carolinas, so they can pose a significant off-season severe weather threat.

For our region, if an HSLC does develop, it is likely to cross our region early in the morning, during the time when instability is weak. This is good news and should limit our regions’s severe potential. The concern remains that a few embedded cells could contain locally strong downdrafts, so isolated pockets of damaging wind are possible.

With strong winds in place just a few thousand feet above the surface, convective downdrafts can transport this horizontal momentum to the surface, producing blasts of high wind.

To recap: This is likely a low-end severe threat, which will occur in the interval from sunrise to about 2 p.m. The severe weather risk, about 15%, is mainly for isolated wind damage and brief, heavy rain, with any wind damage most likely in D.C.’s southern and eastern suburbs.

CWG will update the situation if we determine there are any changes in Friday’s severe thunderstorm threat.