Computer model projection of radar imagery in the D.C. area at 2 p.m. Tuesday.

The likelihood of severe storms late this afternoon and evening has gone down, thanks to persistent cloud cover and a batch of showers set to arrive around midday. There’s still plenty of energy in the upper atmosphere to keep a low-end threat on the table, however.

As shown in the image below (taken at 10:30 a.m.), a significant batch of cloud cover with embedded showers and light rain is set to cross the D.C. region over the next several hours. This batch has developed out ahead of a ripple in the upper level flow.


Clouds and showers as depicted by infrared satellite. (Weathertap.com)

On Monday afternoon, the Storm Prediction Center elevated our severe threat to Enhanced (Level 3 out of 5). That is a rare category for this region, especially one day in advance. Now the persistent clouds and rain are expected to prevent the atmosphere from fully destabilizing under the strong sun, which was enough to warrant a reduction of that threat back to Slight Risk (Level 2 out of 5). Around noon, SPC downgraded the area further to just a “marginal risk" (Level 1 out of 5).

The concern for the most significant storminess has shifted south of the D.C. region, to Fredericksburg and Richmond, where the edge of the cloudiness will permit more hours of solar heating.


Severe storm risks across the region on Aug. 13. (NWS SPC)

With an approaching front late in the day and continued impulses in the upper-level airflow, there are still mechanisms in place for organizing a second round or more of showers and thunderstorms late in the afternoon into evening. For us, that Slight Risk translates into a 2 percent risk of a tornado, and 15 percent chance of damaging winds. In the zone to our south, the wind damage risk jumps to 30 percent.

Some of the high-resolution computer models suggest a breakup of cloud and either filtered or full sun from about 4 to 7 p.m. today. Temperatures will rise several degrees, but the atmosphere probably won’t be able to “rebound” to the point of triggering widespread, severe storms. Instead, some isolated strong to marginally severe cells may still be possible.

What to expect:

After the first batch of showers moves through around lunchtime, we anticipate a few scattered, heavy downpours with embedded lightning in spots, and a few strong gusts. The greatest threat for storminess in the immediate D.C. region will probably be across Northern Virginia, but the threat really ratchets up from Fredericksburg south. The best timing favors 6 to 9 p.m. in all of these areas.

What the newest models show

One of the model runs below (the high-resolution NAM) shows the bulk of heavy weather transiting well south of the D.C. region, during the evening (around 8 p.m.).


Computer model projection for radar imagery on Wednesday early evening. (StormVistaWx Models)

Another very high resolution model simulation at the same time, using the HRRR, is in concordance with the NAM; very few, if any, storms develop in and around D.C.

Capital Weather Gang will keep a keen eye on the evolution of afternoon trends and issue any updates as warranted.