Limited storm threat today as cold front moves through

August 6

With a front moving through our region this afternoon, we anticipate that scattered showers and thunderstorms will develop through the day and persist into evening.   The greatest chance of any strong to severe storms should be to the south and east of the immediate D.C. region.  The most likely time window for storms is between 4 and 10 p.m. – although scattered showers are possible beforehand.

Today’s setup

The Storm Prediction Center (SPC) has placed the Mid-Atlantic in a very slight risk of severe thunderstorms for later today (5%, for damaging wind gusts;  Figure 1).    The morning analysis charts reveal a cold frontal boundary draped across the D.C. metro region (Figure 2).  This front will continue to slowly sag southward, as a piece of energy in the jet flow aloft (shortwave trough) approaches from Ohio late this afternoon and evening.


Figure 1. SPC probability of damaging wind for today.  (NWS)

Figure 2. Surface weather map valid 8 a.m. today.  (marylandwx.com)

Extensive cloud cover is apparent across northern Maryland, and this should prevail for the next several hours (Figure 3).   This will limit instability, particularly along the Mason-Dixon.  Further south, across central Virginia and especially southeast of the immediate D.C. region, solar heating is destabilizing the atmosphere.   The strong gradient of instability, from southeast to northwest, is shown in Figure 4.


Visible satellite image, valid 10 a.m. today.  (weathertap.com)
Figure 4. Surface-based CAPE, valid 10:30 a.m. today.  (NWS)
Figure 4. Surface-based CAPE, valid 10:30 a.m. today.  (NWS)

Wind shear (increase in wind speed with altitude) is modest and will increase through the day, as the upper level disturbance approaches (Figure 5), but the largest values should be just to our south.


Figure 5. Deep-layer wind shear forecast, valid 6 p.m. today.  (twisterdata.com)

The greatest likelihood of thunderstorms, including a few that are locally strong to severe, will be just south and east of the District.    This includes central Virginia and southern Maryland.   There, unstable air and wind shear will assist storm formation in air converging along and ahead of the front.

What the models show

The suite of high resolution models suggest widely scattered convection this afternoon and evening, with the best coverage and greatest intensity to the south and east of the District, during the evening.   Snapshots of these models, valid at various times throughout the evening (indicated in the caption), are shown below.     The evening max in activity is consistent with later arrival of the shortwave disturbance.


HRRR model radar simulation at 6 p.m. (WeatherBell.com)

Figure 7. WRF-ARW model run, valid 9 p.m. this evening.  (NWS)

High resolution NAM model radar simulation at 6 p.m. (WeatherBell.com)

Storms will range from isolated cells to multicell clusters and short line segments.  Any strong to severe storms mainly focused southeast of metro D.C.  may produce torrential downpours, lightning and isolated microbursts.

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Angela Fritz · August 6