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  • Severe thunderstorm watch until 8 p.m. for the entire region
  • Most likely storm timing: 3-6:30 p.m. for D.C. region (within two county radius of the District), 3:45-5:30 p.m. for immediate metro area (within one county radius of the District)
  • HazardsLikely – Heavy rain and lightning; possible – damaging winds, small hail; remote chance – tornado, large hail

6:00 p.m. update: Storms have mostly cleared out of the area. There is still some lingering moderate rain south of D.C. moving into Charles County, however, that should move out to the east soon.

Looking ahead, the threat for storms might not be totally over for the evening. Jeff Halverson writes:

The good news: The pocket of strong shear and shortwave currently over central Pennsylvania and extending to the Mason-Dixon Line continues to move northeast, and should exit by early evening, reducing the likelihood of severe storms across our region.

The bad news: The surface analysis shows a weak area of low pressure (a meso-low) forming across the D.C. metro, central Virginia and northern Virginia. This circulation, while weak, should continue to promote enhanced convergence and uplift of air for several hours. Combined with the unstable and very humid airmass (although CAPE has been reduced by this first round of storms), additional bands or clusters of thunderstorms are likely through the evening, a few of which could be strong and pose a local flash flood threat.

5:45 p.m. update: A new severe thunderstorm warning has been issued for northeast Prince Georges and northern Anne Arundel counties until 6:15 p.m. ET. These storms continue to move northeast out of the D.C. region at 20 mph. Further north, a tornado warning was issued for a storm in northern Baltimore County.

5:05 p.m. update:  The worst of the storm stretches from roughly College Park through just east of downtown south to around Mt. Vernon.  A tentacle then extends to the south along I-95 from Franconia to Dale City.  The storm is moving east at a good clip and will probably take just about 30 minutes to come through.  Next up? The eastern flank of the Beltway from Greenbelt to Suitland by 5:15 p.m. and then the Bowie to Upper Marlboro by 5:30-5:45 p.m.  Note: there are no warnings currently out for this storm but it produces heavy downpours and lots of dangerous lightning.  When thunder roars, go indoors!

4:50 p.m. update: Intense storm moving into the District and downtown areas right now.  Very gusty winds detected on doppler radar, should pass in 30 minutes or so. Storms should move east of the District into College Park to Suitland corridor by around 5:15 p.m.

4:40 p.m. update: Storms have reached the west edge of the Beltway with some front runners even developing inside the Beltway around Arlington and McLean and even into the District.  Storms should take roughly an hour to pass through any location.  The heaviest activity is currently from Oakton to Burke and is moving into the McLean to Springfield corridor.

Also , radar shows some storms in northern Montgomery County and Howard counties headed towards the north side of Baltimore.

4:30 p.m. update: Leading edge of storms has reached Oakton and Burke, headed into Vienna and Annandale next 15 minutes, and inside Beltway (McLean and Falls Church) just before 5 p.m. Warnings currently not in effect but these storms are big thunder and lightning producers and could produce locally strong winds.

4:20 p.m. update: Heavy storm over Centreville headed east along I-66 inside Beltway and into the District over the next 45-60 minutes – just in time for the PM Commute.  Downpours, frequent lightning and strong winds possible.

4:05 p.m. update:  Strongest storm right now is in the vicinity of Gainesville, Bull Run and Manassas and has its eyes on Centreville and Chantilly between now and 4:30 p.m.

3:50 p.m. update: Severe thunderstorm warning for northern Prince William, western Fairfax, southeast Loudoun, and central Fauquier counties until 4:30 p.m.  Storm capable of producing damaging winds  northeast of Warrenton moving northeast at 30 mph.  Warning includes Manassas, Centreville, Chantilly, Oakton, and Dulles Airport.

3:40 p.m. update: A severe thunderstorm warning for central Fauquier County until 4 p.m..  Storms currently moving into Warrenton area contain lightning, very heavy rain and strong winds.

3:00 p.m. update: A severe thunderstorm warning has been posted for northern Fauquier County until 3:30 p.m. for possible damaging winds (60 mph) and hail.

2:45 p.m. update: Strong storms have developed east of I-81 in northwest Virginia and will move into our western areas (Fauquier, Loudoun, and southern Frederick counties) over the next hour.  A cluster of storms is also focused in northern Frederick County, Md, near the Pa border headed into Carroll County. In the immediate metro area (within one county of the District), the most likely time for storms  is from 3:45 and 5:15 p.m. from west to east.

At 2:30 p.m., mesoanalysis (fine-scale analysis) of our region indicates (1) the most unstable air mass is over northern Virginia, the least unstable (but still potent) across north-central MD. This is consistent with a nearly 10 deg spread in temps between these two regions; (2) a pocket of strong wind shear (40-50 knots) is moving into the region north and west of D.C., along and north of the Mason-Dixon Line – associated with a shortwave in the flow aloft. This may prompt more intense and widespread storms across western MD, the WV panhandle, and south-central PA – including possible isolated supercells and bow echoes. Some of the storms in this zone already displaying some rotation, as evident in these pictures/radar views:

1:25 p.m. update: A severe thunderstorm watch has been issued for the D.C. metro region through 8 p.m. tonight. The greatest concern – beyond heavy rain and lightning –  is the potential for  damaging winds. In addition, there’s an outside chance of large hail (1 inch in diameter or greater) and an isolated tornado or two. Remember that a watch means conditions are favorable for severe storms, but not guaranteed – stay alert (yellow light). A warning, on the other hand, means a severe thunderstorm is imminent or occurring – take action (red light).

Area covered by severe thunderstorm watch (National Weather Service)
Area covered by severe thunderstorm watch (National Weather Service)

1:15 p.m. update: The National Weather Service Storm Prediction Center says it is likely to issue a severe thunderstorm watch this afternoon.  Excerpt from its latest discussion:



Original post, 12:53 p.m.: Today and tomorrow are expected to be quite active in terms of thunderstorms, with some severe storms likely. However, we do NOT expect a major severe weather outbreak on either day.

Storms are likely to develop and move through the area in the 2-6 p.m. window this afternoon.  More scattered showers and storms are possible in the evening and again tomorrow.  Storms are likely to produce heavy rain, lightning, and – possibly – strong to damaging wind gusts.

Technical discussion

Today’s thunderstorm setup

There has been a lot of media discussion of the “polar vortex” and we at CWG have discussed what this actually means. Indeed, a pronounced trough in the jet stream with unseasonably cool air is approaching the Mid-Atlantic. Anytime this occurs in mid-summer, there is always the potential for significant severe weather – given the strong winds aloft and dynamic ascent these troughs produce.

The experts at National Weather Service Storm Prediction Center (SPC) have been following this setup closely. Their outlook for today and tomorrow indicates a “slight” risk of severe thunderstorms, with a 15% probability of damaging winds (Figures 1 and 2). (Not shown for today’s outlook: 0% probability of large hail, and <2% chance of a tornado over the immediate DC Metro).  Often, if SPC believes there is the risk of a widespread severe thunderstorm outbreak, they would issue a “moderate” risk outlook…but – thus far – have not raised the threat assessment that high. A moderate risk would portend widespread, highly organized storms including multiple supercells and/or a derecho.

Figure 1. Severe storm wind outlook for Monday, July 14 and the morning of July 15, 2014. (NOAA, SPC)

Figure 2. Severe storm wind outlook for Tuesday, July 15 and the morning of July 16. (NOAA, SPC)

The air mass over the Mid-Atlantic this afternoon and evening will be plenty unstable, but the primary forcing – an approaching cold front – is a slow mover, and will still be located far to our north and west (Figure 3, surface forecast valid 8 p.m. Monday). Furthermore, the core of the strongest wind shear, which is required for highly organized and violent storms (40-50+ kts), will remain to our north and west (winds at 30,000 feet, Figure 4). Additionally, the best jet stream dynamics, including a jet streak (which induces air to rise in its entrance region) will remain over the Great Lakes (green shadings, Figure 5).

Figure 3. U.S. fronts and precipitation forecast for Monday evening. (NOAA, HPC)

Figure 4. Surface to 500mb wind shear. (twisterdata.com)

Figure 5. 300mb wind speed (jet stream) forecast for Monday evening. (unisysweather.com)

The amount of buoyant energy available for thunderstorms, called CAPE (convective available potential energy) is expected to become moderately large, around 2,500-3,000 joules (per kg of air). Deep layer shear, per this morning’s sounding, is weak (around 15 knots) but I expect this will increase to around 30 knots this afternoon. There are several local features that will initiate thunderstorms: a surface pressure trough/convergence zone over the Piedmont (dashed orange line in Figure 3), heating over elevated terrain to our west, and uplift along the outflow boundary left behind from last night’s round of strong storms.

This cool outflow was created by thunderstorm downdrafts, which knocked down the temperature by 10-15 °F, mainly across central Maryland. When I examined the morning temperature fields, the “ghost” of this outflow was still apparent; at 10:30 a.m., temperatures had risen to 86 °F south of D.C., while remaining around 78-80 °F north of the District. The upshot is that areas north of D.C. may destabilize more slowly, and to a lesser degree.

I expect widespread coverage of showers and thunderstorms, possibly coming in several batches. Storm cells will organize into multi-cell clusters and line segments, moving from southwest to northeast at about 20-25 mph. These storms will be strong – with frequent lightning, strong gusts (40-50 mph) and heavy downpours. Some of these line segments could become bow echos, like last night. Locally severe gusts (> 57 mph) are possible, due to wet microbursts. The NWS Sterling forecast office and SPC have highlighted a 2% tornado risk over northeastern Maryland, mainly for small/weak vortices spinning up in bowing segments.

Precipitable water is close to 2 inches area-wide (a sopping wet air mass), meaning localized flash flooding is possible, especially in the places that received heavy rain last night. Flash flood watches are already in effect north of the Mason-Dixon Line. I would not be surprised to see SPC issue a severe thunderstorm watch for our region mid-afternoon.

To round out today’s discussion, Figures 6 and 7 are snapshots from the high-resolution WRF-ARW model, showing the simulated thunderstorm coverage at 3 p.m. and 7 p.m., respectively. In summary, expect batches of strong to severe storm cells from mid-afternoon well into the evening.

Forecast radar from the WRF-ARW model for 3 p.m. ET on Monday. (NOAA)

Forecast radar from the WRF-ARW model for 7 p.m. Monday (NOAA)

Tuesday’s setup

Given today’s activity level, it’s probably best to take this one day at a time. But Figure 8 gives a glimpse at tomorrow’s expected surface setup (8 p.m. Tuesday). The frontal boundary slowly moves through, and a small wave of low pressure develops along it. Wind shear will be stronger aloft. Cloud cover, however, may mitigate some of the destabilization. Tomorrow’s severe potential may be all about timing – the location of the best frontal convergence relative to peak afternoon heating. For now, SPC has shifted the greatest probability of severe storms (15%) south and east of DC. But we should remain vigilant. I expect widespread thunderstorm activity tomorrow, with the possibility of strong to severe storms, and flash flooding – especially in areas that receive heavy rainfall today. Tomorrow we’ll post a detailed assessment similar to today.

U.S. fronts and precipitation forecast for the evening of Tuesday, July 15, 2014 (NOAA, HPC)

By Wednesday, the front will have cleared out of region, and northwesterly winds will begin to usher in a refreshingly cooler and drier air mass, with temps barely making 80 °F to round out the work week!