6 p.m. update: We’re still seeing some strong storms pushing through the region – mainly east of I-95 in Charles, Prince George’s County and southeast D.C. Some general rains are falling in our northwest suburb with a break in the action elsewhere.
Some more showers and storms are still likely to cycle through the region this evening and could produce locally heavy rain. Short-term models suggest rain may start to decrease in coverage and intensity between around 11 p.m. and 2 a.m. Any severe weather (damaging winds and hail) should be isolated from here on out.
The forecast for July 4 looks terrific: sunny skies, low humidity and with highs in the low 80s. It might be a bit breezy at times, with a wind from the north at 15-20 mph, gusting up to 25 mph on the backside of Arthur. Fireworks viewing weather will be excellent, with temperatures in the 70s under clear skies with a bit of a breeze.
We’re going to take a break from frequent updates for the time being. If/when any high impact weather returns to the region or any warnings are issued, we’ll update as needed.
5:38 p.m. update: Here are a couple amazing sky views of the storms as they pushed through:
— midatlanticaerial (@midatlaerial) July 3, 2014
— Richard Barnhill (@wolfpackwx) July 3, 2014
5:30 p.m. update: While the immediate metro area has likely seen the worst of the storms, a severe storm is currently pushing through western Charles County, and has a warning until 6 p.m. This storm is headed into LaPlata and damaging winds are possible.
5:17 p.m. update: Twitter shows some structural damage to a high rise in College Park – this is suggestive of a micoburst.
— TerpWeather (@TerpWeather) July 3, 2014
5:15 p.m. update: Reports of downed trees from the storms earlier are feeding into us via Twitter and email:
— Joe Fox (@SilverSpringJoe) July 3, 2014
— Andy Null (@Andy_Null) July 3, 2014
— Jolleen Wagner (@AlbanyJ04) July 3, 2014
5:00 p.m. update: Some good news for D.C. (bad for Baltimore) is that the most intense storms with potentially damaging winds are moving out of the region north of Columbia. However, regional radar shows plenty of heavy rain and storms to the south to cycle through over the next few hours, which could cause some localized flash flooding. More gusty winds and lightning are possible in these additional rounds of showers/storms, but severe wind and hail should be isolated rather than widespread.
4:50 p.m. update: Radar shows storms with strongest winds now around Laurel, and heading towards Columbia in next 15 minutes. The storms remain severe.
4:45 p.m. update: Alex Liggitt of WJLA shows 57 mph gusts were logged by WeatherBug stations in Rockville and College Park as the storms push through.
4:40 p.m. update: Strongest winds in these severe storms pushing through are from Glenmont to Greenbelt, pushing towards Laurel.
4:30 p.m. update: Severe thunderstorm warning for the District, southeast Montgomery, Howard, northern Prince George’s and northwest Anne Arundel county until 5:30 p.m. NWS says quarter size hail and destructive 70 mph winds possible.
4:25 p.m. update: Doppler wind velocities show most severe winds in Bethesda and northwest D.C., targeting NE D.C., Silver Spring, Takoma Park area next 15-20 minutes.
4:20 p.m.: NWS update to severe t’storm warning for much of metro area says “destructive” winds to 70 mph in storms lined up along west side of Beltway sweeping through.
4:15 p.m. update: Doppler wind velocities show VERY strong wind potential right now in McLean and headed in general direction of Potomac and Bethesda. Please seek shelter in these areas.
4:10 p.m. update: Severe t’storm warning for southern Fairfax County, southern Prince William County, southeastern Fauquier County, and northern Stafford County until 5 p.m.
4:06 p.m. update: Reports of hail in Fairfax, Chantilly and Fair Lakes.
4:02 p.m. update: Doppler radar shows potential for very strong winds around Fairfax and Oakton, potentially moving towards McLean and Potomac up west side of Beltway. Take cover in these areas.
3:53 p.m. update: A severe thunderstorm warning has now been issued until 4:45 p.m. for a good chunk of the immediate metro region, including the District, southern Montgomery County, southeast Loudoun County, much of Fairfax County, Alexandria and Arlington. Storms coming through will produce very heavy rain, lightning, strong (possibly damaging) wind gusts, and perhaps some hail.
3:45 p.m. update: The storm in eastern Frederick County moving into western Carroll County has reportedly produced golfball size hail in Mt. Airy. It’s headed in the general direction of Westminster.
3:30 p.m. update: Severe storms are beginning to erupt across the region, with two severe thunderstorm warnings hoisted to our west: 1) A warning until 4:00 p.m. for southeast Loudoun, eastern Fauquier, western Fairfax and northwest Prince William counties. Manassas and Centreville are in the path of this storm which could produce damaging winds. 2) A warning until 4:15 p.m. for southeast Frederick, northern Montgomery, and northwest Howard counties. Mt. Airy and Eldersburg are in the path of this storm.
Original post from 1 p.m.: The atmosphere is primed today for another round of showers and thunderstorms, triggered by afternoon heating, the approach of a cold front and a potent wave in the upper atmosphere. Hurricane Arthur’s moisture enters the mix as well, creating a flash flood threat this afternoon and tonight.
A severe thunderstorm watch has been issued through 9 p.m., in addition to the flash flood watch in effect through late this evening. Scattered damaging wind gusts to 70 mph are likely, with isolated cases of large hail to 1 inch in diameter.
The most likely window for storms is between 4 and 10 p.m., but isolated or patchy activity could develop prior and linger beyond that window.
Figure 1 below shows the forecast surface chart for 8 p.m. this evening. This suggests two frontal boundaries will act as thunderstorm triggers – a stationary front and cold front. Dark green shading indicates heavy rain potential, as the plume of tropical moisture associated with Hurricane Arthur gets lifted along these fronts.
Our entire region is under a flash flood watch, valid through the afternoon, evening and overnight periods. Copious moisture is in place with precipitable water values of nearly 2 inches – high even for mid-summer D.C. region standards.
The 8 p.m. forecast for upper level flow is shown in Figure 2. Unlike yesterday, we anticipate upper-level dynamics will play a role in generating severe weather, including flash flooding. Note the highly amplified trough (U-shaped feature) in the westerlies, entering the Mid-Atlantic. This will promote large-scale lift of moist, unstable air, area-wide. There is also a pocket of fast winds – called a jet streak – shown by the green shaded region. When air enters the right rear quadrant of this streak, it spreads apart. This “difluent flow” draws up additional air from below, like air rising up a chimney – further enhancing rain and storm generation beneath it.
Based on these two charts alone, we anticipate an active afternoon and overnight period, with plenty of widespread convection. To assess severity of convection, we examine how unstable the air mass will get, and also the wind shear. Air mass instability determines updraft strength, and shear promotes long-lived, organized storm structures.
Figures 3 and 4 show forecast values of CAPE (convective available potential energy – a measure of instability) and wind shear (change in speed with height), based on ensemble forecasts. These charts are called “plume diagrams” and they allow us to assess the probability of different CAPE and shear values, for a number of varying model simulations. You can see a decent spread in forecast CAPE (between 12Z-00Z [8 a.m. – 8 p.m.] July 3, but the mean of all ensembles (solid black curve) points to 2,500-3,000 J/kg of buoyant energy. For shear, 25-30 kts is a safe bet.
What this means: The atmosphere will be plenty unstable for vigorous convection, and there is enough shear to organize storm cells into multicellular clusters and line segments. One concern today and tonight is the possibility of cell training – the repeated tracking of heavy rain cells over the same regions. Figures 1 and 2 show the setup for this: Deep, moist winds blowing from the south, parallel to frontal boundaries. Cells are expected to trigger near and along these boundaries, tracking from south to north in “corridors”.
The Storm Prediction Center (SPC) has much of the Mid-Atlantic region outlooked for 30% chance of severe thunderstorms – including damaging winds (30% probability), large hail (15% probability) and even an isolated tornado (2% probability).
What do the fine-scale (mesoscale models) predict for thunderstorm coverage? Shown below (Figures 5 and 6) are the WRF-ARW and HRRR runs, respectively. They portray much greater storm coverage than what occurred yesterday.
For completeness, I also show simulations from the high resolution NAM model. Figure 7 is simulated reflectivity at 8 p.m. tonight, Figure 8 for 5 a.m. Friday. Note the narrow, multiple corridors of convection active this evening. Even as Arthur makes his closest approach in the early morning, one or more convective bands is ongoing across the DC region.
The bottom line: Anticipate 1-2 inches of rain area-wide, coming in showery bursts, with coverage and intensity increasing through the afternoon into tonight. Localized, strong to severe thunderstorms are anticipated as well, and these may persist into evening. Localized rain amounts of 2-4” with flash flooding are possible. By 8 am Friday, the final rain bands push north and east and drier air arrives during the afternoon.