7:30 p.m. update: Earlier storm activity over the metro area has passed to the east. However, one more broken line of showers and storms has formed west of the Beltway from near Leesburg to Warrenton that, if it holds together, will pass through the metro region over the next couple of hours. We don’t expect severe storms with this batch, but some brief downpours are possible.  Unless severe weather redevelops, this will be our last update. Follow our PM Update for the forecast through tomorrow and beyond.  6:40 p.m. update: A severe thunderstorm warning has been issued for southeast Fairfax and southern Prince George’s County until 7:15 p.m. This storm near Franconia has a history of producing hail and is headed towards Mt. Vernon, Ft. Washington and Clinton over the next half hour.  6:35 p.m. update: Scattered storms continue to track through the metro area, some with heavy rain and gusty winds. The strongest storm is located in south Springfield for which we have seen some reports of small hail. It’s tracking east towards Mt. Vernon over the next half hour.  Scattered showers and storms may continue into the mid-evening – very much hit or miss.  6:15 p.m. update: The earlier severe warning has ended but several strong storms remain across the area. Lots of outflow boundaries as well (blue arrows). That means some gusty winds are possible even away from the main storms. Places under the storms are seeing heavy rain and frequent lightning.  5:43 p.m. update: A severe thunderstorm warning has been issued for the northern Beltway area, including Potomac, Wheaton-Glenmont, Bethesda, and Silver Spring until 6:15 p.m. This storm is moving east at around 25 mph, and has the potential to produce quarter-size hail and damaging wind gusts.  5:25 p.m. update: Storm activity has increased quite a bit over the past hour and numerous storms have developed west of the Beltway in northern Virginia along I-66. While not severe, they’re tending stronger and are likely to produce heavy downpours and gusty winds. The storms are moving east and are on track to move inside the Beltway by around 6:30 p.m.  Another cluster of storms is developing near the Virginia and Maryland border and will track across southern Montgomery County along and just north of the Beltway over the next hour or so.  In general, expect hit or miss storms throughout the area through at least around 8 p.m.  3:40 p.m. update: Radar so far shows only an isolated storm or two developing to our west and short range models suggest widely scattered as opposed to widespread activity developing. We still think chances are good storms will increase some over the next hour or two even if a number of areas don’t see storms. By around 5 p.m., we think radar will be more active, especially from I-95 and to the east. On balance though, the scope and seriousness of the severe weather threat has diminished some. We’ll keep you posted.  Original post from 11:45 a.m.  An unstable air mass, combined with strong wind shear and a cold front portend a risk of severe thunderstorms containing damaging winds and large hail this afternoon and evening.  The most likely location for severe weather is to the immediate south and east of D.C., although we cannot rule out a threat in the District itself.  The setup for possible severe weather  The forecast late-afternoon and evening weather map shown below depicts an active scenario, with a cold front approaching the region. The front does not feature a strong temperature contrast, but a weak wave of low pressure may develop along this front, just to the south of the D.C. region. If so, this would help organize more widespread shower and thunderstorm activity.  At jet-stream level, a weak wave of low pressure is moving through, with strong winds aloft creating significant wind shear.  TheNational Weather Service Storm Prediction Center (SPC) has placed the immediate D.C. region, northeastern Virginia and Delmarva in a Slight Risk category for afternoon severe storms. This includes 5 percent probability of tornadoes and a 15 percent risk of large hail and damaging winds. But as you can judge from the figure below, the bulls-eye of this convective outlook lies to the south and east of the city. The reason for this is the earlier timing of the cold front’s passage through D.C., enabling the atmosphere to our south and east to “stew” longer under the hot sun and become more unstable.  The frontal boundary will help lift the unstable air mass. The SREFS (Short Range Ensemble Forecast System) prediction for instability (shown below) clusters around 2500-3000 J/kg of CAPE (Convective Available Potential Energy), which is a moderately intense value, given the expected strong solar heating and southerly winds this afternoon. This expected value combined with a cold front is a good recipe for at least strong storms.  SREFs plume showing forecast evolution of instability (CAPE) during the afternoon and overnight. (NWS)  But the wind shear (change in wind speed with altitude) is also a significant player, upping the severe ante, an impressive 40 knots (45 mph) as measured this morning by the Dulles weather balloon. As the figure below shows, the model forecast for winds aloft suggests these high levels of shear will be maintained, and may even strengthen a bit toward evening.  Predicted deep-layer shear (knots) valid at 6 p.m. today. (twisterdata.com)  Expected severity, storm coverage and timing  Based on the observations and trends discussed above, the potential exists for organized bands or clusters of multi-cell type storms, some of which may evolve into faster-moving, bowing segments. The combination of strong instability and shear portend the risk of significant hail in spots, and also damaging wind gusts. Shear greater than 40 knots especially favors bow echoes and we cannot rule out a transient supercell-type structure (thunderstorms containing a mesocyclone). Such a mesocyclone would be capable of producing an isolated, weak tornado or waterspout.  The most concentrated storm coverage is expected to be along and east of the I-95 corridor, starting mid-late afternoon and continuing through the evening. Again, this is predicated on the best destabilization out ahead of the cold front, with an axis or corridor of most unstable air setting up along the Delmarva.  A severe thunderstorm watch is anticipated later today for counties to the east of the I-95 corridor, possibly including the immediate corridor counties. Portions of counties further north and west i.e. Loudon, Frederick, Fauquier (and beyond) may escape the severe thunderstorm risk.  One caveat: Locations that experienced significant impact from strong to severe thunderstorms early this morning – namely, Loudon, Montgomery, Howard and Carroll Counties – may be at reduced risk for afternoon severe storms. This is because the boundary or pool of rain-chilled air left by the previous round of storms has stabilized the atmosphere a bit, enough to take the edge off late-day storm severity.  The HRRR and high resolution NAM model both favor areas along and especially east of I-95 for the heaviest and most concentrated storm activity.  The HRRR, shown below, initiates storm activity a bit earlier than the NAM, in the early afternoon, and the storms are generally less not as intense and slower to develop – until they move east and southeast of Washington.  The NAM, shown below, fires up the storms a bit later and the storms are more powerful in the immediate D.C. area:  Given the favorable ingredients for severe storms (discussed above), we think the high resolution NAM model may have a better handle on the overall situation – but it’s a close call as to whether areas along and especially west of I-95 witness significant thunderstorm activity this afternoon.
Most storms have passed through the region, but an isolated or shower storm could linger through 9 or 10 p.m.