Radar courtesy MyRadar | ？ OpenStreetMap contributors
* Severe thunderstorm watch until 9 p.m. *
4:30 p.m. — Storms approaching immediate D.C. area intensify, prompting severe thunderstorm warning
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An intense segment of storms has flared up just west of the Beltway and is poised to charge through the immediate D.C. area with very heavy rain, lightning and burst of potentially damaging winds. The storms stretch from Burke to Potomac and should reach downtown Washington a little after 5 p.m. and the east side of the Beltway between 5:15 and 5:30 p.m.
4:00 p.m. — Storms approaching Montgomery and Fairfax counties
Storms have progressed as far east as Leesburg and near the western Montgomery County border, where they are fairly intense. As they head east they’ll produce heavy rain, lightning and some bursts of strong to damaging winds as they near the Interstate 270 corridor in Montgomery County. These storms do have a history of producing some downed trees.
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To the south, the storms aren’t quite as widespread and intense, but bear watching as they continue eastward.
2:20 p.m. — Severe thunderstorm watch issued until 9 p.m.
The National Weather Service has issued a severe thunderstorm watch for the entire D.C. and Baltimore region until 9 p.m. Heat and humidity are fueling pop-up storms which have already started to erupt along and just east of Interstate 81.
Storms may impact areas within a two county radius of Washington and Baltimore between about 3:30 and 7:30 p.m. before they start to wind down.
Storms are likely to contain heavy downpours and frequent lightning; a few storms could also produce damaging wind gusts.
A severe thunderstorm watch means conditions are favorable for severe storms to form but they won’t necessarily occur everywhere. If a severe thunderstorm warning is issued for your particular location, it means severe weather is imminent and you should seek shelter.
Original article from 1:30 p.m.
Storms on Monday parked themselves on the north and east side of the Washington Beltway, unloading up to five inches of rain in two hours, sending streams rushing over their banks. On Tuesday, storms erupted all over the region, several letting out bursts of winds that toppled trees and damaging lightning bolts.
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Wednesday may mark a third consecutive evening of storminess. The National Weather Service has placed the region in a Level 2 out of 5 risk zone for severe storms and indicated that it may issue as severe thunderstorm watch.
The heat wave that began Sunday is fueling vigorous late-day storms. Until it relents late this weekend, the storms will keep flaring up. Like bubbles of water in a heated pan, they’ll be difficult to predict, popping up somewhat haphazardly. But every afternoon and evening through at least Saturday, it will be wise to monitor the weather and have a way to receive storm warnings and be able to shelter if you’re outside.
Heat index could soar to near 110 degrees Thursday in D.C.
Monday and Tuesday’s storms displayed why caution is warranted.
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Monday’s storms, while not as windy as Tuesday’s, were massive rain producers. The map below shows a wide area of two to five inches of rain in southern Montgomery County into northern Prince George’s County. Stream levels topped flood stage in Takoma Park and along the northeast branch of the Anacostia River north of Hyattsville. Near the Beltway’s intersection with Interstate 95, high water shut down three out of four lanes on the Outer Loop.
On Tuesday, scores of trees came down in eastern Fairfax County and the District as a severe thunderstorm generated intense downburst winds that radar estimated near 60 mph. Some trees fell on homes and blocked roads. Parts of Annandale, Falls Church and McLean were particularly hard-hit, and more than 50,000 customers lost power in Fairfax County. In Montgomery County, lightning bolts struck an apartment building and a home, setting them on fire, while a third bolt struck a tree that toppled and damaged a car. Lightning also started a fire at a home in the District.
D.C. region awakes Wednesday to power outages, cleanup after severe storms
What’s driving these storms?
The storms of the past two days have erupted despite a lack of a triggering cold front or any severe thunderstorm watches issued by the Weather Service.
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But storms have erupted amid a hot, humid and unstable weather pattern creating buoyant pockets of air that accelerate upward. The instability is courtesy of a zone of high pressure near Bermuda that circulates tropical air northward around its western limb.
The storms have formed despite limited wind shear, the increase in wind speed (and at times, direction) with altitude that help sustain storms by tilting their updrafts. The lack of shear is tied to rather weak winds in middle and upper levels of the atmosphere as the jet stream has migrated northward into Canada.
In the absence of much shear, some storm cells have still become severe but only briefly, before their updrafts have become snuffed out by heavy rain. We call these “pulse” storms, and they will continue to dominate the next few days.
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The setup for Monday’s storms
On Monday, numerous localized and short-lived cells erupted just north of D.C. in the late afternoon, across southern Montgomery and northern Prince George’s counties. These cells then clustered together into a ribbonlike corridor roughly parallel to the northern branch of the Beltway (radar image shown below).
The self-assembly of this cluster was facilitated when downdrafts from adjacent cells merged into a larger “cold pool.” Southerly winds scooped up unstable, moist air as those winds ramped up and over the cold pool. New cells were retriggered along the western side of the cold pool, then drifted to the east (pushed in that direction by weak, mid-level winds) — repeatedly. We call this “back-building” or “echo training.”
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The result was a several-hour episode of torrential rain and flash flooding, with localized amounts pushing five inches northeast of Silver Spring, as shown in the image above. The storm cells were not particularly tall, were weak lightning producers and did not generate much wind damage.
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The setup for Tuesday’s storms
On Tuesday, scattered storm cells erupted along the eastern peaks of the Appalachians and drifted over the I-95 corridor by late afternoon. Several of these became locally severe, and the weather hazards included pockets of damaging gusts and particularly intense lightning.
On this day, the approach of a weak wave in the upper-level flow (called a shortwave) along the Mason-Dixon Line helped trigger more widespread activity. But as with the previous day, there was no cold front sweeping through, nor a surface low-pressure system, to help kick off widespread, vigorous uplift of air.
One of these cells pulsed to severe levels over eastern Fairfax County, blasting the region with a downburst of damaging wind. The figure below shows a radar image of the storm cell (left panel) and its damaging outflow of wind (right panel).
Looking ahead
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For the remaining workweek, the region will continue to endure a hot, humid and unstable air mass. Wind shear will remain weak. This means more late-afternoon and evening occurrences of isolated, pulse-type storms.
It’s noteworthy that the forecast models portray only general regions of storm coverage, timing and predominant mode of organization (or lack thereof); they do not help us localize individual cells or small complexes, down to the specific hour. Hours in advance, we do not receive any useful guidance on which of those cells will become severe. This puts forecasters in more of a reactionary mode, waiting to see what pops on radar and how those storms will behave over the next 30 to 60 minutes.
On Saturday, there’s the possibility of more-organized storms, depending on the timing of a cold front moving through the area. If it arrives during the heat of the late afternoon or early evening, it could spawn a line of widespread storms that could be intense; more wind shear will be present compared with previous days. Models differ on the front’s timing.
By Sunday, the front will have probably pushed to our south but may still be close enough to continue the chance of storms in the southern part of our region.