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Hazardous Heat Across the Western US into the Northern Plains; Heavy Rain and Flooding Possible in the South

Hazardous heat will persist over much of interior California, the central Great Basin, northern Rockies and Northern Plains into Thursday. Heavy to excessive rainfall will persist across the Southeast US. through mid-week. The Southwest Monsoon will continue to bring a flash flooding threat to the Four Corners Region this week. Read More >

...Squall Line RIps Area from Eastern WV to the Bay...

Sunday July 25, 2010



Early in the afternoon of Sunday July 25th, 2010, a squall line of thunderstorms formed over eastern West Virginia. With the heat of a near 100 degree afternoon to fuel it, and winds that strengthened to 70 mph higher up in the atmosphere where these clouds were billowing, that line intensified as it moved rapidly east. It would end up being a prolific wind storm for the area, with the northern suburbs and northern D.C. particularly hard hit. Winds gusted up to 90 mph in a few spots that were hardest hit, and there were several measured wind gusts between 60 and 70 mph in areas that received damage. With that destructive wind, hundreds of trees were felled along the entire path of the squall line, power was knocked out to hundreds of thousands, and many roads were blocked. Trees fell onto houses and vehicles. There were some reports of minor roof damage, as was the case in suburban Prince George County at 2 different apartment complexes. The falling trees also killed two: one boy in Loudoun County VA who was walking through a park with his family, and a woman in College Park MD whose van was stopped in its tracks when a tree fell on it. In addition, there was a lightning fatality reported in Rockville, MD when a man was struck outdoors while attending a community celebration. The storm also caused or contributed to the death of a man who was jet skiing on the Chesapeake Bay, a half mile south of the Bay Bridge when lightning struck nearby.


A squall line begins as individual thunderstorms that form along a boundary in the atmosphere. If winds are aligned in a certain way through the atmosphere, these storms will begin to organize and coalesce into a line of thunderstorms - a squall line. This typically happens several times during the course of a mid-Atlantic summer as cold fronts push though warm and humid summertime air.


So why was this one so strong and damaging? In this case, everything was pushed to the extreme. Temperatures rose to near 100 degrees (near record level for the day) with high humidity. That hot, humid air is the fuel for thunderstorms, and it was in abundance. That fuel was combined with winds in the atmosphere that steadily increased during the morning and into the afternoon. While winds were only gusting 10 to 20 mph at ground level, by early afternoon winds around 15,000 feet (the middle of the thunderstorms) had increased to 70 mph. Those strong winds higher up in the atmosphere were tapped in the turbulence of the thunderstorms, and thrown down to the surface in damaging wind gusts.



Below is a time lapse of squall line at 2:30pm, 3:00pm, 3:30pm, 4:00pm. Imagery is from the NWS Doppler Radar located here at the Baltimore/Washington forecast office in Sterling, VA.

radar reflectivityradar reflectivity

radar reflectivityradar reflectivity

While the standard radar pictures above showing the heavy rain does not look too far out of the ordinary for a summertime line of thunderstorms, the NWS radar can look into these storms and see the damaging winds that are associated with them. These next images are from the same four times as above, but instead show the winds within the storms. Arrows point to areas where the radar indicates winds higher than 50 mph, and in some spots more than 70 mph.

radar velocityradar velocity

radar velocityradar velocity



Reported Damage and Impacts

From those damaging winds, trees were downed along the squall line's path. Here is a map of damage reported to the National Weather Service. On this map, every 'W' is a report of wind damage. The blue areas are areas that were issued Severe Thunderstorm Warnings by the National Weather Service.

wind damage area map

This is a map of trees damage just in the District of Columbia alone (dots):

wind damage map for DC



In the main damage area shown in red here...

wind damage area map

...winds gusted 40 to 50 mph on a widespread basis. That was enough to tear leaves and small branches off trees over that entire area, and even knock down some weaker trees. Within that area however, there were spots that received winds of 50-70 mph which was enough to bring down trees and large branches.


Some of those spots that had a wind instrument to measure the wind speed:

63 mph; Stephens City, VA
58 mph; Front Royal, VA
58 mph; North Potomac, MD
64 mph; Laytonsville, MD
58 mph; Sterling Park, VA
62 mph; Gaithersburg, MD
75 mph; Reston, VA
68 mph; Chevy Chase, MD

66 mph; White Oak, MD
60 mph; Glenn Dale, MD
62 mph; Annapolis, MD
60 mph; Eastport, MD


And then there were a few isolated spots where this squall line threw down its most extreme wind gusts of 80-90mph. In those spots, large trees snapped, even power poles were pushed over or snapped, not from the force of a tree falling on them, but from wind force alone.



Danger Brewing (Outlook -> Watch -> Warning)

NWS meteorologists were monitoring the developing dangerous weather conditions during the late week, and by Friday morning began to include that threat in the Hazardous Weather Outlook with thunderstorms that "may produce strong gusty winds." That threat continued in the outlook through the weekend. As NWS meteorologists became even more confident on during the morning and midday hours on Sunday, a Severe Thunderstorm Watch was first issued for an areas including northern MD, northwestern VA, and northeast WV. That was followed by another Severe Thunderstorm Watch for an area just south of the initial one, including Washington D.C.

severe thunderstorm watchsevere thunderstorm watch


As the squall line moved rapidly across the region, Severe Thunderstorm Warnings were to be issued. A Severe Thunderstorm Warning was issued for the area around Winchester, Front Royal, Berryville, and Charles Town at 2:01pm.


That was followed by a warning for the outer northwest D.C. suburbs at 2:36pm.


And then one for Washington and central MD at 3:07pm.


(in the above maps, red 'pins' are reported damage locations)

Other warnings were issued as well. All in all that Sunday, eight Severe Thunderstorm Warnings were issued, 7 Special Marine Warnings for the Chesapeake Bay and its major estuaries, and one Tornado Warning for Howard and Montgomery Counties. As of the following Wednesday morning, NWS Storm Survey teams dispatched for that warning area could find no conclusive evidence of a tornado, only strong damaging winds associated with the squall line.

Always remember the NWS uses the watch/warning program to alert you to ANY weather threat! The watch/warning system is much like a traffic light. Green is OK. If there is a possible threat, the NWS will issue a watch saying there is the potential for that threat to happen, so KEEP A WATCH OUT (a yellow light, caution)! If we move up to a WARNING (a red light), that means we expect something to occur, so take action!

In this case, a Severe Thunderstorm Watch means we have the potential for severe thunderstorms, so be ready and KEEP A WATCH OUT! A Severe Thunderstorm Warning means we expect a severe, damaging thunderstorm to occur in a targeted area, within a short period of time (generally 30-60 minutes) TAKE ACTION AND SEEK SHELTER!


For future events, if you would like to better receive warnings from the National Weather Service, you can do any or all of the following:

* Purchase an inexpensive NOAA Weather Radio.

* If available, sign up for county text alerts from your county. Just internet search out your county's webpage, or your county's emergency management webpage for information.

* Many private weather companies offer public text/PDA alerting as well.

* Check the Hazardous Weather Outlook each day. That will alert you to any potential weather threats being tracked for the next 7 days.

* As always, keep tabs on your weather forecast at Click anywhere on that map for a forecast specific for the point you click.


Photos and Video

A few selected damage photos from the squall line:

damage photo

Above: Jefferson County WV power poles pushed over by wind. (Photo courtesy of Jefferson County Emergency Management)


damage photo

Above: Crane destroyed after being pushed over by wind at Montgomery County MD waste transfer station. (Photo courtesy of Montgomery County Emergency Management)


damage photo

Above: The reason for many power outages across the area. (Photo courtesy of Montgomery County Emergency Management)


damage photo

Large tree uprooted in Loudoun County VA (photo provided by NWS).

Also, here is a YouTube post of the storm from Maryland (the first few seconds show the worst for that spot)...