A shelf clound advancing over Fairfax, Virginia, May 27, 2011. I also took a time lapse video of this shelf cloud. (Kevin Ambrose)

Late in the afternoon last Friday (one week ago), scattered thunderstorms developed west and north of Washington and moved north-northeast. By 6:00pm, a line of thunderstorms developed to the southwest of Washington and began racing northeastward at over 40 mph. One of the storms near Stafford was severe and a funnel cloud was observed. A tornado warning was issued in eastern Prince William County and southern Fairfax County.

As the storms moved north in Fairfax County and Arlington they weakened a bit. The arrival of the thunderstorm was proceeded by an ominous-looking shelf cloud that heralded the arrival of gusty winds which was followed several minutes later by heavy rain. I observed about a dozen lightning strikes from my location at Fairfax Town Center, but nothing that I would classify as severe weather. The shelf cloud did look cool as it pushed through the area.

Read below to see a time lapse video of this shelf cloud and to learn more about the shelf cloud.

Time lapse video from Fairfax Town Center showing the shelf cloud moving northward.

A shelf cloud is a low, wedge-shaped cloud that often proceeds the arrival of a thunderstorm. It is created by the thunderstorm’s downdraft which advances ahead of the parent thunderstorm. The cool air of the downdraft undercuts the warm air at the surface and condenses a cloud that looks like it is rolling, or plowing, ahead of the thunderstorm. The shelf cloud is accompanied by a gust front which can range from a cool breeze to a violent wind squall. In severe cases, short-lived vortices resembling tornadoes can touch the ground with the shelf cloud. These vortices are called gustnadoes.