Tornado Potential Near Washington D.C.

A line of thunderstorms from southern Pennsylvania to northern West Virginia moved southeast into Virginia and Maryland around the Washington, D.C. area. For much of the afternoon, the main threat was from damaging wind and hail.

As the line moved through northern Virginia, it began to interact and eventually merge with a couple of isolated severe thunderstorms that formed earlier in the afternoon ahead of the main line. A field of broken cumulus clouds in between the line and the isolated cells developed very rapidly into a line of severe thunderstorms that produced a number of reports of large hail and damaging straight-line winds.

As the northern end of the line approached the Washington, D.C. area, the southern end of the line surged southeastward into the area south of the isolated cells. The northern end of the line slowed as the cells merged with the northern end of the line. As a result, a very nice mesocyclone became very evident in the NEXRAD imagery as it passed near the Sterling radar site. Tornado warnings were issued, although no touchdowns were reported (a funnel cloud was observed at the Sterling office).

Of particular interest to me was the increase in lightning as the merging between the cells and the line took place, followed by a decrease in lightning as the mesocyclone formed. Also evident was the progression of overshooting cloud tops along the line as the mergers took place, with the overshooting tops traveling in an east-northeasterly direction toward the DC area while the line of storms moved steadily to the southeast. as the more robust thunderstorm activity moved southeast of the DC area, you could see how by using the NEXRAD data along with the satellite and lightning data the tornado threat might have been on the increase toward the DC area, while a more significant wind, hail, and heavy rain threat was favoring areas to the south.

Finally, the ProbTor product took that into account and while not overly high, showed an increase to about 30% as the storms moved toward Washington, D.C., then decreased very rapidly after that as the mesocyclone became rain-wrapped and fell behind the main line of thunderstorms as they headed south of the DC area.

Gary Cannalte