On Thursday, October 13, a small outbreak of tornadoes and severe thunderstorms developed during the afternoon hours. There were several sightings of funnel clouds and tornadoes across Northern Virginia. The image below shows reports of tornadoes received by the NOAA Storm Prediction Center (tornado locations are shown as small red triangles):
Additional, vigorous uplift of air was generated by an approaching trough in the jet stream. The trough took on a "negative tilt", which means that its axis (shown below by the dotted magenta arrow) is oriented from NW to SE, as opposed to N-S. When a trough becomes negatively tilted, the upward flow of air intensifies downstream of the trough (in the case, across the Mid Atlantic). This helps to invigorate thunderstorms erupting upward from the unstable air mass.
A radar loop of the heavy thunderstorms moving through the DC-Baltimore region is shown below:
But tornadoes were not the only severe weather story this day. Intense rains accompanied the strong thunderstorms. In some cases, repeated movement of cells over the same location, called "echo training", dumped 2"-3" in some locales. To generate flash flooding, the air mass must be very moist. The map of "total precipitable water" (a measure of the amount of water vapor in the air column) below indicates a plume of 1.5"-2.0" values south of the warm front. This very humid Atlantic air was streaming toward the warm front at low levels, where it became lifted into narrow corridors.
A radar loop showing these narrow corridors of heavy rain - the "rain train" - is shown below: