Monday, October 17, 2011

October 13 2011: Mini Severe Weather Outbreak in the Mid Atlantic

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):
This was a rapidly developing severe weather situation, and unusual for its small geographic coverage.  The surface weather map (below) shows several classic elements came together to produce rotating, supercell thunderstorms.   Warm, humid air was being lifted along a warm front draped from west to east across Northern Virginia, assisted by air converging into a region of low pressure approaching from over West Virginia.   The small pocket of warm, humid air - a moderately unstable air mass - is shown by the shaded magenta region.  This pocket was located just along and to the south of the warm front.   Warm and humid air was being transported toward the warm front by low-level southeasterly flow (thin orange arrow).
The region was thus primed for the development of thunderstorms.  However, supercell thunderstorms, which contain a vertical core of rotating wind, acquire their rotation because of wind shear.  Wind shear is the change in wind speed and direction with increasing altitude.   Tornadoes develop when low-level winds both increase in speed, and veer - that is, turn clockwise - with height.  The large yellow arrow shows the wind direction at 18,000 ft, from the southwest, and stronger than at the surface.   A more detailed view of these veering and strengthening winds is provided by an instrument called a wind profiler, located at Beltsville, MD:
Warm fronts are common breeding grounds for tornadoes in the Mid Atlantic.  The winds are often east-southeasterly close to the frontal boundary, along the surface, but rapidly veer to southwesterly winds overhead. 

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:
video


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:
video

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