The 29 June 2012 Derecho: A Microscale Study

During the afternoon of 29 June 2012, a complex of strong thunderstorms developed over Illinois and Indiana and then tracked
southeastward over the Ohio Valley and central Appalachian Mountains by evening. As the convective storm complex moved over and
east of the Appalachian Mountains at a forward speed of 45 to 50 knots, the leading storm line re-intensified and eventually produced
widespread significant severe winds (> 65 knots) over northern Virginia and the Washington, DC metropolitan area. In addition, as the
severe thunderstorm line tracked over the tidal Potomac River region south of Washington, DC, numerous severe downbursts were
generated. The passage of the thunderstorm gust front and downbursts embedded in the thunderstorm line were recorded by an
ultrasonic wind sensor on the (Intellicheck Mobilisa) Potomac River buoy located 30 miles south of Washington, DC, and WeatherFlow
observing stations in the Dahlgren, Virginia area. This convective system satisfied all criteria to be classified as a “derecho”, defined as
a family of downburst clusters produced by an extratropical mesoscale convective system (MCS).

The progressive pattern of severe winds and associated wind damage as marked in the map above is reflected in a five-hour animation
of composite Terminal Doppler Weather Radar (TDWR) and WV - IR brightness temperature difference (BTD) imagery between 2245 UTC
29 June and 0345 UTC 30 June:
Enhanced wind severity was likely associated with a spearhead echo that developed over northern Virginia near 0215 UTC and tracked
over the Potomac River during the following hour. The Geostationary Operational Environmental Satellite (GOES)-sounder Microburst
Windspeed Potential (MWP) algorithm, based on convective available potential energy (CAPE), and vertical temperature and humidity
lapse rates, and vertical difference in equivalent potential temperature (theta-e difference, TED) between the surface and middletroposphere
(between 10,000 and 20,000 feet above ground level) observed favorable conditions for severe thunderstorm-generated
winds over the tidal Potomac River region associated with the line echo wave pattern (LEWP).

Comparison of Geostationary Operational Environmental Satellite (GOES) sounder derived TED (magenta) and MWPI (white) parameters at
2256 UTC 29 June 2012 with overlying radar reflectivity image from Sterling, Virginia NEXRAD at 0252 UTC 30 June (top); Rapid Refresh
(RAP) model-dereived TED at 0300 UTC 30 June 2012 with overlying radar reflectivity image from Sterling, Virginia NEXRAD at 0301
UTC (bottom) marking the location of the NEXRAD-TED cross section (white line). The white triangle marks the location of the Potomac
River buoy near Dahlgren, Virginia.

The early evening (2256 UTC) GOES sounder Theta-e Difference (TED) and Microburst Windspeed Potential Index (MWPI) parameters
showed high values, greater than 40, south of the Washington, DC metropolitan area nearly four hours prior to the onset of the severe
thunderstorm line. Figure 1 compares theta-e difference (TED, magenta) and MWPI (white) values during the late afternoon where MWPI
and TeD over 40 indicated a high risk of severe downbursts, due to the presence of a prominent mid-tropospheric dry-air layer. Figure
1 also shows a RAP model-derived theta-e difference (TED) product image where red shading indicates values between 20 and 30 that
correspond to a high risk of downbursts. The overlying NEXRAD reflectivity displays a well-derfined spearhead echo that indicates a
severe downburst in progress as the leading convective storm line is moving east over the Potomac River near Dahlgren, Virginia. Near
this time, gust front winds of 50 to over 70 knots were recorded by the Potomac River Buoy and nearby Weatherflow observing stations,
followed by downburst wind gusts of 38 to 60 knots associated with the passage of the spearhead echo. Wind histograms from the
buoy and Baber Point Weatherflow station captured these two distinct wind events associated with the derecho. Note the highly
turbulent nature of the winds associated with the derecho as recorded with 10-second time resolution by the Potomac River Buoy.

Wind histograms from the Potomac River Buoy (top) and Baber Point Weatherflow station (bottom).

A theta-e-NEXRAD cross section animation, near the time of severe downburst occurrence, displayed a prominent dry-air notch on the
eastern (leading) flank of the convective storm and a foot-shaped signature near the surface that likely indicated a downburst in
progress:

The cross section effectively illustrates the process of downburst generation that includes evaporation of precipitation, generation of
negative buoyancy, and subsequent intense storm downdraft development.

Vertical cross section of theta-e and radar reflectivity from Sterling NEXRAD at 0300 UTC 30 June. Dark blue shading indicates the
presence of the driest (lowest theta-e) air interacting with the heavy rainfall of the leading convective storm line.

This extraordinary derecho event ultimately resulted in nearly a thousand severe wind reports from northern Illinois to the Atlantic
Coast. The combination of satellite, radar, and numerical model resources effectively described the evolution of this convective storm
system and can serve as an example of how to use this data in forecasting microscale severe wind events (i.e. downbursts) embedded
in larger-scale derechos.



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