West Texas Downbursts: 13 August 2008

A downburst event over western Texas during the afternoon of 13 August 2008 served as another good example of the coordinated use of two predictive models: the Geostationary Operational Environmental Satellite (GOES)-sounder derived Microburst Windspeed Potential Index (MWPI) and GOES imager microburst algorithms, as compared to microburst risk algorithm output using 1 km MODIS data. During the afternoon of 13 August, a cluster of convective storms developed along the dryline over the Texas-New Mexico border. Several strong downbursts, produced by the convective storm cluster, occurred in close proximity to elevated GOES MWPI (Pryor 2008) and imager microburst risk (Pryor 2009) values. The imager microburst algorithm output, generated with MODIS data, indicated the strongest signal for downburst potential over western Texas. A downburst wind gust of 62 knots was recorded at Amherst West Texas Mesonet station while wind gusts of 43 to 48 knots were recorded by West Texas Mesonet stations in the Lubbock area. The ambient thermodynamic environment over western Texas was typical of the southern Great Plains with a deep, well-mixed boundary layer that favored evaporatively initiated downbursts. The MWPI algorithm incorporates relevant parameters for downburst potential, including convective available potential energy (CAPE) and boundary layer temperature and humidity gradients.

The images above are a GOES sounder-derived MWPI product and corresponding imager microburst risk product at 2000 UTC 13 August 2008. Apparent in the MWPI product image is a band of enhanced cumulus clouds and convective storm activity developing near the dryline over the Texas-New Mexico border that would evolve into a storm cluster over western Texas between 2100 and 2300 UTC. Based on a study by Weiss and Schultz (2006), a surface dewpoint difference of 1C was observed between Paducah, Texas and Dora, New Mexico (West Texas) mesonet stations at 2300 UTC, establishing the presence and location of the dryline. Associated with the convective storm cluster were downburst wind gusts of 38 to 62 knots (plotted in image) that were recorded by West Texas mesonet stations. Note that the downbursts occurred in close proximity to elevated MWPI values. The imager microburst risk product, with overlying radar reflectivity data from Lubbock, Texas NEXRAD (LBB) at 2215 UTC, displays a downburst-producing convective storm over Amherst, where a wind gust of 62 knots was observed. The convective storm cluster subsequently produced downburst wind gusts of 43 to 48 knots in the Lubbock area between 2325 and 2355 UTC. The Amherst downburst occurred in close proximity to high microburst risk (orange shading). High downburst risk was more apparent in the 2010 UTC MODIS image below as a darker orange shading immediately downstream of the convective storm over Amherst.

The GOES sounding above displayed an ambient thermodynamic environment typical of the southern Great Plains with a steep temperature lapse rate and a well-mixed boundary layer that favored the development of intense convective downdrafts as rain shafts descended into the subcloud layer. Comparison of the GOES sounder, imager and MODIS microburst products to the above sounding revealed that high risk values are associated with an "inverted V" profile (Johns and Doswell 1992). The dryline served as an initiating mechanism for convective storm activity and fostered a favorable environment for downbursts by enhancing vertical mixing (Ziegler and Hane 1993). Overall, the MWPI, imager, and MODIS microburst products effectively indicated the potential for strong convective wind gusts over western Texas during the afternoon of 13 August.

REFERENCES

Johns, R.H., and C.A. Doswell, 1992: Severe local storms forecasting. Mon. Wea. Rev., 121, 1134–1151.

Pryor, K.L., 2008: An initial assessment of the GOES Microburst Windspeed Potential Index. Preprints, 5th GOES Users' Conf., New Orleans, LA, Amer. Meteor. Soc.

Pryor, K.L., 2009: Microburst windspeed potential assessment: progress and developments. Preprints, 16th Conf. on Satellite Meteorology and Oceanography, Phoenix, AZ, Amer. Meteor. Soc.

Weiss, C.C., and D. M. Schultz, 2006: Synoptic and mesoscale influences on west Texas dryline development and associated convection. 23^rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc.

Ziegler, C.L., and C.E. Hane, 1993: An Observational Study of the Dryline. Mon. Wea. Rev., 121, 1134–1151.