17 June 2009 Dryline Downbursts

During the early afternoon of 17 June 2009, strong convective storms developed along the dryline over the Texas/New Mexico border and then propagated northeastward over the Texas and Oklahoma Panhandles. Ziegler and Hane (1993), in their observational study of a western Oklahoma dryline, found that the environment is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both flanks of the dryline. The dryline resembles a “mixing zone” that slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Elevated GOES imager microburst risk values (output brightness temperature difference (BTD) >40K) east of the dryline in the vicinity of downburst occurrence over western Texas serve as evidence of the presence of a well-developed mixed layer and a favorable environment for downbursts resulting from sub-cloud evaporation of precipitation.


Strong downbursts occurred over the Texas/Oklahoma Panhandle region between 2000 and 2100 UTC. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 1700 UTC imager microburst product. Locations of observed downbursts (FAS, HOOK, HES) are plotted on the web version of the GOES imager microburst product displayed above.




The images above are GOES imager microburst products generated at 1700 UTC 17 June with overlying radar reflectivity imagery near the time of downburst occurrence at Friona (top) and Hereford (bottom) West Texas Mesonet stations. Downburst wind gusts of 42 knots and 62 knots were recorded at Friona and Hereford, respectively, at 2010 and 2030 UTC. The radar reflectivity images show the high precipitation content associated with the convective storms that initiated strong downdrafts. It is also important to note that the stronger downburst observed at Hereford was associated with significantly higher output BTD, greater than 60K (red shading).

The 1700 UTC RUC analysis sounding profile over Hereford indicated the presence of a deep and dry convective mixed layer with a steep temperature lapse rate (red curve) below the 600 mb level. Large CAPE signified the favorability for the development of strong updrafts and heavy precipitation. The presence of a deep and dry subcloud layer fostered the development of intense downdrafts due to the evaporation of precipitation and resulting negative buoyancy. The combination of high reflectivity (>60 dBZ) associated with these storms and near dry-adiabatic subcloud lapse rates in the storm environment resulted in the classification of these downbursts as "hybrid" type.

Schafer (1986) has noted that the dryline plays a significant role in convective storm climatology in the Southern Plains region in which convective storm activity develops on 70 % of dryline days. The presence of the dryline had a major impact on convective storm coverage and downburst severity over western Texas during the afternoon of 17 June. Enhanced boundary layer mixing, expecially along the eastern periphery of the dryline, established favorable conditions for strong convective outflow winds. This conceptual model was verified by the occurrence of two strong downbursts over the Texas Panhandle during the late afternoon of 17 June.

References

Schaefer, J.T., 1986: The dryline. In Mesoscale Meteorology and Forecasting. P.S. Ray (Ed.), American Meteorological Society, Boston, 549-572.

Ziegler, C.L., and C.E. Hane, 1993: An observational study of the dryline. Mon. Wea. Rev., 121, 1134–1151.