Oklahoma Downbursts: 12-13 May 2009

During the evening of both 12 and 13 May 2009, strong convective storms developed over western and central Oklahoma, respectively, and produced severe downbursts. The GOES imager microburst product was evaluated for this significant convective wind event. The vertical environmental temperature profile for the late afternoon was characterized as a "loaded gun" profile (Johns and Doswell 1992) for both downburst days. The imager microburst product was found to effectively indicate downburst potential for both events (especially the 12 May event), with severe downbursts occurring in proximity to local maxima in output brightness temperature difference (BTD) and corresponding microburst risk.




The images above are GOES microburst products derived from GOES-12 sounder image data at 2300 UTC 12 May 2009 (top) and 2200 UTC 13 May 2009 (bottom). Measured downburst wind gusts from Oklahoma Mesonet observing stations are plotted over the images. The 2300 UTC image displays wind gusts of 54 knots that occurred at Erick and Altus at 0155 UTC and 0235 UTC 13 May, respectively. The 2200 UTC image displays a wind gust of 48 knots that occurred at Red Rock 0100 UTC 14 May. Both images show local maxima in microburst risk in proximity to the locations of the observed downbursts. The RUC sounding profiles displayed below, generated at the valid times corresponding to the above microburst products and at locations of downburst occurrence on 12 May (top) and 13 May (bottom), underscore the favorable environment for severe convective storms that included a deep, mid-tropospheric unstable layer.





Both sounding profiles above are variants of the "loaded gun" sounding described in Johns and Doswell (1992). These profiles feature a deep, dry adiabatic layer based in the mid-troposphere overlying a shallow, capped moist boundary layer. However, the 13 May profile
over north-central Oklahoma near Red Rock mesonet station exhibited a slightly deeper and drier mixed layer. Interestingly, higher microburst risk and resulting convective wind gusts were associated with the 12 May sounding profile. This finding indicates the importance of the mid-tropospheric dry adiabatic layer (> 300mb deep) in forcing strong convective downdrafts, especially with the supercell storm that produced the severe downburst at Erick during the evening of 12 May. The weaker downburst (48 knot wind gust) that occurred at Red Rock during the evening of 13 May was produced by a multicell storm in an environment with a more shallow mid-tropospheric unstable layer. Thus, as apparent with this two-day convective storm outbreak, the mid-tropospheric thermodynamic structure had a greater influence on downburst strength than did the boundary layer structure.

REFERENCES

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