Reanalysis of 26 August Oklahoma Downburst Event

During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation as described in a previous entry. New RUC model graphical guidance effectively indicated the potential for strong downbursts near and west of Oklahoma City. Parameters calculated include 850-1000mb temperature lapse rate (LR), 850-1000mb relative humidity difference (dRH), precipitable water (PW), and surface dewpoint depression (DD).



Figure 1. RUC derived temperature lapse rate and radar reflectivity with overlying surface dewpoint depression (DD,top), precipitable water (PW, middle), and vertical humidity difference (dRH, bottom) at 2200 UTC 26 August.

The table below lists three strong downbursts that occurred over central and western Oklahoma between 0020 and 0040 UTC 27 August and associated RUC-derived microburst parameters from 2200 UTC 26 August:
Time (UTC)-Station-Wind Gust (kt)-Lapse Rate (K/km)-dRH (%)-DD (K)-PW (mm)
0020-Kingfisher (K)-43-8.2-9-13-40
0030-Weatherford (W)-41-8.6-13-18-35
0040-El Reno (E)-50-8.5-17-16-40

As displayed in Figure 1, and noted in the table above, the strongest downburst recorded at El Reno, overall, was associated with local maxima in all of the listed parameters. In general, stronger downbursts were associated with steeper sub-cloud temperature lapse rates, higher storm precipitable water content, and larger surface dewpoint depressions. This suggests that a combination of sub-cloud evaporational cooling in a more well-mixed boundary layer and precipitation loading was a factor in the generation of downdraft instability and resulting strong downbursts. These conditions were effectively indicated by RUC analysis-derived parameters over two hours prior to downburst occurrence.