Cold Front and Downbursts in Oklahoma: 26 August 2009

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. Elevated Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values and Microburst Windspeed Potential Index (MWPI) values in the vicinity of downburst occurrence over western Oklahoma served as evidence of the presence of a well-developed mixed 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. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2200 UTC GOES microburst products.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product with overlying radar reflectivity from Oklahoma City NEXRAD (KTLX) at 0033 UTC 27 August (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 2200 UTC 26 August 2009 (bottom), with the location of Oklahoma mesonet observations (i.e BESS, WEAT, etc.) of downburst wind gusts plotted on the MWPI image. Both product images displayed convective storms developing along the cold front over western Oklahoma. Convective storm activity increased in coverage near the cold front during the following three hours. Downburst wind gusts between 41 and 56 knots were recorded by the Oklahoma Mesonet stations plotted in the MWPI image above between 0000 and 0100 UTC 27 August.
The following are significant downbursts recorded by the Oklahoma Mesonet during this event:
Station - Time (UTC) - Wind Gust (knots)
Bessie - 0005- 50
Kingfisher - 0020 - 43
Weatherford - 0030 - 41
El Reno - 0040 - 50
Medford- 0055 - 56

OO33 UTC NEXRAD reflectivity overlying the imager microburst product displayed downburst-producing convective storms in progress west of Oklahoma City in a region of elevated microburst risk (orange shading). Also important to note the general increase in MWPI values from southwest (BESS) to northeast (MEDF) associated with a progression from hybrid to stronger wet type downbursts. Downbursts over western Oklahoma were predominantly "hybrid" type, while over north-central Oklahoma (MEDF, BREC), downbursts were "wet" type associated with heavy rainfall.


The two RUC sounding profiles above at 2200 UTC at Weatherford (top) and Medford (bottom), respectively, show contrasting downburst environments over Oklahoma. The Weatherford sounding, an "inverted v" profile, indicates an overall deeper and drier mixed layer over western Oklahoma that favored the development of intense downdrafts due to the evaporation of precipitation in the sub-cloud layer. The Medford sounding, with a shallower, moister mixed layer and larger CAPE, indicated that precipitation loading was a significant factor in downdraft generation. Thus, this cold front downburst event demonstrates that favorable environments can vary over a relatively small geographic region.