Panhandle Downbursts: 18 August 2009

During the afternoon of 18 August 2009, two mesoscale convective systems developed over the western High Plains and merged during the evening over the Oklahoma/Texas Panhandle region. Embedded convective storms produced downbursts during and after the MCS merger between 2300 and 0200 UTC. The first two recorded downburst wind gusts (40 knots) over Cimarron County, Oklahoma, at Kenton and Boise City mesonet stations were "dry" type, associated with radar reflectivity between 20 and 30 dBZ. After the MCS merger, downbursts that occurred over western Texas were "hybrid" type with reflectivities greater than 40 dBZ. Between 2000 and 2300 UTC 18 August, the GOES-11 imager microburst product indicated elevated risk (yellow shading in product images), expressed as output brightness temperature difference (BTD), immediately downstream of the downburst producing convective storms.


The images above are a GOES-11 imager microburst product at 2000 UTC with overlying radar reflectivity imagery from Amarillo, Texas (KLBB) NEXRAD (top) and corresponding Rapid Update Cycle (RUC) model sounding profile over Kenton. The radar reflectivity image at 2258 UTC shows a cluster of strong, high-reflectivity convective storms over northeastern New Mexico merging to form an MCS. Interestingly, the downburst at Kenton, in progress at this time, was associated with much lower reflectivity. In accordance with the RUC sounding profile above that indicates the presence of a well-developed mixed layer and steep temperature lapse rate below 700mb level, the microburst product displayed elevated values (yellow shading) immediately downstream of the low reflectivity region of the MCS.

Above is a GOES-11 microburst risk image at 2100 UTC 18 August with overlying radar reflectivity at 0055 UTC 19 August. After 0000 UTC 19 August, the MCS evolved into a solid, high-reflectivity convective storm line as it propagated into the western Texas Panhandle. By 0055 UTC, a strong embedded storm produced a downburst wind gust of 38 knots at Hereford (West Texas) mesonet station. In a similar manner to the Kenton downburst, elevated (yellow shading) risk values were indicated immediately downstream of the storm line as shown in the above product image.


About one hour later, a stronger downburst occurred at Dimmitt, with a wind gust of 45 knots recorded at the mesonet station. A RUC sounding over Dimmitt (above) at 2300 UTC displayed a more prominet "inverted V" profile. The steep lapse rate below 700 mb and deeper, dry mixed layer signified that sub-cloud evaporation of precipitation and subsequent cooling and generation of negative buoyancy promoted stronger forcing of convective downdrafts. Also noteworthy in the sounding are a Lifted Index (LI) of -3 and K-index (KI) of 40, both parameters indicating the potential for widespread convective storm development. The above 2300 UTC GOES-11 microburst risk image above at 2300 UTC, with overlying reflectivity at 0155 UTC 19 August, shows the convective storm line over Dimmitt with a downburst in progress. Note higher risk values (orange shading), indicating wind gust potential of 45 to 50 knots, immediately downstream of the convective storm. Overall, the GOES-11 imager microburst risk product accurately indicated wind gust potential with a mean difference between output BTD and wind gust speed of 0.28.