1-2 June 2009 High Plains Downbursts

The early part of the first week of June 2009 was convectively active over the southern High Plains. During the afternoon of 1 June, clusters of multicell convective storms developed over southeastern Colorado and the Texas Panhandle and converged over the Oklahoma Panhandle by early evening. Embedded cells within the convective storm clusters produced strong downbursts that were observed by Friona (West Texas) and Hooker, Oklahoma mesonet stations. During the afternoon of 2 June, convective storms developed along a cold frontal boundary that was tracking southwestward across the Texas Panhandle. A variety of storm types, including single cell, multicell and supercell, developed along the boundary. As noted in previous events and observed during the afternoon of 2 June, the strongest downburst was produced by a supercell storm. Strong downbursts were recorded at Levelland, Reese Center, and Lake Alan Henry mesonet stations between 2100 and 2200 UTC. The GOES imager microburst risk product indicated elevated output brightness temperature differences (BTD) in proximity to the location of observed downbursts during the afternoon of both 1 and 2 June.


During the afternoon of 1 June, the first recorded downburst occurred at 2235 UTC at Friona, Texas with a wind gust of 38 knots. The 1900 UTC GOES imager microburst product above indicated high risk values (>50K, orange shading) in proximity to Friona. RUC sounding profiles over Friona (above) and Hooker (below) at 1900 and 2200 UTC, respectively, indicate that the pre-convective environment was favorable for hybrid microbursts with a steep temperature lapse rate and well-developed mixed layer below 600 mb. The inverted-v sounding profiles signified that downdraft instability was driven by sub-cloud evaporation of precipitation and resulting negative buoyancy. Overlying radar reflectivity imagery from Amarillo NEXRAD displayed clusters of multicell convective storms over the Texas Panhandle, with well-defined rear-inflow notch (RIN) associated with the cell over Friona at the time of downburst occurrence.


The stronger downburst at Hooker (42 knots) occurred nearly two hours later, at 0020 UTC 2 June. By this time, the environment was even more favorable for downbursts, with output BTD greater than 50K and a more well-defined inverted-v profile. Parent storm reflectivities for both downbursts were comparable (near 40 dBZ), suggesting that downbursts were driven by a combination of precipitation loading and negative buoyancy processes.

In contrast, the downbursts that occurred during the afternoon of 2 June over western Texas were produced by convective storms that evolved in environments more typical for wet microbursts as illustrated in the above 1900 UTC RUC sounding over Lubbock. The prototype wet microburst sounding profile featured large CAPE overlying a shallow, moist mixed layer (ML). The first two downbursts of the event, with associated wind gusts of 38 and 37 knots, respectively, occurred near Lubbock at Levelland and Reese Center mesonet stations at 2120 UTC. These downbursts occurred near the cold front boundary in a region identified as having high microburst risk (output BTD>50K, orange shading) in the 1900 UTC product image below. Large output BTD most likely reflected steep temperature lapse rates below the 500 mb level.


Overlying radar reflectivity imagery from Lubbock NEXRAD displayed single and multicell convective storms over Levelland and Reese Center, respectively, at the time of downburst occurrence. These storms contained reflectivities greater than 50 dBZ, consistent with wet microburst-producing storms. At 2145 UTC, a much stronger downburst, with a wind gust of 61 knots, occurred at Lake Alan Henry. The parent storm of this downburst can be classified as a supercell based on reflectivity imagery. Although this downburst occurred in a region with lower microburst risk (output BTD of 43K, yellow shading), forcing typically associated with supercells provided a much larger contribution to downdraft energy. Specifically, the mesocyclone within the supercell most likely added a downward directed dynamic pressure gradient force to the rear-flank downdraft (RFD) that contributed greatly to downburst severity.

The downbursts observed on 1 and 2 June 2009 over the High Plains demonstrated the varying favorable environments and convective storm types that occur during the summer. The 2 June event illustrated the tendency for supercell storms to produce severe downbursts even when the GOES imager microburst product indicates lower risk in the vicinity.