20-21 July 2009 Western U.S. Downbursts

During the week of 20 to 24 July 2009, widespread convective storm activity occurred over the western United States. Severe downbursts occurred during the evening of 20 July over northwestern Oklahoma followed by strong downbursts that occurred over southern Arizona during the evening of 21 July. The pre-convective environment in the vicinity of the locations of downburst occurrence was unstable with a well-mixed convective boundary layer and a steep temperature lapse rate below the 600mb level. High Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values in the vicinity of downburst occurrence over northwestern Oklahoma and southern Arizona served as evidence of the presence of a convective mixed layer. Strong downbursts that were recorded by Alva, Oklahoma Mesonet station (61 knots) at 0405 UTC 21 July and Gila Bend, Arizona ALERT station (46 knots) at 0514 UTC 22 July 2009, respectively, resulted from sub-cloud evaporation of precipitation. These downbursts occurred in proximity to high microburst risk values as indicated in the 0200 UTC GOES-11 imager microburst product.

The images above are Geostationary Operational Environmental Satellite (GOES)-11 imager microburst products at 0200 UTC 21 July (top) and 0200 UTC 22 July 2009 (bottom), with overlying radar reflectivity imagery from Vance Air Force Base (KVNX) and Phoenix, Arizona (KIWA) NEXRAD, respectively. A downburst wind gust of 61 knots was observed at Alva, Oklahoma at 0405 UTC 21 July (top) and a wind gust of 46 knots was observed at Gila Bend, Arizona at 0514 UTC 22 July 2009 (bottom). These downbursts occurred in proximity to high microburst risk values as indicated by the orange shading in the respective 0200 UTC GOES microburst products. Radar imagery shows that these downbursts were produced high-reflectivity storms, thus establishing these downbursts as "hybrid" type.

High microburst risk values, corresponding to high GOES-11 output brightness temperature difference, indicated that these downbursts resulted from evaporational cooling as heavy rain descended into the dry sub-cloud layer. This favorable environment was best illustrated in the above RUC analysis soundings at 0200 UTC 21 and 22 July 2009 over Alva, OK and Gila Bend, AZ, respectively. Common for both soundings is the "inverted V" profile; deep, dry mixed layer; and steep temperature lapse rate below the 600 mb level.