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.

Oklahoma Panhandle Downbursts: 14 July 2009

During the afternoon of 14 July 2009, a cold front tracked through the Oklahoma Panhandle, bringing relief from a week-long heat wave. The cold front also triggered strong convective storms and resulting downbursts. The pre-convective environment downstream of the cold front over the panhandle was unstable with a well-mixed convective boundary layer and a steep temperature lapse rate below the 600mb level. 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 the Oklahoma Panhandle served as evidence of the presence of a convective mixed layer. Strong downbursts that were recorded by Boise City and Beaver, Oklahoma Mesonet stations at 2015 and 2315 UTC, respectively, resulted from sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 1800 and 1900 UTC GOES microburst products.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product at 1800 UTC (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 1900 UTC 14 July 2009 (bottom), with the location of Oklahoma mesonet observations (BOIS, BEAV) of downburst wind gusts plotted on the images. The GOES microburst products displayed convective clouds developing along a weak cold front over the western Oklahoma Panhandle that would soon trigger strong convective storms. It is apparent in the product images that elevated risk values are associated with the classic "inverted V" profile as displayed in the image below, a Rapid Update Cycle (RUC) model analysis sounding over Boise City, Oklahoma at 1800 UTC. Downburst wind gusts of 48 knots and 40 knots were observed at Boise City and Beaver at 2015 and 2315 UTC, respectively.

Elevated risk values indicated the presence of a dry subcloud mixed layer with a steep temperature lapse rate up to 600 mb that favored the development of intense downdrafts due to the evaporation of precipitation and resulting negative buoyancy. This favorable environment was most effectively shown in the above sounding profile over Boise City at 1800 UTC.

Cold Front and Downbursts in Oklahoma

During the afternoon of 30 June 2009, strong convective storms developed along a weak cold front as it was tracking southward 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 2100 UTC and 0000 UTC July 1 resulted from sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2000 UTC GOES microburst products.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 2000 UTC 30 June 2009 (bottom), with the location of Oklahoma mesonet observations (WEAT, CHEY, MANG) of downburst wind gusts plotted on the images. The GOES imager microburst product displayed convective clouds developing along a weak cold front over western Oklahoma that would soon trigger strong convective storms. Downburst wind gusts between 39 and 44 knots were recorded by the Oklahoma Mesonet stations plotted in the images above between 2100 UTC and 0000 UTC July 1.

The Oklahoma Mesonet meteogram above displays surface characteristics of the strongest downburst that occurred at Mangum at 2350 UTC. Classic downburst signatures are apparent in the meteogram including a sharp peak in wind speed (51 mph/44 knots), significant temperature decrease (27F), and a pressure jump of 4 millibars (mb).

The 2200 UTC GOES imager microburst product with overlying radar reflectivity imagery from Frederick, OK (KFDR) NEXRAD above displays a small, well-defined bow echo associated with downburst occurrence at Mangum. Note that this downburst occurred in a region of elevated output BTD (near 40K, orange shading), signifying the presence of a mature convective mixed layer (ML) as illustrated in the sounding profile below.


Elevated risk values indicated the presence of a dry subcloud layer with a steep temperature lapse rate up to 600 mb that favored the development of intense downdrafts due to the evaporation of precipitation and resulting negative buoyancy. This favorable environment was most effectively shown in the above sounding profile over Mangum at 2200 UTC.