End of Summer Downbursts in Oklahoma and Validation Results

On the last full day of the Summer season, 21 September 2009, strong convective storms developed along a cold front as it was tracking eastward over Oklahoma. In general, the pre-convective environment downstream of the cold front over eastern Oklahoma was moist and very unstable with a shallow, well-mixed boundary layer. These conditions favored convective storms that produced strong outflow winds (downbursts) associated with heavy rain. Elevated Geostationary Operational Environmental Satellite (GOES) microburst index values, indicated in the 1800 UTC product images in the vicinity of downburst occurrence over eastern Oklahoma, served as evidence of the presence of this favorable environment. Strong downbursts that were recorded by Oklahoma Mesonet stations between 2100 and 0000 UTC 22 September resulted from a combination of heavy precipitation and sub-cloud evaporation of precipitation.

Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 1800 UTC 21 September 2009, with the location of Oklahoma mesonet observations of downburst wind gusts plotted on the image.

Figure 2. Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product at 1800 UTC 21 September 2009, with the location of Oklahoma mesonet observations of downburst wind gusts plotted on the image.

Figures 1 and 2 display elevated microburst risk (yellow color) ahead of a cold front tracking eastward over Oklahoma. The cold front served as a trigger for convective storms over eastern Oklahoma during the afternoon and evening of 21 September 2009. Elevated Geostationary Operational Environmental Satellite (GOES) microburst index values displayed in the 1800 UTC product images in the vicinity of downburst occurrence over eastern Oklahoma, indicated wind gust potential of 35 to 50 knots. Downbursts wind gusts between 39 and 50 knots were recorded by the Oklahoma Mesonet stations plotted in Figures 1 and 2 between 2040 and 0000 UTC 22 September.

Validation results for the 2007 to 2009 convective seasons have been completed for the MWPI and imager microburst products. GOES sounder-derived MWPI values have been compared to mesonet observations of downburst winds over Oklahoma and Texas for 168 events between June 2007 and September 2009. The correlation between MWPI values and measured wind gusts was determined to be .62 and was found to be statistically significant above the 99% confidence level, indicating that the correlation represents a physical relationship between MWPI values and downburst magnitude and is not an artifact of the sampling process. Comparison of GOES-11 imager microburst risk values (output brightness temperature difference (BTD) in degrees K) to measured downburst wind gusts for 61 events in Oklahoma between June and September 2009 yielded a correlation of .40. This correlation was higher than the correlation computed between MWPI values and downburst wind gusts (.27) for the same time period. The correlation between output BTD and measured wind gusts was determined to be statistically significant at the 82% confidence level, indicating a high confidence that the correlation represented a physical relationship between output BTD values and downburst magnitude.

Figure 3. Scatterplot of Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma and Texas between June 2007 and September 2009.

Figure 4. Scatterplot of Geostationary Operational Environmental Satellite (GOES) -11 imager output BTD values (K) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma between June and September 2009.

Figures 3 and 4 are scatterplots of MWPI values and GOES-11 imager output BTD values (K) versus observed downburst wind gust speed as recorded by mesonet stations in Oklahoma and Texas. The MWPI scatterplot identifies two clusters of values: MWPI values less than 50 that correspond to observed wind gusts of 35 to 50 knots, and MWPI values greater than 50 that correspond to observed wind gusts of greater than 50 knots. Similarly, the GOES-11 imager microburst risk scatterplot identifies two clusters. The dominant cluster contains output brightness temperature difference (BTD) values less than 50K that correspond to observed wind gusts between 35 and 50 knots. The scatterplots illustrate that both microburst products demonstrate effectiveness in distinguishing between severe and non-severe convective wind gust potential.