Cold Front and Downbursts in Oklahoma: 26 August 2009

During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward 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 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 2200 UTC GOES microburst products.


The images above are a Geostationary Operational Environmental Satellite (GOES) imager microburst product with overlying radar reflectivity from Oklahoma City NEXRAD (KTLX) at 0033 UTC 27 August (top) and a corresponding GOES sounder Microburst Windspeed Potential Index (MWPI) product at 2200 UTC 26 August 2009 (bottom), with the location of Oklahoma mesonet observations (i.e BESS, WEAT, etc.) of downburst wind gusts plotted on the MWPI image. Both product images displayed convective storms developing along the cold front over western Oklahoma. Convective storm activity increased in coverage near the cold front during the following three hours. Downburst wind gusts between 41 and 56 knots were recorded by the Oklahoma Mesonet stations plotted in the MWPI image above between 0000 and 0100 UTC 27 August.
The following are significant downbursts recorded by the Oklahoma Mesonet during this event:
Station - Time (UTC) - Wind Gust (knots)
Bessie - 0005- 50
Kingfisher - 0020 - 43
Weatherford - 0030 - 41
El Reno - 0040 - 50
Medford- 0055 - 56

OO33 UTC NEXRAD reflectivity overlying the imager microburst product displayed downburst-producing convective storms in progress west of Oklahoma City in a region of elevated microburst risk (orange shading). Also important to note the general increase in MWPI values from southwest (BESS) to northeast (MEDF) associated with a progression from hybrid to stronger wet type downbursts. Downbursts over western Oklahoma were predominantly "hybrid" type, while over north-central Oklahoma (MEDF, BREC), downbursts were "wet" type associated with heavy rainfall.


The two RUC sounding profiles above at 2200 UTC at Weatherford (top) and Medford (bottom), respectively, show contrasting downburst environments over Oklahoma. The Weatherford sounding, an "inverted v" profile, indicates an overall deeper and drier mixed layer over western Oklahoma that favored the development of intense downdrafts due to the evaporation of precipitation in the sub-cloud layer. The Medford sounding, with a shallower, moister mixed layer and larger CAPE, indicated that precipitation loading was a significant factor in downdraft generation. Thus, this cold front downburst event demonstrates that favorable environments can vary over a relatively small geographic region.

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.

12 August 2009 West Texas Downbursts

During the afternoon and evening of 12 August 2009, isolated convective storms developed over western Texas and produced severe downbursts in the Lubbock area. The GOES-11 imager microburst risk product indicated elevated values (brightness temperature difference, BTD) in proximity to the location of the downbursts. The first downburst wind gust (53 knots) was observed by Floydada (West Texas) mesonet station at 2135 UTC. Another severe downburst was observed at Reese Center (57 knots), west of Lubbock, at 0010 UTC 13 August. Both downbursts were produced by small multicell storms with high radar reflectivity, > 55 dBZ. Output BTD between 45 and 50K indicated convective wind gust potential of 45 to 50 knots, based on a previously derived statistical relationship. Afternoon microburst risk (1900 UTC) corresponded well with an "inverted V" RUC sounding profile that indicated the presence of a relatively deep and dry convective mixed layer with steep temperature lapse rates below 600 mb. Convective storms had a high precipitation content as indicated by radar reflectivity, signifying that loading was significant factor in downdraft intensity.


The images above are GOES-11 imager microburst products at 1900 UTC (top) and 2000 UTC 12 August 2009 (bottom), with overlying radar reflectivity imagery from Lubbock, Texas (KLBB) NEXRAD. Both images show high radar reflectivity (>50 dBZ) associated with the storms that are located in regions of elevated microburst risk (tan shading). The stronger downburst at Reese Center was associated with a rear-inflow notch (RIN) (Przybylinski 1995). The RIN indicates the channeling of dry air into the convective downdraft and subsequent precipitation evaporation, generation of negative buoyancy, and downdraft acceleration. Further negative buoyancy is generated when the precipitation descends below the cloud base into the mixed layer. The final result is a very strong downdraft that impacts the surface. The favorable environment for downbursts is best illustrated in the RUC sounding profile below.

Above is a RUC analysis sounding at 1900 UTC 12 August over Floydada. This sounding is a typical hybrid microburst sounding (Ellrod 1989): "inverted V" profile; large CAPE; deep, dry mixed layer (ML); and steep temperature lapse rate below the 600 mb level. Although sub-cloud evaporation of precipitation and subsequent negative buoyancy generation fueled strong convective downdrafts, heavy precipitation provided further downdraft forcing as a result of the contribution of the loading process.

References

Ellrod, G. P., 1989: Environmental conditions associated with the Dallas microburst storm determined from satellite soundings. Wea. Forecasting, 4, 469-484.

Przybylinski, R.W., 1995: The bow echo. Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.

3 August 2009 Downbursts and Heatburst

During the evening of 3 August 2009, convective storms developed along the dryline over the Oklahoma Panhandle and produced both dry and hybrid downbursts. The pre-convective environment in the vicinity of the locations of downburst occurrence was very unstable with a deep, well-mixed convective boundary layer and a steep temperature lapse rate below the 500mb level. High Geostationary Operational Environmental Satellite (GOES) imager brightness temperature difference (BTD) values in the vicinity of downburst occurrence over the panhandle served as evidence of the presence of a deep, sub-cloud dry adiabatic lapse rate layer. Strong downburst wind gusts that were recorded by Boise City, Oklahoma Mesonet station (41 knots) at 2340 UTC 3 August and by Hooker mesonet station (55 knots) at 0330 UTC 4 August 2009, respectively, resulted from sub-cloud evaporation of precipitation. Although these downbursts occurred in similar environments, the Boise City downburst was associated with a higher reflectivity storm (>50 dBZ) and could be classified as a "hybrid". The Hooker downburst was associated with a heat burst. A heat burst is defined as a "localized, sudden increase in surface temperature associated with a convective storm often accompanied by extreme drying"(courtesy AMS Glossary of Meteorology).

The images above are GOES-11 imager microburst products at 2200 UTC 3 August (top) and 0000 UTC 4 August 2009 (bottom), with overlying radar reflectivity imagery from Amarillo, Texas (KAMA) NEXRAD.The Boise City and Hooker downbursts occurred in proximity to high microburst risk values as indicated by the orange shading in the respective GOES-imager microburst products. Radar imagery shows that the Boise City downburst (top) was associated a classic spearhead echo signature, typically associated with wet microbursts. Interestingly, the stronger downburst observed at Hooker (bottom) was associated with a storm that appeared more innocuous on radar imagery with reflectivity below 30 dBZ. Also noteworthy was the occurrence of a heat burst at Hooker between 0300 and 0400 UTC. The Hooker meteogram below clearly shows this heat burst as a 10F temperature increase and 15F dewpoint decrease near the time of downburst occurrence.


This favorable environment for a heat burst was best illustrated in the above RUC analysis sounding at 0000 UTC 4 August over Hooker. This sounding is similar to the heat burst sounding identified in Conder et al. (2006): "inverted V" profile; deep, dry mixed layer (ML); and steep temperature lapse rate below the 500 mb level. Although sub-cloud evaporation of precipitation and resulting negative bouyancy fueled strong convective downdrafts, subsidence warming associated the the dry downburst at Hooker was primarily responsible for the anomalous heat burst.

References

Conder, M.R., S. R. Cobb, and G. Skwira, 2006: West Texas Mesonet observations of wake lows and heat bursts across northwest Texas. Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc.