17 June 2009 Dryline Downbursts

During the early afternoon of 17 June 2009, strong convective storms developed along the dryline over the Texas/New Mexico border and then propagated northeastward over the Texas and Oklahoma Panhandles. Ziegler and Hane (1993), in their observational study of a western Oklahoma dryline, found that the environment is dominated by vertical mixing that maintains a convective boundary layer (CBL) on both flanks of the dryline. The dryline resembles a “mixing zone” that slopes eastward from the surface dryline location, then becomes a quasi-horizontal elevated moist layer above the CBL east of the dryline. Elevated GOES imager microburst risk values (output brightness temperature difference (BTD) >40K) east of the dryline in the vicinity of downburst occurrence over western Texas serve as evidence of the presence of a well-developed mixed layer and a favorable environment for downbursts resulting from sub-cloud evaporation of precipitation.


Strong downbursts occurred over the Texas/Oklahoma Panhandle region between 2000 and 2100 UTC. These downbursts occurred in proximity to moderate to high microburst risk values as indicated in the 1700 UTC imager microburst product. Locations of observed downbursts (FAS, HOOK, HES) are plotted on the web version of the GOES imager microburst product displayed above.




The images above are GOES imager microburst products generated at 1700 UTC 17 June with overlying radar reflectivity imagery near the time of downburst occurrence at Friona (top) and Hereford (bottom) West Texas Mesonet stations. Downburst wind gusts of 42 knots and 62 knots were recorded at Friona and Hereford, respectively, at 2010 and 2030 UTC. The radar reflectivity images show the high precipitation content associated with the convective storms that initiated strong downdrafts. It is also important to note that the stronger downburst observed at Hereford was associated with significantly higher output BTD, greater than 60K (red shading).

The 1700 UTC RUC analysis sounding profile over Hereford indicated the presence of a deep and dry convective mixed layer with a steep temperature lapse rate (red curve) below the 600 mb level. Large CAPE signified the favorability for the development of strong updrafts and heavy precipitation. The presence of a deep and dry subcloud layer fostered the development of intense downdrafts due to the evaporation of precipitation and resulting negative buoyancy. The combination of high reflectivity (>60 dBZ) associated with these storms and near dry-adiabatic subcloud lapse rates in the storm environment resulted in the classification of these downbursts as "hybrid" type.

Schafer (1986) has noted that the dryline plays a significant role in convective storm climatology in the Southern Plains region in which convective storm activity develops on 70 % of dryline days. The presence of the dryline had a major impact on convective storm coverage and downburst severity over western Texas during the afternoon of 17 June. Enhanced boundary layer mixing, expecially along the eastern periphery of the dryline, established favorable conditions for strong convective outflow winds. This conceptual model was verified by the occurrence of two strong downbursts over the Texas Panhandle during the late afternoon of 17 June.

References

Schaefer, J.T., 1986: The dryline. In Mesoscale Meteorology and Forecasting. P.S. Ray (Ed.), American Meteorological Society, Boston, 549-572.

Ziegler, C.L., and C.E. Hane, 1993: An observational study of the dryline. Mon. Wea. Rev., 121, 1134–1151.

10 June 2009 High Plains Downbursts

During the afternoon of 10 June 2009, isolated convective storms developed over the High Plains of New Mexico, Oklahoma, and Texas in association with an upper-level disturbance. Downburst wind gusts between 38 and 53 knots were recorded over the Oklahoma and Texas Panhandles and eastern New Mexico during the two-hour period, 2155 and 2355 UTC. The Geostationary Operational Environmental Satellite (GOES) imager microburst product indicated elevated risk values (output brightness temperature difference (BTD) >40K) in proximity to the location of the downbursts one to three hours prior to each event. In general, the pre-convective environment over the southern High Plains was favorable for hybrid microbursts, characterized by a deep convective mixed layer with a steep temperature lapse rate, especially below the 600-mb level.

The images above are Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2000 UTC 10 June 2009 (top), with mesonet observations of downburst wind gusts plotted on the image, and a corresponding Rapid Update Cycle (RUC) model analysis sounding at Hereford, Texas. The GOES imager microburst product displayed scattered cumulus cloud (black shading) development in the unstable air mass over western Texas near the New Mexico border. Moderate to high microburst risk, indicated by yellow to orange shading, was in place over western Texas, near Hereford, where strong downbursts would occur about two hours later. The corresponding Rapid Update Cycle (RUC) model sounding profile over Hereford exhibited a classic “inverted-V” and indicated that the environment over the Panhandle region was favorable for hybrid microbursts with a steep temperature lapse rate and well-developed mixed layer below 600 mb. Near 2200 UTC, downburst wind gusts of 38 and 50 knots were recorded at Dimmitt and Hereford (West Texas) mesonet stations, respectively. Note that the stronger downburst recorded at Hereford was associated with higher output BTD near 50K (orange shading).

The images above are Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2200 UTC 10 June (top), with mesonet observations of downburst wind gusts plotted on the image, and a corresponding Rapid Update Cycle (RUC) model analysis sounding at Kenton, Oklahoma. The GOES imager microburst product displayed convective storm activity in progress over the Texas Panhandle and eastern New Mexico, including downburst-producing storms near Hereford and Dimmitt. The corresponding Rapid Update Cycle (RUC) model sounding profile over Kenton exhibited a more well-defined “inverted-V” profile. The product image and sounding profile signified that the environment over the Panhandle region was especially favorable for hybrid microbursts. Downburst wind gusts of 40 and 48 knots occurred between 2300 and 0000 UTC 11 June at Boise City and Kenton, Oklahoma mesonet stations, respectively, where a moderate to high risk of microbursts was indicated at 2200 UTC. In a similar manner to the 2000 UTC product image, this image indicated a local maximum in output BTD near the location of the downbursts that occurred over the western Oklahoma Panhandle.


The above images are GOES microburst products with overyling radar reflectivity imagery from Amarillo NEXRAD. These images, visualized by McIDAS-V software, display that downbursts were associated with multicell storms that occurred in regions of elevated microburst risk (orange shading). For this High Plains convective storm event, the GOES imager microburst product effectively indicated downburst potential with a correlation between output BTD and wind gust speed of .64 and a mean difference between output BTD and wind gust speed of 1.2.

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.