South Texas Severe Weather Event on March 26, 2009


Introduction


Scattered thunderstorms affected portions of South Texas Thursday March 26th 2009 bringing only the second significant rainfall to the region since last fall. A few of the storms became severe and produced damaging winds, large hail, and frequent lightning. The worst damage was associated with a supercell thunderstorm that developed over central Live Oak County near George West. The supercell moved east-southeast across the extreme southern portion of Bee County into the northern portions of San Patricio County before weakening. This storm produced a 5 to 10 mile swath of damage mainly due to high winds. A couple of semi-trucks were turned over. Several power poles were snapped or leaned over along with highway signs being blown over. The winds were estimated to be 60 to 70 mph with gusts as high as 80 mph. The storm also produced large hail ranging in size from quarter to golf ball size.


Early Morning Elevated Thunderstorms over South-Central Texas


Surface pressure analysis and observations [Storm Prediction Center (SPC)] with warm front (red) and dryline (brown) shown

Figure 1 - Surface pressure analysis and observations [Storm Prediction Center (SPC)] with warm front (red) and dryline (brown) shown.

A frontal boundary sagged south into the South Texas Wednesday night and was lifting slowly north as a warm front early Thursday morning. The frontal boundary was located from north of Victoria to south of Cotulla at 7 AM CDT (Figure 1). South Texas was in the warm and humid sector south of the front and east of the dryline in northeast Mexico. The frontal boundary lifted slowly northward during the afternoon while the dryline moved to a Cotulla to Hebbronville line.

The scattered strong to severe thunderstorms formed north of the frontal boundary during the morning hours south of the Texas Hill Country. The thunderstorms were elevated in nature, above the shallow cool airmass north of the warm front. The activity on the western edge of the complex of storms northwest of Uvalde, during the mid-morning hours (Figure 2 shows the reflectivity image at 1522 UTC/1022 AM CDT from the Corpus Christi WSR-88D {KCRP}), would intensify by Noon CDT over southern Medina County south of Hondo. This storm complex would move southeast along the frontal boundary into Atascosa County to the south of Pleasanton into the northern portions of Live Oak and Bee Counties before decreasing in intensity over Goliad and Victoria Counties. Damaging winds and periods of hail from marble to golf ball size occurred with this complex of thunderstorms from south of Hondo to north of Beeville.


Corpus Christi, TX WSR-88D (KCRP) Reflectivity image at 1522 UTC March 26, 2009

Figure 2 - Corpus Christi, TX WSR-88D (KCRP) Reflectivity image at 1522 UTC March 26, 2009.


Ingredients for Severe Weather in South Texas


Showers and thunderstorms struggled to develop over South Texas during the morning hours. The moisture profile over South Texas was adequate according to the upper air observation from Corpus Christi at 12 UTC/7 AM CDT (Figure 3) with precipitable water value around 1 inch. But there was a significant capping inversion inhibiting thunderstorm formation, as temperature increased with height on the profile from 3 to 5 thousand feet (sharp bend of the red line on the sounding).


Sounding/Hodograph for Corpus Christi, TX 12 UTC March 26, 2009 (SPC)

Figure 3 - Sounding/Hodograph for Corpus Christi, TX 12 UTC March 26, 2009 (SPC)


However, the airmass over the region was potentially very unstable due to a strong decrease in temperature with height above the cap. Temperatures were expected to reach the upper 80s along with dewpoint temperatures in the upper 60s to near 70 degrees. Strong instability was expected with Surface Based Convective Available Potential Energy (SBCAPE) values reaching as high as 3500 J/kg. Another ingredient needed for strong rotating updrafts in supercell thunderstorms is wind shear, the change of wind speed and direction with height in the atmosphere. The wind shear profile in the lower atmosphere (below 10 thousand feet) was weak across South Texas Thursday morning. However, the wind speeds increased above 10 thousand feet providing wind shear in the surface to 6 km layer of nearly 60 knots. Generally, shear in the surface to 6 km layer above 40 knots is sufficient for supercell development.

500 MB Analysis (height/temperature) with short wave trough indicated by blue dashed line

Figure 4 - 500 MB Analysis (height/temperature) with short wave trough indicated by blue dashed line.

Moisture moving northward from South Texas along with an approaching upper level disturbance moving out of northeast Mexico (Figure 4) produced the area of strong to severe thunderstorms over south of the Texas Hill Country during the morning hours. The upper level disturbance would eventually move to the east-southeast into South Texas by the late afternoon hours. This would maintain a sufficient wind shear profile over the region during the afternoon for severe thunderstorms.


Supercell Thunderstorm from west of George West to north of Sinton


The outflow winds from the area of thunderstorms south of San Antonio moved into South Texas during the early afternoon hours. More thunderstorms developed near this outflow boundary from west of George West to near Freer. These thunderstorms were to the south of the warm front and formed in an environment where the storms were rooted in the boundary layer with very strong instability. The thunderstorm near George West quickly became severe. This severe thunderstorm, originating just south of Choke Canyon Reservoir, moved east-southeast across the northern portions of Lake Corpus Christi. The storm then moved through southern Bee County affecting the communities of Tynan and Skidmore. The storm moved into the extreme north central portion of San Patricio County north of Sinton before weakening. Figure 5 shows an approximate path of the damaging winds associated with this storm.


Map showing path of wind damage associated with severe storm March 26, 2009

Figure 5 - Map showing path of wind damage associated with severe storm March 26, 2009.


Reflectivity data indicated the mesocyclone, area of rotation within the supercell thunderstorm, was strong enough to produce a tornado as a "hook" shaped echo (Figure 6) was evident just to the north of Lake Corpus Christi at 1942 UTC (242 PM CDT).


KCRP WSR-88D radar reflectivity image for 1942 UTC 03/26/09

Figure 6 - KCRP WSR-88D radar reflectivity image for 1942 UTC 03/26/09.


Using the Four-dimensional Stormcell Investigator (FSI) from the Advanced Weather Information and Processing System (AWIPS), the structure of the supercell was evident. The lower left quadrant of FSI (Figure 7) shows a cross section of the storm using the line show in the upper left quadrant as the storm approached the community of Tynan at 1946 UTC (246 PM CDT). The cross section shows a bounded weak echo region with strong reflectivity held aloft by a very strong updraft. This higher reflectivity, greater than 50 dBZ, aloft is also shown in the upper right quadrant depicting reflectivity at a constant altitude of almost 28 thousand feet. Hail from marble to golf ball size was reported around 2 miles west of Tynan around 253 PM CDT along with very heavy rainfall. Rain amounts of 2 inches occurred in about 15 minutes that produced flooding in some fields and low lying areas.


Four-dimensional Stormcell Investigator image of supercell at 1946 UTC March 26, 2009

Figure 7 - Four-dimensional Stormcell Investigator image of supercell at 1946 UTC March 26, 2009.


However, strong outflow winds associated with this storm prevented the tornadic circulation from developing at the surface. The KCRP radar indicated a line of very strong winds on the velocity data. Figure 8 shows the velocity image from KCRP WSR-88D with the area in light green from near Tynan to east of Mathis depicting winds toward the radar in excess of 60 mph. Several power poles were blown down along State Highway 359 near Tynan due to these winds.


KCRP WSR-88D radar velocity image from 1955 UTC March 26, 2009 with damaging winds southeast of Tynan

Figure 8 - KCRP WSR-88D radar velocity image from 1955 UTC March 26, 2009 with damaging winds southeast of Tynan.


Radar and Satellite Data (Click images to zoom in)


Reflectivity Loop Velocity Loop
Visible Satellite Infrared Satellite Water Vapor Satellite Image

Surface and Upper Air Analysis Data (Click images to zoom in)


300 MB Upper Air Analysis 850 MB Upper Air Analysis 925 MB Upper Air Analysis
Storm Total Precipitation Local Analysis Prediction System Loop

Rapid Update Cycle Model Loop


13-km Rapid Update Cycle Loop of surface and upper level features

Damage Photos





Summary


Strong to severe thunderstorms formed north of a warm front boundary as an upper level disturbance moved across northern Mexico into South Texas during the morning hours of March 26th, 2009. The complex of severe storms moved south of San Antonio to north of Beeville sending another boundary of outflow winds into South Texas. A supercell developed near George West and moved east-southeast to north of Sinton. The storm produced widespread wind damage, marble to golf ball size hail, and very heavy rain. There were some reports of tornadoes but were likely gustnadoes along the outflow boundary.


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