May 6, 2001 Severe Thunderstorm

BELL COUNTY:

Sunday, May 6th began with the Storm Prediction Center outlook calling for a Slight Risk of severe storms in Central Texas. The risk category was upgraded to Moderate Risk at 11:30am CDT. The synoptic weather situation involved a surface low in the Texas and Oklahoma panhandles with the dryline southward near a Borger to Dryden line. The Del Rio sounding at 7am CDT reflected a build-up of low-level instability with CAPE in excess of 3000 J/kg at that hour. Forecasts from the morning ETA and RUC model runs indicated that CAPE would exceed 3500 J/kg and that the lifted index would be better than -8 across western portions of Central Texas by early afternoon with little or no convective inhibition. The same forecast models indicated a moderate westerly flow at mid and upper levels above a light to moderate surface flow from the south-southeast.

At 12 noon, I updated my forecast/outlook page with this information:

The airmass over our area northward into southern Oklahoma has become very unstable overnight as warm, moist air has gradually increased on a light but steady flow from the warm waters of the Gulf of Mexico. Overhead, the atmosphere has been gradually cooling as a series of upper air impulses rotating around the strong upper air low in the northern and central plains bring cooler air into the area from the west and northwest. The next impulse is currently over New Mexico, headed right toward Central Texas. The combination of strong low-level moisture and cooling and upward motion aloft will combine with a surface dryline moving in from western Texas to create an explosive mix over portions of Central and Northern Texas this afternoon and evening. Please do pay attention to the weather later today and tonight, and stay in touch with later watches and warnings from the NWS.

Right now, I would expect storms to initiate by early afternoon along a line from Wichita Falls to near Junction, then move mostly to the east. Any storm that develops today will have the potential to become severe in a short time, and may produce damaging winds and very large hail. Some tornadoes may also occur, particularly in the area north of a Brownwood to Corsicana line.

At 2:11pm CDT, the Storm Prediction Center issued a Tornado Watch that included Bell County:

THE STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF CENTRAL TEXAS EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 245 PM UNTIL 900 PM CDT (in an area) ALONG AND 110 STATUTE MILES EAST AND WEST OF A LINE FROM 50 MILES SOUTH OF JUNCTION TEXAS TO 55 MILES
NORTHWEST OF WACO TEXAS.
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DISCUSSION...WITH AIR MASS NOW VERY UNSTABLE...CAPES TO 4000 J/KG..AND WEAKLY CAPPED...SEVERE THUNDERSTORMS ARE EXPECTED TO DEVELOP RAPIDLY  WRN PORTION OF WW.  VEERING SHEAR PROFILES SUPPORT ISOLATED SUPERCELLS ENHANCING LARGE HAIL POTENTIAL ALONG WITH ISOLATED TORNADOS. SEVERE THUNDERSTORMS WILL PROPAGATE E/SE INTO THE EVENING HOURS.

Utilizing 1km visible imagery and NWS radar, the genesis of this storm can be traced back to at least as early as 11am CDT, fully six hours before the first tornado warning was issued! By early afternoon, visible satellite imagery depicted a growing cumulus field over the Texas Hill Country and Edwards Plateau, with good insolation underway. Two individual cumulus clusters stood out from the field of growing cumulus clouds. One of these was in northern Mason County, southeast of San Angelo.

High resolution visible image from GOES-8 at 11am CDT 5-06-2001.

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NWS NEXRAD reflectivity (0.5 deg elevation) at 12:08pm CDT on 5-06-2001.

This radar image was taken about 40 minutes before the visible satellite image (above), and shows a weak return over central Mason County.

NWS NEXRAD reflectivity (0.5 deg elevation) at 12:49pm CDT on 5-06-2001.

About 40 minutes later, the radar indicated that the cell had moved east-northeast to near the San Saba county line, and was intensifying, with core returns of ~45dbz indicated. Animation of the base reflectivity images from San Angelo (KSJT) (not shown) demonstrate that between 1pm and 3pm CDT, this cell continued to move to the east-northeast across southern San Saba county, then along the county line between Burnet and Lampasas counties. The cell briefly weakened during that time but was continuously identifiable.

A Severe Weather Statement issued about 4:20pm said spotters had reported hail 1-inch in diameter 7 miles west of Killeen. The storm continued intensifying and at 4:34pm CDT, the KGRK (Fort Hood-Central Texas) nexrad radar presented this (see above) image of the storm. Note the classic "flying eagle" configuration of the storm, with a weak echo region in the center (sometimes called a "V-notch"). This is believed to be caused by the displacement of hydrometeors (rain, hail, graupel) at mid and upper levels of the storm as the enviromental winds (from the west-southwest) are forced to go around (to the north and south) of the very intense updraft that is feeding the storm and extends to or near the tropopause. This radar presentation is sometimes considered a characteristic of the "classic" supercell type.

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Enlarged views of the 4:34pm CDT (2134UTC) base reflectivity and base velocity images from KGRK.

(The white symbol seen on the southwestern side of the storm is the location of the Robert Gray Army Airfield, which shares the KGRK identifier with the radar site, which islocated at the right lower corner of both images.)

Up until about 4:30pm, velocity data from both KGRK and the radar at KWTX (antenna located near Moody, about 25 miles from the storm) had indicated that rotation was present but was mostly elevated above the 0.5 degree tilt. Between 4:30pm and 5:00pm, the velocity couplet developed at lower levels (detected as low as 0.2 degrees by the KWTX radar), and at 5:00pm, the NWS in Fort Worth issued a Tornado Warning for Bell County, stating:  "At 500pm CDT National Weather Serrvice doppler radar detected a tornadic thunderstorm over Harker Heights moving east at 25mph." This information was updated at 5:14pm with a Special Weather Statement reporting "at 514 pm CTD a tornado was observed by  trained spotters in Harker Heights moving east at 15mph." (To my knowledge, this report was not been verified by ground-truth survey. In fact, other trained spotters in the immediate vicinity of this report did not see a tornado.)

NWS NEXRAD reflectivity (0.5 deg elevation) at 5:44pm CDT on 5-06-2001.

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Enlarged views of the 5:44pm CDT (2244UTC) base reflectivity and base velocity images from KGRK.

At this time, the storm is clearly transitioning from classic supercell to "high precipitation (HP)" type. Although the "flying eagle" is still somewhat visible, the storm is beginning to show signs of developing strong outflow. Note the development of a well-defined gustfront from near the southern edge of the storm southwestward. The storm is also in the process of "turning right" or to the southeast, after moving mostly eastward for several hours.

At 6:10pm, NWS Fort Worth updated with a Severe Weather Statement that said, in part: "A large dangerous tornadic thunderstorm was centered near Salado moving southeast at 15mph." The corresponding radar data (from 6:pm) follows:

Note that the velocity data suggests that the outflow has raced southeastward ahead of the precipitation area, with inbound flow very close to the cone of silence at the radar site while the precipitation remains perhaps 5-10 miles to the northwest and the only remaining velocity couplet is on the east-northeast side of the storm. At about this point in time, the storm was producing damaging straight line winds over southeastern Bell County.

Persistent strong outflow continued to race ahead of the storms it moved southeastward, and the tornado threat ended as NWS Fort Worth issued a Severe Thunderstorm Warning at 6:30pm for Bell and Milam counties. The corresponding radar image appears below:

Coming soon!

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