March 26, 2000 Left-Moving Supercell
BELL AND FALLS COUNTIES
I had been on vacation for ten days, returning on Saturday
morning in time for the Bell County Skywarn storm spotter training session.
The weather picture was characterized by very warm, moist air over much
of the eastern half of Texas with the dryline oscillating across west central
Texas and a strong jet stream flow overhead. Non-severe thunderstorms were
in progress Saturday morning before sunrise in the Wichita Falls-Abilene
area as the jet I was on flew into DFW airport from the west. The Storm
Prediction Center outlooks placed a slight risk over parts of Central Texas
for Saturday afternoon, but no severe storms developed in our area until
the evening hours, when thunderstorms moved southeastward out of the Red
River valley area and brushed the northeastern counties (Navarro, Limestone
and Freestone).
By Sunday morning, the SPC outlook was for a risk of severe storms over Central Texas by evening. Most of Sunday was a gorgeous day in Central Texas, with sunny skies and warm south winds. My 3pm CST surface analysis depicted a broad trough of low pressure from a storm center on the Iowa-Kansas border north of Kansas City southwestward to near Del Rio. Several mesoscale low pressure centers could be inferred from the surface data, with one center near Oklahoma City and another near Childress. A cold front extended from the main low center to near a Wichita, KS - Amarillo - Tucumcari, NM line. The dryline was not particularly well-defined but was generally along a line from east of Childress to east of Abilene to east of San Angelo to west of Del Rio. Three-hour pressure falls were most pronounced in the zone from San Angelo to Waco, where falls >5mb were observed. Here's my 3pm CST surface analysis:
Surface analysis at 3pm CST on March 26, 2000.
There was weak moisture convergence in a broad zone ahead of the dryline, with a localized maxima near Llano. Click here to view the 4pm CST moisture convergence analysis from CAPS.Because of the warm, moist air mass and strong insolation between noon and 3pm, CAPE values were approaching 3000 j/kg over a broad area from San Antonio to Dallas-Fort Worth. Click here to view the 4pm CST CAPE analysis from CAPS. About 4pm, I was at the Killeen airport, and the air had that "explosive" feel I've come to recognize as being a precursor to rapid development of intense storms. Although only scattered cumulus clouds dotted the sky, I noticed that they leaned strongly eastward as they developed upward into the atmosphere. This confirmed that there was strong vertical shear, with the winds changing directions from south to west and increasing in velocity with altitude. This observation is confirmed by the CAPS upper wind analysis (250mb, approximately 34,000 feet above ground) for 4pm, which can be found by clicking here. This map not only shows that very strong winds were blowing over our area, but that the winds were out of the west-northwest. The impact of this flow should have taken developing storms to the southeast.
The first convection appeared on NWS nexrad radar around 4pm CST. The 4:01pm CST composite reflectivity image () shows the initial storm has developed in Mason County. By 4:50pm, additional storms were developing over the eastern Texas Hill Country, including a storm in central Burnet County:
4:51pm CST image from NWS nexRAD at Fort Worth.
Between 4:50pm and 5:31pm, the Burnet County storm split. It is not unusual for storms to split on days when there is strong vertical wind shear. What is somewhat uncommon is for the left-moving storm (the storm which deviates to the left of the mean wind vector) to become the dominant storm. Nevertheless, that is precisely what occurred in this case. At 5:31pm, the Fort Worth radar was showing a very strong left-moving storm in northeastern Burnet and northwestern Williamson counties. Although it is hard to see at the resolution shown here, the storm already had a 65dBz core, indicating the likelihood of large hail. The NWS's Austin-San Antonio office shortly issued a Severe Thunderstorm warning for Williamson County. Here's the image::
5:31pm CST image from NWS nexRAD at Fort Worth.
By 5:50pm CST, the storm was moving northeastward into southwestern Bell County. The NWS office at Fort Worth issued a Severe Thunderstorm Warning for Bell County. The 65dBz core became more prominent as it expanded and as the storm moved a little closer to the radar at Fort Worth. Here's the image:
5:50pm CST image from NWS nexRAD at Fort Worth.
Note the pronounced weakening of the right-moving portion of the original cell (southwestern Williamson County). Of passing interest is the radar thin-line which appears northeast to southwest between Mineral Wells and Fort Worth.
A short time later, the 6pm CST composite reflectivity image from Fort Worth indicated additional storms developing south of the Bell County storm along the Balcones Escarpment toward New Braunfels. Here's a link to that image. By 6:30pm CST, the left-moving storm had crossed much of southern Bell County, producing large hail and very strong winds, which caused widespread damage in the communities of Salado, Heidenheimer, and Little River-Academy. Note that the core of the storm is now showing 70dBz reflectivity. Here's that image:
6:30pm CST image from NWS nexRAD at Fort Worth.
The storm continued rapidly east-northeastward at near 40mph, and by 6:50pm CST was in southwestern Falls County near Rosebud. Here's the 6:50pm CST image, still showing a small core where the reflectivity is 70dBz or higher:
6:50pm CST image from NWS nexRAD at Fort Worth.
The 7:00pm CST composite reflectivity image from Fort Worth shows the continued evolution of the severe storms between Austin and San Antonio. The storm near New Braunfels would continue southeastward to near Seguin and produce a reported tornado there before 8pm CST. Click here to see the 7pm CST composite reflectivity image.
By 7:29pm, the storm was moving over northeastern Robertson County east of Bremond. Bremond reported quarter-size hail covering the ground and very strong winds. The 7:29pm CST Fort Worth nexrad image follows:
7:29pm CST image from NWS nexRAD at Fort Worth.
The last radar image shows the storm weakening as it crosses Leon County about 7:50pm CST. Here's that image:
7:49pm CST image from NWS nexRAD at Fort Worth.
This storm traveled more than 120 miles over a period of 3 hours or so, averaging close to 40 mph. The mean wind favored a motion to the east or east-southeast, but supercell storms often deviate from the mean wind. The rapid motion and left deviation of this storm suggested that storms moving to the right of the mean wind would be relatively slow-movers. That is exactly what happened later in the evening as supercell storms pummeled the San Marcos, New Braunfels and San Antonio areas while moving to the southeast at 15mph.
ETA model forecast images were obtained via the courtesy of Gilbert Sebenste and the Storm Machine
RUC output was obtained in gridded format from the NWS OSO server and processed on PC-Gridds.
Radar plots, maps and analysis from CAPS was obtained from the CAPS server and from the archived files at CAPS.
CAPS is The Center for Analysis and
Prediction of Storms (CAPS), located at the University of Oklahoma in Norman,
and is a National Science Foundation Science and Technology Center
whose mission is to demonstrate the practicability of small-scale numerical
weather prediction with an emphasis on deep convection.
