March becomes tornadic on the plains
March 12, 1999 Texas Hill Country chase


10:15 am. Visible satellite.

By Gene Moore
San Antonio, TX

Storms began early on Friday, March 12, 1999, with multiple supercells firing shortly after sunrise near San Angelo, Texas. These storms were worthy of tornado warnings, but no damage occurred. The activity formed north of a stationary front in the cool air and east winds. Overhead, a strong subtropical jet stream had spread cirrus across the sky while a potent upper low brought a storm in from the west. At the surface, conditions were dominated by the stalled front draped across south Texas. This pattern led to wide spread low ceilings, fog and drizzle with visibility restricted to less than a mile. Chasing would have to wait until the fog thinned and skies cleared. Around noon the visibility improved with pockets of sunshine heating portions of west Texas. Meanwhile a surface low pressure system developed south of San Angelo.


1:40 pm. San Angelo radar.

Chase Strategy
The morning forecast and target area favored a spot north of the front in Mason County (just south of Brady, TX). This region was agreeable for supercells moving in from west Texas to become tornadic. Here they would intersect the warm air and higher dew points streaming north from the Gulf of Mexico. Since this was my first chase of the season, I was a bit rusty and made some errors.

The chase
Upon entering Mason County I incurred visibility problems and called for radar help. The radar information I received was misinterpreted and my next decision threw me off in the wrong direction. I compounded that error by getting two highway numbers confused. These were minor mistakes, but when combined with poor visibility they proved to create big problems. I had moved to western Mason County when the clouds broke, revealing a wall of hard convection speeding away to the east. The 1:40 PM radar image from San Angelo (see left) shows the two McCulloch county storms as they moved into San Saba County to produce a damaging tornado. Consequently, I decided to let the first storm go. In retrospect, had I tried to recover northeast I would have probably missed the tornado and wound up wasting time on a decreasing cell. The tornado threat ended after the first touch down. I knew when to cut my losses.


2:15 pm. Visible satellite.

It was early in the afternoon, about 2:30 PM, and my clearing sky was again becoming obscured. This time it was from an anvil stretching across the sky from a developing storm north of Del Rio. The cooling effect of the anvil was killing the cumulus towers along the front. The anvil was coming from my #2 target area in southwest Texas. This is a beautiful area to visit with its rugged terrain and clear streams. However, it can be a very difficult area to chase storms. There the land is covered in scrub-oak and the relief fluctuates from hills as high as the Ozark mountains to intermittent deep canyons. The trees and hill tops obscured visibility to the horizon much of the time.


4:00 pm. Visible satellite.

A cell containing a severe thunderstorm warning was coming out of Val Verde County and moving southeast. It was part of a short line of convection positioned just northeast of the surface low. Adding to my problems was the cyclic nature of the storms. While driving into Edwards County I observed a stunning laminar bell shaped cloud with a long inflow tail flowing into the lead storm. This is the kind of storm structure that usually produces a tornado. Unfortunately, it died as I got there. Later, a new cell formed and produced a large wall cloud over Barksdale in Edwards County. The cell dropped a strong gust front and moved rapidly southeast hammering Uvalde with golf ball hail and wind. Subsequently, I exited the Uvalde storm due to a broad area of straight line winds on the storm's leading edge. The wall cloud/mesocyclone had wrapped back into the gust front. The storm had became one of those "wound-up green hail monsters".


5:27 pm. Radar.

Looking at the Uvalde storm from a distance, a long area of high and mid level clouds extended southwest from the Uvalde storm to a developing convective base much further west in northwest Zavala county. This area was associated with an extensive outflow boundary (OFB). Some very light rain was falling in this region, but it was generally rain free and well illuminated with sunlight. The new base on the southwest edge formed a wide shallow wall cloud with a tail cloud extending to the north-northeast. At this point I had little faith that this developing storm would get going before sunset, but I stayed and waited. This period is shown on the 5:28 pm frame. At this time the main storm is shown over Uvalde and a warning was in effect there.


5:51 pm. Radar.

The 5:51 pm radar image depicts a dramatic inflow notch on the Uvalde storm. Also note from this image a band of low level reflectivity extending to the southwest to the new Zavala county cell. This feature was observed visually as a thin rain curtain extending across the northwest horizon. A few narrow streamers of precipitation could be seen near the wall cloud along with occasional rising dust. Note the green reflectivity area extends to the red core on the southwest edge. This was a 95 percent rain free updraft.


5:51 - 6:14 pm. Radar loop.

Over the next 30 minutes the Zavala county base grew dark and heavy. The 6:03 pm radar frame showed that a hook echo was forming during this period, but to me the cloud did not appear "threatening." The 6:08 pm frame shows the large Zavala and Uvalde cores joining. At this time the wall cloud was quickly destroyed by the rear flank down draft (RFD) and the convective base pushed southeast. Fearing this was a negative development I got closer to observe the main inflow region. I moved to a position 2 miles west of Batesville (in northwestern Zavala Co). The 6:14 pm image shows the storm rapidly intensifying. This was reflected under the cloud base by a dramatic increase in the outflow from the core. The wall of precipitation that was miles to my northeast had now extended to a point just northeast of Batesville. The outflow from the core of the storm picked up considerable dust. Winds became more easterly and the temperature dropped. I drove east through the small town and begin to proceed south. More wind from the northeast core area surged across the road and wind speeds were up to 50+ kts. This was much like a "wet" downburst; although little rain was falling at my location. A new core area was now developing to my southwest on the edge of the cloud. My route south appeared clear of precipitation with only obscuring dust a problem. Unfortunately, this massive amount of dust was beginning to obscure what was going on at cloud base and would lead to problems. My goal was to proceed south in order to regain visibility. A huge mass of dirt lifted out of the field to my east and crossed the highway rocking the vehicle. I looked back northwest and a narrow arm of striated precipitation was moving rapidly southwest.


6:20 pm. Radar.

If you have figured this out by now you are way ahead of where I was while chasing. These things can happen fast during a chase! If I could have viewed the 6:20 pm DFX radar image at the time, I could have seen it all come together nicely. The hook was quite obvious on radar. Furthermore, note that the hook is on the southeast edge of the core -- not the southwest.


6:20 - 6:49 pm. Radar loop.

The massive wall of dirt crossed the road, killing my visibility of the road ahead. Then this annoying dust whirl spun up at the side of the road. It gathered so much dirt it turned totally black. Determined to make it further, I drove through the edge of the swirl and proceed south. It seemed too strong for a gustnado. I could see black suction spots developing as I looked over my right shoulder. Anyway, I was free -- or, so I thought. Another huge mass of dirt lifted on what was about 70 kt inflow winds. It proceeded across the road and did a complete 360 degree turn in front of my vehicle. During this last incident I took another overview; hanging my head out the window. The whole scene now made since. The long arm of precipitation -- the dirt stopping here. The mesocyclone I was under is depicted by the DFX base velocity image from 6:26 pm (left).


6:26 pm. Base velocity.

I felt much as if a deer in the headlights as the inflow to my south increased and the second vortex southwest grew larger. I was between and just east of both of them. With the south route blocked by tornadic inflow, I had no choice but to bail out. I was not in immediate danger, but remaining in the same spot would have engulfed me in rotating rain and dust. Looking north, the rain curtain feeding into the circulation was spaced with small dust vortices about 250 meters apart. I proceeded north between two of them and into a light hail shaft. After getting free of the hail, I watched the tornado from a few miles north until the circulation rain wrapped. The first vortex disappeared in about one minute and the second in about 3 minutes. Neither vortex visually moved east out of the field or crossed TX route #117. Before returning home I set up behind the storm for lightning pictures.

Spring tune-up lessons learned and mostly relearned:

1. Cold outflow won't stop a tornado from forming when the circulation is present.

2. The tornado can form after the RFD destroys the visible wall cloud.

3. See where the dust is going. Is there a focal point even though the wall cloud is gone?

4. Getting too close can obscure the overview of what's going on. It can also bury the photographer in so much dust and wind that photography is impossible.

5. Be extra cautious when dust obscures the cloud base.

6. A well formed radar hook (striated rain curtain) may be almost invisible, especially when seen during a low sun angle.

7. When the main updraft collapses southwest of the mesocyclone it may force the circulation to the southeast edge of the storm.


Gene Moore is a veteran chaser from San Antonio, Texas. His web site is available at http://www.chaseday.com/.


Return to STORM TRACK 1999 logbook