STORM TRACK: September 30, 1984 (Volume 7 Issue 6)

(Editor's Note: Since the 1984 season turned out relatively flat for most storm chasers, it is not inappropriate to reach back to last year for some good chase accounts. A report on the May 20, 1983 Plainview, Texas tornadoes appeared, with photos, in the May, 1983 issue of Storm Data. It was a very interesting storm, which produced three tornadoes -- all without visible condensation connection to cloud base. Such occurrences are somewhat rare but not uncommon in the western states (Panhandle, western Kansas, etc.) and are illustrative of what to look for if you're in that part of the country during storm season. Some of the photo based illustrations here were not reproduced in Storm Data.)

The early 1983 storm season appeared to be divided in two, with an early tornado outbreak in March, followed by the typical peak in May. The year started off great, when I tracked two tornadoes near Clarendon, Texas on March 3 (See ST Vol. 6, No. 6). Following a return to winter-like conditions in April, Roy Britt, Dr. Jim McDonald and I filmed three tornadoes near Plainview on May 20. Roy and I later filmed a unique supercell thunderstorm on May 24, and my wife and I filmed a small tornado under a flanking line storm near Winters on June 5.

The May 20 tornado outbreak had classic synoptic conditions for severe weather: a surface low near Roswell, New Mexico; a dryline bulge extending toward Lubbock; and a warm front extending from the low to Childress. Upper air patterns included a closed low over New Mexico and a strong diffluent axis over the Texas panhandle ("difluent axis' or diverging winds which tend to reduce pressure fields in the upper atmosphere and, thereby, enhance vertical development of vigorous cumulus towers), The morning sounding at Amarillo had a classic bell-shaped appearance. Though the air mass was unstable, the temperature inversion was weak, and the convective temperature of 80 deg F looked like it would be reached early in the day (Figure 1).

Figure 1

The Lubbock area appeared most favorable for severe weather. Throughout the morning, surface winds remained 150 deg (from the south-southeast) at 10 to 15 MPH. A strong temperature gradient was observed along the front with surface wind vectors perpendicular to the isotherms, maximizing isentropic lifting. About 1:00 PM, a line of towering cumulus developed northwest of Lubbock. The towers shot straight up and glaciated rapidly. Roy, Dr. Mac and I headed northward toward the towers. Now, we figured we had some extra time, so we stopped at Abernathy Allsups store to pick up one of those tasty, day old barbeque sandwiches (sad face symbol). We continued northward and observed a small, isolated Cb, developing west of Plainview, with a small cumulus flanking line extending southwestward. The storm had a tilted, highly sheared updraft leading up to a rigid cumuliform anvil (Fig. 2).

Figure 2

A small line of stratocumulus was seen to the east, moving toward the updraft. The storm structure looked good, and I had a feeling it would produce. As my excitement increased, Dr. Mac casually sat back in his seat and began reading a manuscript. Though it was his first chase, he knew the odds of seeing a tornado were slim. We pulled underneath the rain free base and munched on our sandwiches, just as the EBS tone went off on the radio. SELS issued a tornado watch at 2 PM for the area. The cloud base to the north was smooth in appearance, with a small precipitation shaft further north. A farmer in a pickup stopped and told us that he encountered golfball-sized hail to the north in 'downtown Edmonton' (downtown Edmonton? - pop. 220).

Within a few minutes, the updraft passed overhead, and the wind shifted to a northeasterly direction, bringing cooler air. We moved eastward to Plainview and called the National Weather Service (NWS) in Lubbock at 2:55 PM. They said the Plainview storm was VIP level 4 (intensity scale based on radar), but that this level had dropped within the last few minutes. Another more intense VIP 6 storm was located about 30 miles to the northwest near Hart. I decided to stick with the Plainview storm, since it, was the furthest south -- and remembered that a sudden drop in VIP levels could mean the storm was becoming tornadic instead of dissipating.

We moved northward and were under the rain free base five minutes later. Scud formed rapidly beneath the cloud base and moved upward, forming a wall cloud (Figure 3). We stopped at our first film site at 3:08 PM CDT, about two to three miles north of the wall cloud (See Figure 4). The updraft was very elongated, extending southwestward and then abruptly curving southeastward at the updraft edge. Surface winds were NE at 20 MPH, with gusts to 25. The air temperature was 69 deg F, with a wet bulb temperature of 60 deg. Three minutes later, there was a sudden wind gust, and the wall cloud began rotating. The rotation intensified and moved southward along the cloud base to the end of the updraft. At 3:15 PM, a thin horizontal funnel appeared aloft, with two dust whirls at the ground below (Figure 5).

Figure 3

Figure 4

Figure 5

Looking south, the horizontal gray final extended from a mall cloud mass, rotating in a clockwise direction. Two dust whirls rotated about a common center, across a plowed field. We moved eastward with the tornado, and as it closed in we could see a third subvortex at times. (Figure 6). By 3:40 PM, we noticed a wedge of dust just south of the tornado, moving rapidly eastward. The dust wedge demarcated the rear flank downdraft boundary at the surface. After the downdraft occluded the tornado, the circulation dissipated within a few minutes. Though the condensation funnel never reached the ground, the dust whirl circulation remained in contact with the ground for nearly ten miles. Cool north winds persisted throughout the time we were filming.

Figure 6

Roy put the pedal to the floor, and we drove eastward across dirt roads, doing eleven miles in eleven minutes! By this time, the clear slot had pushed well to the east, and another wall cloud was developing ahead of it. I crossed my fingers as we passed directly underneath the rotating wall cloud, and the car suddenly jolted as we encountered a strong low-level jet of southeasterly winds. Seconds later, Dr. Mac looked back and saw a large, column-shaped vortex on the road. He said, 'Is that a tornado?' I looked back and saw the town of Providence obscured in dust. We jumped out of the car and began filming, hardly believing we were back there a minute ago. The dust whirl was beneath a small, laminar shaped updraft at the leading edge of the clear slot (Figure 7).

Figure 7 and 8

A line of dust extended in a north-south direction behind the tornado, depicting the gust front. We watched the tornado move northward along the dust boundary and widen to a cylinder shape (Figure 8). Southeast winds were quite strong, making it difficult to stand and hold the movie camera. In contrast, the sun was shining, and an occasional raindrop fell from the anvil overhead. The dusty vortex continued to widen and then went through a turbulent period, be subvortices were present within a minute of the transition. Each subvortex formed on the south edge of the circulation and intensified as it moved around to the east side, finally dissipating on the north side. Sometimes a condensation funnel formed above esch subvortex. The dust circulation continued to widen, and the dust motion slowed, dissipating at 4:06.

The storm and dusty outflow boundary continued eastward and became stationary near Lockney. The first updraft had moved to the northeast, and we began watching a second updraft develop to the south. At 5:16 PM, a small funnel formed at the end of the upraft again, and a single dust column appeared at the surface. We watched this tornado for several minutes before it dissipated. By this time, the entire storm system appeared to be gusting out. Looking northward at the first updraft, we could see that it had lowered and occluded in precipitation. We raced northward and watched the wall cloud become obscured in precipitation. We had to watch the storm go off in the distance, as we met the Caprock escarpment at Cedar Hill. At times, it appeared that the base reached the ground, but it was difficult to confirm due to poor visibility. Most of the severe activity had ended by 5:30 PM. As we drove home, Dr. Mac could hardly believe he had seen tornadoes on his first chase and kept saying, 'Were those really tornadoes'

It was interesting to note the following: (1) The drop in VIP levels for the Plainview storm was followed by tornado activity about thirty minutes later. Erik Rasmussen has documented similar drops in the VIP levels, associated with the Tulia tornado outbreak in 1980. (2) The funnels appeared to originate along a, horizontal plane at the very edge of the updraft. (3) Condensation never reached the ground in any of the three tornadoes. Dust whirls accompanied the funnels aloft, and the dust whirls remained in contact with the ground throughout the funnels' lifetimes. (4) The circulation of the first and third tornadoes were well-contained within a certain diameter, whereas the second tornado began as a single cell, widened into a multi-vortex and widened further until it spun out."