STORM TRACK: May 31, 1987 (Volume 10 Issue 4)

On the morning of April 19, 1986, a strong tornado ravaged the West Texas community of Sweetwater. The storm struck with what was apparently little or no warning to its residents and inflicted one fatality and caused approximately 15 million dollars in damage. The morning forecast alluded to a 40 percent chance of thunderstorms during the day, and there was no mention of the possibility of severe weather. Yet, at 7:15 am CST, a large, multi-vortex tornado touched down gust southwest of the city.

The following article addresses the lifecycle of the tornado, the synoptic setting that induced the formation of the parent supercell, and the plausibility of recognizing such a severe storm scenario. Could this have been a forecastable situation?

A severe thunderstorm developed northeast of a surface low at the intersection of a dryline and a cold front to the southwest of Roscoe around 7:00 am. The Sweetwater tornado touched down approximately 1.5 miles southwest of the I-20 and Robert Lee Road intersection. The storm moved to the northeast, inflicting minor roof damage to a farm residence just east of the Santa Fe railway, a mile south of I-20. At this time, electric power lines were severed from transmission towers. Empty oil tanks were lifted and carried approximately 400 yards impacting and destroying a small house just south of I-20 at Robert Lee Road.

The funnel crossed the south-central section of town and intensified as it approached Highway 70. In this older part of town, many of the aged and weakly built structures, both residential and business, were unroofed or had exterior walls collapse. After the tornado crossed Highway 70, it strengthened further. Brick-veneer residences in the area sustained increasingly severe damage from the intersection of Arkansas and Lubbock Streets northeastward. An 86 year old man was killed in his residence by flying debris on Neff Street about one block north of Alabama.

From that point northeastward about a mile, many residences were unroofed with part or all of the exterior walls removed. A mobile home park about a quarter mile north of Alabama and Newman Streets sustained severe damage, with many homes totally destroyed and pieces thereof removed from the site several hundred yards. The damage track (FO) varied from 400 yards wide at Highway 70 to 700 yards wide in the area of most severe damage. The most intense damage was rated F3 near the site of the fatality. The tornado continued through Sweetwater and moved out into open country.

At 500 AM CST, a surface cold front extended from western Arkansas through southeastern Oklahoma and northwest Texas to a low near Midland (Fig. 1). A strong dryline extended southward from the Midland low to just west of Sanderson, at the mouth of the Pecos River. The cold front curved from the Midland low northwest through eastern New Mexico. The airmass south of the cold front and east of the dryline was very warm and moist, with temperatures from the 60's to the middle 70's and relative humidities between 90 and 100 percent. The airmass west of the dryline was mild and quite dry with dewpoints in the teens. North of the cold front, temperatures dropped 20 degrees in 200 miles. A light south to southeast wind prevailed in the moist air while a 10 knot northeast wind filtered in north of the front.

The 600 AM CST, intense warm, moist advection resulted from a 35 knot low level jet at 850mb (Fig. 2). Note the jet maximum to be situated just west of Sweetwater and just east of the front/dryline triple point. The wind field induced strong moisture convergence.

Figure 1

Figure 2

Wind veered some 60 degrees between 850 mb and 700 mb at Midland, but maintained a speed of around 30 knots throughout the layer. Cold advection was moving into western Texas, implying continued deepening of the 700 mb trough affecting eastern New Mexico and Trans-Pecos Texas. A negative tilt trough was more exaggerated at 500 mb (Fig. 3), where 30 meter height falls occurred west of Midland. Very cold (-17C) temperatures at 500 mb combined with the lower level tropical airmass to produce a steep lapse rate that rendered the airmass highly unstable.

At 300 mb (Fig. 4), the polar jet curved from northeastern Arizona through southern Mexico into the Texas Panhandle and northwestern Oklahoma. Upper wind speed analyses placed the eastern Permian Basin, including the Sweetwater area, in the right-rear quadrant of the jet maximum. This provided substantial speed diffluence across the area of developing thunderstorms.

Figure 3

Figure 4

Both Midland and Del Rio soundings revealed a moist and explosively unstable environment over the southern part of western Texas. The combination of ambient lifted indices of between -6 to -8, vigorous sub-cloud veering of wind, strong low-level warm and moist advection, and upper level diffluence, created a classical tornadic situation. Unfortunately, by the time this sounding information and the 600 AM analyses were available, the Sweetwater tornado had already occurred.

A Severe Thunderstorm Watch was issued at 5:52 AM CST by SELS, valid until noon. The watch included the town of Sweetwater. At 7:15 AM CST, golfball-sized hail began about 12 miles west of Sweetwater. Three minutes later, the hail report was received at the Abilene NWS. The office began immediately to prepare a Severe Thunderstorm Warning for Nolan County which includes Sweetwater. The warning was disseminated via NOAA Weather Radio at 7:24 AM. Only after 7:30 AM, was the information of a tornado having occurred relayed to the WSO. The Abilene Weather Service acted as quickly as possible in reacting to the situation.

It was learned during the post disaster survey that four telephone company power poles with phone lines were downed about 20 miles southwest of Sweetwater around 700 AM CST, 15 minutes prior to the Sweetwater tornado. The parent supercell evidently produced a tornado prior to the multi-vortex twister. Had this information been relayed to law enforcement or NWS personnel, a Tornado Warning could have been issued in time to prepare Sweetwater.