May 10  1991 West Texas Tornadoes

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Brief Overview                       

                        At least 2 significant tornadoes occurred northwest of Lubbock on May 10, 1991. The first tornado touched down
                        1 mile north of Pep on the border between far northwest Lamb county and far southwest Hockley county and moved
                        northwest into southeast Bailey county. This tornado evolved into a large multi-vortex 1/2 mile wide tornado. The tornado
                        passed across Bula and lifted northwest of town. Around 20 homes suffered damage(some of  the damage was "heavy"). 

                        Another tornado moved very slowly and very erratically about 3 miles northwest and 4 miles north of Lazbuddie.
                        One home was demolished and another damaged. This tornado occurred in a very rural area. Hail up to baseball size
                        was reported with this storm.

                        Severe storms also occurred south of Wink, TX with 6" hail reported in Ward county. Hail up to 5" in diameter was
                        reported in the northwest Nebraska panhandle. A tornado was reported in Dallam county TX  with no damage report.
                        Golfball sized hail also occurred in Dallam and Oldham counties.

Synoptic Sequence of events 

                        At 12 UTC May 8 , an upper level low pressure system was located over Western Oklahoma. Fairly cool
                        500mb temperatures were noted even along the Gulf Coast(-10 to -13C). This kind of pattern is conducive to
                        widespread thunderstorms across the southern plains and Gulf Coast. Indeed, the effective frontal boundary
                        was located along the immediate Gulf Coast by 21 UTC May 8.  By 12 UTC May 9, the upper low had
                        moved into southeast Texas and was closing off from the westerlies. Thunderstorms were widespread across the
                        Gulf Coast region and northern Gulf.  By 21 UTC May 9th, the effective frontal boundary was located across the
                        northern Gulf into south and southwest Texas. However, cool dry air was nowhere to be found across the southern
                        plains. With lowering heights in the west, some lee troughing was resulting in return flow up the high plains. The
                        real warm sector with tropical moisture was trapped in the southern plains though.  The upper trough persisted
                        even on 12 UTC May 10. At 15 UTC May 10, the effective frontal boundary was still draped across deep
                        south Texas. This kind of surface pattern leads to more backed winds on the southern high plains. Upslope flow
                        generally results across West Texas in this pattern. Interestingly, the very sharp elevation gradient in west central
                        Texas lead to the development of an upslope driven thermal gradient between Midland and Amarillo or in this
                        case just north of Lubbock. North of this boundary, temperatures were cooler but dewpoints still high. Storms
                        Temperatures and especially surface potential temperatures became very warm across eastern New Mexico around
                        Clovis and Tucumcari. Storms apparently developed near the synoptic warm front and the cool upslope flow.
                        The shear profile was also enhanced just on the cool side of the upslope driven boundary. I am convinced this
                        boundary is often occurs in west Texas as a result of that very sharp terrain gradient east of Lubbock. When
                        the low level flow is southeasterly or easterly, a thermal gradient often sets up around the Lubbock area.
                        By 19 UTC May 10, The T/TD at Lubbock were 84F/68F while it was cooler but still moist 70F/63F at Amarillo.
                        As in the April 21, 1957 case, southerly mid level flow existed across the high plains on the leading edge of the
                        westerlies. Deep upper level lows were present in both cases in the southwest with light southerly mid level flow.
                        despite this fairly innocuous mid to high level pattern, terrain features and resultant low level thermal gradients
                        created a favorable  mesoscale storm environment. The 500mb pattern on May 11, 1991 at 00z(near the time of
                        tornadoes) can be found here.

                        Surface charts:
                        21 UTC May 8

                        21 UTC May 9
                        15 UTC May 10

                        17 UTC May 10

                        19 UTC May 10
                        21 UTC May 10

                        23 UTC May 10

                        500mb charts:
                        12 UTC May 8

                        12 UTC May 9
                        12 UTC May 10

                        11 UTC May 11

                        250mb charts:
                        12 UTC May 10

                        00 UTC May 11

                        700mb charts:
                        12 UTC May 10

                        00 UTC May 11