March 26  1975 Lefors, TX Tornado

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Brief Overview

                        A  strong tornado hit Lefors, TX around 1140 pm CST on March 26, 1975. About 1/3 of the town of Lefors was destroyed  and 2
                        people were killed. Two-inch hail occurred in Groom, TX(24 mi. SW of Lefors) around 1110 pm from apparently the same storm.

Sequence of events 

                        The 12 UTC March 26 500mb chart showed  a deep trough over the southwest US with strong flow across the plains states. 

                        At 15 UTC, a 991 mb surface low was located over western Colorado with another low over northern, NM. A warm front stretched
                        from south Texas northwestward to south of Amarillo to southern Colorado.

                         At 21 UTC a strong "pacific" cold front was moving east into central NM. A 986mb surface low was located over central Colorado.                         
                        Two drylines were apparent. The westernmost dryline stretched from near east of Amarillo to east of Lubbock to near Midland.
                        But the really rich low level moisture was quite a bit further east and confined to the east of a line from east of  Del Rio to near Abilene.          
                        A front that was mainly stationary or moving very slowly northward stretched from south of Houston to north of Abilene to east of
                        Amarillo, then north into western Kansas. However, persistent rain into a dry, cool airmass had resulted in a cooler airmass over far
                        northern Texas into much of Oklahoma and Kansas. The leading edge of this airmass was charging westward through western Kansas.
                        I chose  not to label this boundary as a front but as a dashed pink line.

                        By 00 UTC the warm/stationary front stretched from near Houston to just south of Waco to southwest of Childress. The cold front
                        was still charging east into eastern NM. A surface low was over central CO with another low along the front in New Mexico.

                        The  00 UTC 500mb  showed a longwave trough over the western US.  Shortwave troughs were rotating around the main trough
                        and one of these was approaching the southern plains.  500mb temps were very cool--around -18C in the Texas panhandle. There was
                        a 110kt 250mb jet streak  across the TX panhandle. The 700mb winds were on the increase and up to 45 kts across the panhandles.
                        The 02 UTC surface chart showed a the pacific cold front charging east  into the western Texas panhandle. Two drylines were still
                        evident with the western most one located west of midland and barely east of Lubbock. A warm front stretched from south of Childress to
                        south of Waco. A rain cooled airmass still persisted further north and the boundary associated with this airmass was located over far
                        north Texas.  An arctic front was barreling down the front range of Colorado. Note the lower 60sF dewpoints north of the warm front
                        advecting back to the NW through Childress. The 02 UTC ob from Amarillo showed south winds at 35kts with gusts to 50kts. Thus the
                        850mb winds had increased at least to 50 kts at  Amarillo between 00z and 02z. Undoubtedly a strong low level jet was in place across
                        the central and eastern panhandle region.

                        At 03 UTC the pacific cold front was just west of Amarillo and about to overtake the dryline. The cold front actually overtook the dryline
                        between Amarillo and Lubbock around 4 UTC--perhaps in western Swisher county. The offending storm apparently developed in this
                        location around 4 UTC and moved rapidly NE. The movement of the front(to the east at 30 mph) allowed the storm(moving northeast at
                        ~ 50 mph) to stay very near or just ahead of the front. The 05 UTC surface map can be found here. Note that the warm front had
                        progressed through Childress. By 0540 UTC, the cold front was located very close to Lefors, which also corresponds to the time of a
                        strong tornado. Also note that the warm front was located just southwest of Lefors at 5 GMT. Thus, it appears that the storm developed
                        near the location where the cold front was overtaking the dryline between Tulia and Happy, then moved rapidly NE and encountered the
                        intersection of the cold front and warm front near Groom around 5 GMT. The winds immediately north of this front were very backed and
                        dewpoint depressions were near zero. T/TD values were probably around 61F/60F at 887mb based on the obs at Childress and adjusted
                         for elevation. I adjusted for elevation since in a upslope regime, temps on the higher terrain will usually be a little cooler at night with no insolation.
                        The elevation of Childress is only 1800ft while Lefors is about 3000ft. Using 700/500/400/300/250/200mb temps. of 3/-18/-30/-44/
                        -52/-60C yields a surface based CAPE of from 2500 to 3000 j/kg and lifted index from -10 to -13!!!  It would be nice to have upper air charts
                        from 06 UTC. It is very hard to fathom this much instability in the Texas Panhandle around midnight in March!! But it certainly appears that this
                        was indeed the case. It would also be nice to have a few more surface observations in the region of interest.  Any conservative estimate would
                        probably be greater than 2200j/kg CAPE and LI of lower than -9. Who knows what the mean layer CAPE would be in this instance. There is no
                        way to approximate this with the data given. But there is no reason to believe that the mixing ratio dropped off significantly with height along and
                        north of a warm front.  Surface based CAPES were around ZERO in western OK. This storm did not produce any more significant tornadoes
                        after the Lefors tornado. Apparently the storm crossed the northern boundary and encountered very stable cold air over the eastern Texas
                        Panhandle.  The positions of the boundaries at varying times and location of the tornado and large hail are shown  here. I suspect that the Lefors
                        tornado did not touch down in Lefors and lift immediately after leaving Lefors. But there were probably no structures to hit ne and sw of Lefors.                

                        The 12 UTC 500mb chart can be found here.  The 700mb winds were 65 to 80kts by 12z.  The wind profile around 06 was  probably similar to
                        the following:

                        surface    12025kt
                        850mb    18060kt
                        700mb    21060kt
                        500mb    22070kt
                        250mb    21110kt