The famous F5 Lubbock tornado occurred in a synoptically quiecient weather pattern. The flow at mid and upper levels was
                                fairly light. This tornado occurred on the extreme SE edge of the westerlies--well out ahead of any mid level cooling or forcing
                                associated with the polar jet. A weak subtropical jet was noted across northern Mexico into the Gulf of Mexico. A strong
                                shortwave trough was exiting the central Rockies into the northern Rockies and northern plains during the day. A nearly
                                stationary front was draped from Iowa into central Kansas and then into Colorado. There was no significant shortwave
                                trough approaching west Texas. 500mb heights were on the rise througout the region from 12 UTC  May 11 to 00 UTC May 12.
                                This was partly due to the exiting shortwave trough as well as to afternoon elevated heating. The 500mb winds at ELP and
                                ABQ at 00z might lead one to believe that a shortwave trough was approaching. The 500mb wind was backed at ABQ. But 
                                the veered wind(270 degrees) at ELP right at 500mb may have been an error since the wind direction above and below this
                                level at 600, 550, and 450mb was from 240 to 250 degrees instead of 270 degrees. In fact, the  00 UTC 400mb chart does
                                not show any well defined shortwave trough. The AMA and MAF soundings at 12 UTC and 00  UTC show steepening low
                                level lapse rates due to diurnal heating.  The west Texas region was located near the middle of large 700mb warm plume. Of
                                course this is to be expected since we are dealing with a dryline, with no surface fronts near west Texas. The bottom line is
                                that if there was an approaching shortwave trough then it was fairly weak. This dryline retreated after 4 pm which is rather early
                                in the afternoon and possibly indicative of an approaching shortwave trough. Typical dryline retreats occur after 5  or 6 pm.
                                I am sure that one could claim the existence of a shortwave trough in the southern Rockies/Plains. Sometimes it seems that
                                 every thunderstorm that pops up is blamed on a shortwave trough. This is of course absurd as mesoscale forcing is several  
                                orders of magnitude larger than synoptic scale forcing.  Mesoscale and smaller dryline features and terrain features are often
                                key in west Texas storm initiation in the absence of strong or even weak synoptic scale forcing. 

                                At 22 UTC, the southwestern end of the dryline was retreating to the northwest.  Surface dewpoints west of the dryline near
                                Lubbock were in the upper 30s and lower 40sF. Surface dewpoints east of the dryline were in the upper 50s to mid 60sF 
                                depending on elevation. There could have been more than 1 convergence line or dryline across west Texas, but we will never
                                know since surface observations have been traditionally tied to aviation in this country and have not been positioned according to
                                meteorological need. 

                                The dryline continued to retreat after 22 UTC and was located in the vicinity of Lubbock by 00 UTC. By 02 UTC the dryline
                                extended from just east of Hobbs to west of Reese AFB to just south of GAG. Due to the lack of surface data and since the
                                low levels were altered dramatically by ongoing convection in the Lubbock area, it is very difficult to assess the instability.
                                Nevertheless, the 02 UTC observations from LBB and REE showed plenty of moisture with lower 60s F surface dewpoints.
                                Instability undoutbedly varied tremendously across the Lubbock region due to small scale convective effects. Thunderstorms 
                                were ongoing in the Lubbock area from 630 pm through the evening. While the storm scale and mesoscale effects are not known,
                                these effects could have turned an ordinary looking severe weather day into a violent tornado situation.  I can make a rough estimate
                                of surface based instability at 01 and 02 UTC based on the surface data at LBB and surrounding RAOBS. At 01 UTC the T/TD 
                                were 86F/56F at 897mb while at 02 UTC they were 71F/63F. Actually these observations are almost identical in terms of
                                theta-e. Surface based cape was around 3000 j/kg. Of course some prefer to use a mean layer CAPE value. But, I am not a
                                huge fan of this. The reason why I am not a fan is simple. How do we know what the vertical distribution of moisture is like
                                given the wide spacing of  radiosonde stations? The answer is that we do not know. If we do not know, then why use it? 
                                For convection that is ingesting low air at the lowest levels(such as supercell storms), I think that surface based cape is a
                                decent proxy of instability. Also, low level moisture tends to "pool" and be locally deeper in convergent areas near thunderstorm
                                updrafts. 

                                A timeline of the event can be found here.