Lubbock F5 Tornado
 May 11, 1970

Jonathan D. Finch

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Detailed Tornado Cases for  Amarillo and Lubbock

                                The famous F5 Lubbock tornado occurred in a synoptically quiescient 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.  But inspection of the actual upper air data
                                shows that the winds at 500mb at El Paso were only veered at 500mb and not immediately below and above 500mb.
                                So it is not clear if the due west 500mb wind at ELP is indicative of a shortwave trough of some kind of measurement error.
                                Also, thunderstorms were occurring around ABQ near the time of the upper air release and this likely affected the winds.
                                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.
                                The cooler temperature at 700mb at Midland at 00z was due to intense surface heating, vertical mixing and low level convergence
                                immediately ahead of  the dryline. This was despite general 700mb warming across the region. So the 700mb temp. at Midland
                                held steady in spite of the 700mb warming that was occurring at ABQ, AMA, ELP and DYS. Note that the moist layer on
                                the MAF sounding extended up to around 700mb. Modern day  numerical models often show 700-500mb cooling/moistening in
                                the vicinity of the dryline during model convective initiation. In fact, sometimes the models don't develop precip but show a narrow
                                axis of cooling/moistening around 700mb. That superadiabatic layer just above 700mb is the result of the wet bulbing effect and the
                               data in this layer is in error.

                                There may have been a weak shortwave trough exiting the southern Rockies and moving into the central plains during the day of the
                                11th. There was a 700mb cold pool at ABQ with +4C at 12 UTC. It looks like this cold pool either weakened or shifted northeast
                                 into the central plains by 00z. But this feature did not affect 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.  But the strong shortwave troughs
                                traversing the northern plains may have resulted in a further east position of the dryline in Kansas and northern Oklahoma compared to
                                Texas. This could be why the southern end of the dryline started to retreat before the northern end did. 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 Rockie/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
                                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.  Again, the strong shortwave troughs traversing the northern plains may have resulted in a further east position of the
                                dryline in Kansas and northern Oklahoma compared to Texas.

                                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 and shear.
                                Nevertheless, the 02 UTC observations from LBB and REE showed plenty of moisture with lower 60s F surface dewpoints.
                                Instability and shear 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.  For low top storms, a lower level lifted index or low level CAPE would probably be better.

                                Keep in mind that this tornado just happened to hit a city with lots of people. Undoubtedly lots of other tornadoes in west Texas
                                have been as intense as this one but just didn't hit anything. One example that comes to mind is the huge tornado that moved northwest
                                across Hockley and Lamb counties back on April 21, 1957.
                               The surface observations from Lubbock for May 11, 1970 can be found here.

                                A timeline of the event can be found here.