Instability Approximation for Woodward Tornado



                                The raob network was in its infancy in the late 1940's. Upper air data were fairly sparse and undependable. Unfortunately, NCDC
                                has lost these data. I did manage to obtain some UA maps for 03 UTC for the late 1940s from Bobby Prentice. These maps are
                                fairly hard to read, but I still managed to reconstruct the 500mb chart for 03 UTC April 10, 1947.

                                However, it is possible to approximate instability on the western plains and high plains in some cases without the assistance of upper
                                air data. For details about how this is accomplished, see my page on calculating instability on the high plains in the absence of
                                upper air data. This technique that I developed has to be used with extreme caution. 

                                To accomplish this for the 1947 Woodward tornado event, we will make an assumption about the thermal profile
                                over the southern Rockies and southern front range. Namely, when very strong mixing is occurring with partly cloudy to clear skies,
                                the thermal profile trends toward dry adiabatic from the surface to 500mb by noon local time. Be cautious when using this method.
                                One must make sure that gusty winds and abundant sunshine are present, along with station elevations generally above 5000ft and
                                surface dewpoint depressions > 40F. The method works best for stations above 5000ft. Using stations below 5000ft can result in
                                500mb temperatures that are too cool by 1 to 2C. This method works best for stations such as Santa Fe, Albuquerque, Laramie,
                                Alamosa, Rawlins, Eagle, Rifle, Cheyenne and Rock Springs with surface dewpoint depressions >40F.

                                At 1930 UTC(close to local noon) on April 9, 1947, gusty south to southwest winds were noted across most of New Mexico and
                                the Big Bend region of Texas, implying strong SW flow at mid-levels. The station pressure for each observation is shown in pink.
                                For example, the surface temperature and station pressure at Guadalupe Pass, TX were 76F and 824 mb. The surface winds was
                                from the southwest at 50 mph. Now take this temperature up the dry adiabat to 500mb and see that the 500mb temperature at GDP was
                                approximately -15C. This same procedure was done for all the stations shown on the surface map that were south of the warm front.
                                Further west and north, I found 500mb temps of -17.5 at Engle, -18 at Albuquerque and  -20C at Acomita and Santa Fe. The warm
                                front was just moving north through Tumumcari at noon, so deep mixing was accomplished a little later there. So the 2030 UTC surface
                                observation was used there to yield a 500mb temp of -17C. There was a 500mb temperature gradient from NW to SE across
                                New Mexico. Also, note that surface winds were generally gusty from the southwest, with dewpoint depressions generally from 40 to
                                50F. These mid-level temperatures were undoubtedly representative of conditons downstream over the Texas Panhandle. So, in my
                                estimation, the 500mb temps in the affected area that evening were between -15C and -18C.

                                A tornado developed WSW of Pampa at 541 pm and passed north of Pampa by 610 pm. The surface observations around this
                                time showed fairly high surface dewpoints in the upper 50s and lower 60s with surface temperaures on the rise south of the warm
                                front. At 530 pm and 630 pm the T/TD were 64F/61F and 69F/59F respectively. The elevation of Pampa is about 3250ft. The
                                station pressure was 886mb. Using a skew-T log-P diagram, one can easily find the lifted index at Pampa assuming 500mb temps
                                between -15C and -18C. Lifting 64F/61F at saturation up the dry adiabat and then up the moist adiabat to 500mb yields
                                -7C. So the lifted index ranged from -8 to -11 at Pampa when the storm was to the WSW. After the storm moved north the
                                T/TD were 69F/59F, which has nearly the same theta-e as before.  Further east at Gage(elevation 2200ft or 920mb),
                                the T./TD were 67F/63F at 730 pm. The theta-e was almost identical at Gage(340K). Even though surface potential
                                temperature was slightly lower at Gage than Pampa, the mixing ratio was slightly higher.