The Front Range Tornado
May 7-8  1965
rough draft form
Jonathan D. Finch

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Overview


                            On May 7, 1965 around 10 pm, a thunderstorm developed near Boulder, Colorado. This storm moved north-northeast
                            and  became severe, producing tornado(es) and large hail from just northeast of Fort Collins to near Cheyenne, Wyomimg.
                            The cloud to ground lightning strikes with this storm could be seen from Denver  (starting at 10 pm). Most tornadoes in                            
                            this part of the country occur later in the spring and summer. Also, this severe weather occurred late at night, which is
                            extremely rare for the front range. Greater than golfball sized hail broke out house windows southeast of Cheyenne.
                            This event was a precursor to a  major tornado outbreak in Nebraska and South Dakota later on May 8, 1965.

                           

Meteorological Discussion


                            The 21 UTC May 7 1965 surface map shows a warm front from southeast Colorado into northwest Kansas and then a
                            stationary boundary extending into Minnesota. A dryline extended south from the front into west Texas. By 00 UTC, this
                            boundary was north of Lamar and La Junta. An area of thunderstorms developed across northern and northwest Kansas
                            and produced copious amounts of cool outflow. This outflow surged to the west-northwest through the evening hours and
                            carried moisture-laden air all the way back to the front range of north central Colorado. This outflow can be seen in the
                            surface observations from Goodland and McCook.


                            The 00 UTC upper air charts showed a very impressive upper level storm system over the southwest US. 500mb, 400mb,
                            300mb temperatures were fairly cool (-17C, -29C and -42C along the front range). By 12 UTC May 8 1965(5 hours after
                            the tornado), this system was a little further east. 500mb, 400mb, 300mb temperatures were around -19C, -30C, -43C. 
                            Therefore, only slight mid to high level cooling occurred from 00 to 06 UTC. 700mb temperatures in the areas of interest
                            were around 6C at 00 UTC and down to around -3C by 12 UTC.  By 03 UTC, the dewpoint at Limon, CO was 47F and
                            from 03 UTC  to 04 UTC to 05 UTC  the dewpoint at Akron, CO jumped from 40F to 49F to 51F. I had a difficult time
                            analyzing the fronts in Colorado. By 05 UTC, Akron, CO appears to be in or close to the true warm sector since the
                           surface theta-e met or exceeded warm sector theta-e values in northeast Kansas.

                            I constructed an approximate sounding for the tornado affected area. The elevation of the tornado varied from 5600ft to 
                            6100ft with an average surface pressure of 808mb. I came up with several possibilities of surface bases CAPE based
                            on best case, average case and worst case scenarios. The surface based CAPE likely ranged from 1000-1600 j/kg.   
                            So how did I arrive at these soundings? Well, the 500-300mb temperatures did not vary much from 00 UTC to 12 UTC
                            (there was a slght cooling at 500mb) and the Denver soundings were not contaminated. I took an average of the 500mb
                            temperatures at 00 and 12 UTC (-18C). The 700mb temperature was about +6C at 00Z and then dropped -3C by 12 UTC.
                            I estimated the 700mb temp. to be around +3C. I also reasoned that the low level air over Limon and Akron had time to
                            advect back to the front range(northeast of Fort Collins) by 06 UTC.  However, upslope cooling from Akron (4700ft or
                            845mb) to Nunn, CO(5600ft or 818mb) and areas southeast of Cheyenne(6100ft or 800mb) would result in 5-6F cooling.
                            I also allowed for a cooler airmass along the immediate front range (3F). Using upslope and some cooling along the front
                            range I came up with 61 - 9 = 52F.  The dewpoint was 47F at Limon at 03 UTC and between 49 and 51 at Akron from 4
                            to 6 UTC.  Since the dewpoint lapse rate( assuming conservation of mixing ratio) is 1F/1000ft, I allowed for a 1F drop in
                            dewpoint. I also lowered the dewpoint another 2F (from 50F at Akron) to arrive at a 47F dewpoint for my medium case
                            scenario.  For the worst case scenario, I lowered the temperature an additional 2F and the dewpoint 1F.
                                     
                            One commonly misunderstood concepts in meteorology is elevated heating and its affect on theta-e. Dewpoints on the high
                            plains and front range are typically lower than on the low plains. However, we all know that severe thunderstorms and even
                            significant tornadoes occur on the high plains and front range. These often hit nothing, but they can still be very powerful.
                            At 03 UTC, the T/TD at Limon, CO and Topeka, KS were 62F/47F and 75F/61F respectively. Even though the T/TD
                            were 13F/14F lower at Limon than at Topeka, the theta-e was about the same at the 2 locations(only very slightly lower at
                            Limon). The mixing ratio at Topeka was 42% higher than at Limon. Actually a 47F dewpoint at Limon has the same moisture
                            as a 52F dewpoint at Topeka. So if one normalizes for sea level, the dewpoint was only about 9F higher at Topeka instead
                            of 14F. However, one can easily see that the higher potential temperature at Limon (92.4F) compensated for the lack of
                            moisture to yield a similar theta-e. Even though the actual temperature 13F lower at Limon, the potential temperature
                            at Limon was 14F higher!!

03 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Limon, CO 5500 820 997 62
 47 8.4 92.4 332.5
Topeka 880 978 1009.8 75 61 11.9 78.4 333.7

                         

                        At 05 UTC the theta-e at Akron, CO was almost as high as Topeka. 

05 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Akron, CO 4700 845 1000.1 61
 51 9.5 86.6 332.1
Topeka 880 978 1009.9 74 61 11.9 75.4 333.1

          
                         
                            By 07 UTC (around the time the tornado was south of Cheyenne), the T/TD were 11F/7F lower at Akron than
                            Topeka. However the 54F dewpoint at Akron is about the same as a 58F dewpoint at Topeka. So in real terms the
                            dewpoint was only 3F lower at Akron. Even though the temperature was 9F lower at Akron, the potential temperature
                            was 11F higher. The theta-e was actually higher at Akron at Topeka.
                                
07 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Akron, CO 4700 845 998.7 63
 54 10.6 86.7 335.4
Topeka 880 978 1009.4 72 61 11.9 75.4 331.8


                        The observations from Cheyenne are VERY interesting.