The Front Range Tornado
May 7-8  1965

Jonathan D. Finch

Related items of interest

Historical Tornado Cases for the Cheyenne Warning Area

Detailed Tornado Cases for the Cheyenne Warning Area      

Historical Tornado Cases for the Boulder Warning Area

Historical Tornado Cases for the United States

Elevated Mixed Layer

Elevated Heating

High plains and front range topo maps


                            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). Severe storms along
                            the front range are most common from June through early September, but can occur as early as April. Severe storms 
                            typically occur between noon and 6pm. Storm initiation tends to earlier than on the low plains due to elevated heating and
                            terrain effects. But on May 7, 1965, storm initiation was in the late evening, with severe weather occurring until after
                            midnight. This is very rare. 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

            Upper Air May 7 1965  12 UTC

                            A large upper trough was located over the Rockies at 12 UTC May 7. Rich gulf moisture was streaming up the high plains
                            with > or = 13C 850mb dewpoints at Dodge City, Amarillo, Midland and Del Rio. 500mb temperatures were fairly cold
                            across the high plains. A 700mb warm plume was in place across the plains, with steep 700-500mb lapse rates.

            Afternoon Surface Features

                            At 18 UTC, a cold front was stalling out and becoming stationary from south Grand Junction, CO to south of Trinidad, CO
                            to south of Goodland, KS. A pacific cold extended from eastern Utah into eastern Arizona. A surface dryline was located
                            from the Texas panhandle into western Kansas.

                            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.  A pacific cold front
                            front extended from west of Grand Junction into eastern Arizona. A secondary cold front was located across central Wyoming
                            Thunderstorms were developing ahead of the surface dryline in western Kansas. The following surface observation sheets
                            show the warm frontal passage: Grand Junction(20-21Z)    Trinidad(18-19Z)    La Junta(22-23Z).  The warm frontal
                            passage at Goodland is complicated by the strong southeasterly thunderstorm outflow surge.

                            By 00 UTC, the warm from was north of Lamar and La Junta, but south of Pueblo. An area of thunderstorms across
                            northern and northwest Kansas was producing copious amounts of cool outflow. The pacific cold front stretched from near
                            Rifle, CO to just west of Gallup, NM. The secondary cold front now stretched from near Philip, SD to south of Rawlins
                            and Evanston, WY.  Snow was falling across the high country of southern Wyoming.  The surface dryline still stretched from
                            far western Kansas to the central Texas panhandle.


            Upper Air May 8 1965  00 UTC

                            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). It appears from the height and wind
                            fields that a lead shortwave trough was ejecting north and northeast into the central high plains at 00 UTC. This likely
                            explains the widespread convection across western Kansas in the late afternoon. Also, it was highly unstable along and
                            ahead of the dryline with dewpoints near 60F, temperatures in the 80sF and 500mb temperatures around -15C. A 700mb
                            warm plume extended from Old Mexico to Denver. In fact, 700mb temperatures were around 5C at Cheyenne. How do
                            I know this? Well, because the midday surface temperature at Cheyenne reached 60F despite upslope southeast winds. So
                            when I analyzed the 700mb chart, I extended the +5C dashed red line into southeast Wyoming. There was also a surge of
                            warm air ahead of the pacific cold front. At 21 UTC this front was west of Grand Junction where it was 69F. At 00 UTC
                            the front was bearing down on Rifle where it was still 61F. It was a balmy 66F at Montrose(5760ft or 808mb). So the
                            700mb temperature must have been about 6-7 ahead of the pacific cold front. Since is was 69F at Grand Junction prior to
                            frontal passage, the +5C isotherm at 700mb must have been located near the north-south front at 00 UTC.  Also, snow was
                            falling at Rawlins, WY at 00 UTC, so the 700mb temperature must have been 0C or colder there. So the judicious use of
                            surface observations on the higher terrain allowed me to draw a more detailed 700mb thermal field than one could draw by
                            simply analyzing the upper air data. There was a 13C temperature drop across this front from Albuquerque to Winslow. The
                            700mb front was sharper at 00 UTC compared to 12 UTC since elevated heating ahead of the front enhanced the thermal
                            gradient.  A 90 to 100kt jet was located from central AZ into western Colorado.

             Evening Moisture Surge and Surface Boundaries

                            Outflow from widespread thunderstorm activity in western Kansas 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. By 03 UTC, the dewpoint at Limon, CO was 47F. 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 appears to be in or close to the true warm sector. The pacific cold front progressed
                            through Albuquerque at 2105 MST (0405 UTC). The observations at Eagle, CO showed strong pressure rises just before
                            03 UTC, so the front was probably east of there. The front was not through Alamosa and Gunnision by 03 UTC and these
                            sites stopped giving observations at this time. Although the lack of quality surface observations hindered the pacific frontal
                            analysis after 03 UTC, I did the best I could in frontal placement. I drew the front from west of Albuquerque to east of Eagle
                            at 03 UTC, but kept the front west of Gunnision and Alamosa. The warm front was still not through Pueblo at 03 UTC. In
                            fact, a wedge of cool air is apparent along the front range.

            Late evening surface analysis

                            Frontal analysis was very difficult on the evening of May 7, 1965. The northward movement of the warm front was
                            complicated by a strong outflow surge from widespread afternoon and evening thunderstorms over Western Kansas. Further
                            complicating matters was the lack of surface observations between the Limon/Akron area and the Fort Collins/Cheyenne
                            area. I believe that the moist surge progressed back to the front range and that the storm in question intensified upon
                            encountering this moisture. But there can be no proof this. At 05 UTC, a canadian cold front stretched from near Valentine
                            NE to immediately north of Scottsbluff to just south of Laramie. To pinpoint this front at 05 UTC, I used the surface
                            observations at Scottsbluff that show thunderstorms developing around 04 UTC, followed by colder air and a windshift to
                            the northeast. There was still a wedge of cooler air along the front range. Moisture was surging back into eastern New
                            Mexico, but was still east of Amarillo. The dryline extended from east of Amarillo north-northwestward into eastern
                            Colorado. Although observations were lacking, the pacific cold front was located north to south through central Colorado.
                            The surface warm front that had been progressing northward all evening was now stationary from south of Sidney, NE to
                            south of North Platte to near Norfolk, NE. I opted to analyze the front at the northern extremity of the outflow surge.
                            The observations at Imperial and Akron indicated moist air moving upslope into northeast Colorado. At 07 UTC, when
                            the tornado was located south of Cheyenne, the observations from Cheyenne indicated hail with thunder with a surface
                            temperature of 35F. But if my analysis is correct, the relatively warm and moist air was not far from Cheyenne. I still didn't
                            know how to analyze the cool wedge along the front range at Denver, Colorado Springs and Pueblo.

            Elevated Heating

                            One commonly misunderstood concept in meteorology is elevated heating and it's effect 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 there. 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
                            was 13F/14F lower at Limon than at Topeka, the theta-e values were nearly the same(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) offset the lack of moisture
                            so that theta-e values were similar. Even though the actual temperature was 13F lower at Limon, the potential temperature
                            at Limon was 14F higher than at Topeka. See the table immediately below for details.

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 despite the T/Td being 13F/10F lower at Akron.
                            Again keep in mind that a 51F dewpoint at Akron has about the same mixing ratio as a 55F dewpoint at Topeka. So on
                            a level playing field, the dewpoint was only 6F lower at Akron than Topeka. Even though the surface temperature at
                            Akron was a "cool" 61F, the surface potential temperature was 86.6F. The potential temperature was only 75.4F at
                            Topeka.  See the table immediately below for details.
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 9F/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. See the table immediately below for details.
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


            Upper Air May 8 1965  12 UTC

                            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 area of interest were around 6C at 00 UTC and around -3C by 12 UTC. Most of this cooling
                            likely occurred after the tornado.

            Surface Based CAPE

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 based 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. As you can see,
                            I was rather conservative in my estimations of T/TD in the tornado affected area. So 1000-1600 j/kg can be regarded as
                            a conservative estimate of surface based CAPE. It is possible, but unlikely, that the storm was elevated over the cold low
                            level air. Albiet rare, there have been documented cases of tornadoes over a shallow cold dome (2-22-75 and 4-5-72).                               

Vertical Wind Shear                     

                            I really don't know what the vertical wind shear profile was like with any accuracy. This tornado occurred  between the 00
                            and 12 UTC radiosonde launches. Of course, the available data suggest that the mid to high level flow was southerly, with
                            increasing speeds with height. There were no surface observations close to the storm. Assuming that this storm was not in the
                            cold air (like Cheyenne), the surface winds were likely from the east or southeast. Windspeed and direction above the surface
                            up to 600mb is impossible to estimate with any accuracy. That said, it appears that the wind profile was characterized by
                            veering, with generally increasing wind speeds with height.