The Cheyenne Ridge Tornado
April 23 1960

Jonathan D. Finch

Historical Tornado Cases for the Boulder Warning Area

Historical Tornado Cases for the Cheyenne Warning Area

Historical Tornado Cases for the United States

May 7-8  1965 Front Range Tornado

June 14-17  1965 Front Range Superstorm

High plains and front range topo maps



Overview


                                        On April 23, 1960 , a thunderstorm developed near Fort Collins, Colorado. This storm moved north-northwest and
                                        and became severe, producing tornado(es) in western Laramie county in Wyoming.  The tornado touched down
                                        about 23 miles west-southwest of Cheyenne or about 3.5 miles west-southwest of Granite, WY at 7500ft elevation.
                                        The tornado moved northwest or north-northwest into eastern Albany county (elevation 7900ft). 
                           
                           

Meteorological Discussion


                                               
                                        The 00z April 23 1960 500mb chart showed 2 branches of the westerlies, one from Old Mexico into the southern
                                        plains, and another from southern California into western Utah. Moisture was streaming north from the Gulf of
                                        Mexico with dewpoints in the 50sF as far north as Nebraska. Elevated moisture was present north of the surface
                                        front across the nothern plains with 750mb dewpoints/mixing ratios around > 5C / 7.5 g/kg).

                                        The 00 UTC April 23,  a weak cold front was about to push through Cheyenne where the dewpoint jumped into
                                        the upper 30s by 01 UTC, along with a wind shift to the northeast.  The airmass north of this boundary was
                                        actuallly moderately moderately unstable in the afternoon, but was becoming less so by 00 UTC. Dewpoints were
                                        in the 40sF in the Nebraska panhandle.  A sharper frontal zone was located from southern Montana into northern
                                        SD. The surface dryline eas rather ill-defined in Colorado but was more pronounced in west Texas, bulding east of
                                        Amarillo. A cold surge was apparent west of the Big Horns. A pacific cold front was located over western Utah.

                                        By 03 UTC April 23, convective outflow was apparent in western Kansas, eastern Colorado and southwest
                                        and southwest Nebraska. This area of outflow eroded from the south overnight with southerly low level winds
                                        shoving the rain cooled air further north.

                                        By 06 UTC, the weak cold front had apparently pushed through Denver as the sea level pressure jumped and
                                        the dewpoint jumped into the upper 30sF. The northern cold front was progressing south into southwest SD.

                                        By 09 UTC, the northern cold front was approaching Scottsbluff. Drier air had filtered into Chadron and Rapid
                                        City.

                                        The northern cold frontal passage at Cheyenne around 12 UTC was accompanied by low clouds and strong pressure
                                        rises. This front was overtaking the southern front.
                                       

                                        By 12 UTC April 23, two branches of the westerlies were still evident. A deep 500mb trough was still parked in
                                        the western US. The flow over the Rockies was meridional with the eastern edge of the strong flow from central AZ
                                        into western Colorado and central Wyoming. Ample elevated moisture was still in place north of the surface front, with
                                        700mb/750mb  dewpoints from 3 to 7 C (6 to 8.5 g/kg).

                                        The 15 UTC surface chart showed a cool surge in eastern Wyoming and western Nebraska. But surface dewpoints
                                        in the cool air were still 40F at Cheyenne(mixing ratio 6.5 g/kg) and 42F (mixing ratio 6.5 g/kg) at Sidney, NE.
                                        Dewpoints were in the mid 40s to lower 50sF immediately north of the surface front. For example, Imperial, NE and
                                        Akron, CO had  dewpoints of  53F and 46F respectively (mixing ratios 9.2 g/kg and 8.3 g/kg. Thus, given that the
                                        mixing ratios at the surface and 750mb were between 7.5 and 9 g/kg east of Cheyenne  in the morning, strong
                                        upslope flow would tend to advect this higher theta-e air onto the front range later in the day. The surface pressure
                                        at Cheyenne was about 800mb, so the lowest 50mb of moisture should have an average mixing ratio of 8 g/kg
                                        later in the day.  

                                        By 18 UTC, the surface winds were beginning to turn upslope in western Nebraska, northeast Colorado and southeast
                                        Wyoming. By 21 UTC  the dewpoint at Cheyenne had dropped to 38F due to vertical mixing. However, dewpoints
                                        just southeast and east of Cheyenne still ranged from 49 to 53F.  I do not know if the surface observation at Fraser,
                                        Colorado is reliable, but Fraser seems to be north of the front at 23 UTC. I am suspicious of this dewpoint temperature.

                                        At 23 UTC, there was a narrow corridor of higher dewpoints from southwest Nebraska and the southern Nebraska
                                        panhandle into northeast Colorado. The dewpoints at Sterling, CO, Imperil, NE and Sidney, NE were was 54F,
                                        51F and 49F respectively. The surface pacific cold front was still west of Vernal, UT and Rock Springs, WY and
                                        not into Colorado yet. I chose to draw the dryline just east of Grand Island, NE since the moisture was mixing
                                        out immediately ahead of the front. This is not the only solution. The surface cold front in Nebraska was now surging
                                        south. In fact the western end of the frontal push was around North Platte. Another baroclinic zone appeared to be
                                        entering the northern Nebraska panhandle.

                                        I was able to calculate the 500mb temperatures across much of Wyoming and Colorado at local noon based on
                                        a technique that I have been using for several years. This technique was partially based on Dr. Toby Carlson's
                                        pioneering work with the elevated mixed layer(EML) in the mid and late 1960s. Carlson was to first to document
                                        the existence of the EML. He used local noon surface temperatures over the dry and well mixed western US to
                                        calculate the dry adiabat that the well mixed temperature fell on. Hence, if you know the local noon surface
                                        temperature, you can estimate the potential temperature and 700mb temperature since the low to mid levels are
                                        dry adiabatic. In fact, I found that I was able to estimate the 500mb temperatures in very deep mixing situations.
                                        The surface dewpoint depression should be at least 50F and the station must be in the warm sector.
                                         I used a variation of the local noon temperature. In some cases, stations mixed out after noon or before noon.
                                         Soundings tend to become dry adiabatic through a deep layer by local noon. In the case of April 23, 1960, 
                                         I was able to calculate the 500mb temperature at Denver, Rawlins and several  other places by taking the
                                        midday mixout temperature and taking this up to 500mb. I compared this to the 00 UTC upper air chart and the
                                        values were very similar.
                                       

                                        By 01 UTC the surface dewpoint at Cheyenne was up to 46F (mixing ratio 8.3 g/kg). With breezy east-southeast
                                        to southeast surface winds (and surely even stronger boundary layer winds), this moisture had time to make it
                                        onto the higher terrain west of Cheyenne by 0210 UTC (time of the tornado). Since the terrain was 1500-2000ft
                                        higher west of Cheyenne where the tornado occurred, the dewpoint would be 1.5 to 2F lower at the tornado
                                        location since the dewpoint lapse rate is 1F/1000ft. Also, strong mixing and slightly lower 700mb dewpoints
                                        on the morning 700mb chart over the northern plains probably resulted in surface dewpoints around 42F (5.6C)
                                        where the tornado occurred (mixing ratio 7.5 g/kg). The station pressure at Cheyenne at 01 UTC was 801 mb.
                                        Meanwhile, at Dallas, TX the temperature and dewpoint were 80F/60F, with a station pressure of 995mb. But the
                                        surface theta-e was slightly higher at Cheyenne compared with Dallas.  The potential temperature and mixing ratio at
                                        Dallas/Cheyenne were 80.8F/98.3F and 11.2 /8.3 g/kg respectively. So despite the mixing ratio being 35% higher
                                        at Dallas and the temperature being 16F higher, the theta-e was actually higher at Cheyenne(335.7K vs 333.5K).
                                        The best way to accomplish high theta-e on high terrain in April is to have deep, southerly flow at and above 700mb,
                                        with upslope flow underneath. This helps steepen lapse rates with warm, moist low-levels and cool upper levels.
                                        This clearly demonstrates the importance of elevated heating on elevated terrain.                                     


01 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Cheyenne 6140 801 999.6 64 46 8.3 98.3
335.7
Dallas 487 995 1011.9 80 60 11.2 80.8 333.4



                                        At 02 UTC, the mixing ratio was 70% higher at Galveston than Cheyenne and the temperature was 7F warmer
                                        at Galveston. The dewpoint was 21F higher at Cheyenne.  However, the potential temperature was 99.5F at
                                        Cheyenne compared to 69.6F at Galveston. Therefore, the theta-e was higher at Cheyenne than at Galveston.


02 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Cheyenne 6140 800 998.4 65 46 8.3 99.5
336.6
Galveston 6 1016.6 1016.1 72 67 14.1 69.6 334.5



                                        The 00 UTC upper air charts showed the leading edge of the strong 500, 400mb, 300mb and 200mb flow
                                        somewhere between Denver and Grand Junction. To construct approximate soundings for Cheyenne 
                                        and  7700ft elevation to the west of Cheyenne, I used the 00 UTC thermal profiles at Rapid City, Denver and
                                        Glasgow, and 500, 400 and 300mb wind/temperature comparisons at 00 and 12 UTC.  Since the mid  to high                           
                                       level winds increase between 00 UTC and 12 UTC, and since the tornado occurred atter 02 UTC, I interpolated
                                        the winds from the surrounding upper air sites and then adjusted a little based on higher winds progressing east
                                        between 00 and 12 UTC.  I made use of  500, 400 and 300mb wind/temperature/front comparisons as well. One
                                        important thing to note is that the 500, 400, 300mb and 200mb temperatures changed very little by 12 UTC in
                                        southeast Wyoming.  Here are sounding comparisons at Denver, Rapid City and Glasgow which also show
                                        how the 500-300mb temperatrues changed very little after 00 UTC.

                                        The surface front was well west of the surface location at 00 UTC. Given the dry adiabatic profiles (that are
                                        implied from the surface to 500mb in the dry air at Rawlins), 500mb cold advection would have required surface
                                       cooling as well.

                                        As already discussed, 700mb temperatures were known with fairly high accuracy. Of course, surface dewpoint,
                                        surface temperature and surface pressure at Cheyenne were known quantities. I made a few assumptions to find
                                        the T/D at 7700ft out to the west of  Cheyenne. Surface based CAPE values were about 2000-2400 j/kg. 

                                        The surface wind was from 120 deg. at 20 kts.  The 500mb wind (3 km) was from 180 deg. at 35 kts. The 5km
                                        wind (400mb) was from 180 deg. at 50kts. The 7 km wind (300mb) was from the south-southwest at 60kts. Thus,
                                        the vertical wind shear was excellent.