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.