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
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). 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.