Severe Thunderstorm Outbreak
May 24  1990


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



Severe Weather Occurrences

                               
                                        May 24, 1990 was a very active severe weather day. Tornadoes occurred in 
desolate areas of Wyoming and
                                        Montana. The most violent tornadoes occurred in central Kansas. Several tornadoes occurred in Laramie county
                                        in southeast Wyoming. The early ones in southeast Wyoming were apparently weak and there are no photographs
                                        of these to my knowledge. The later ones in southeast Wyoming were very picturesque and one of these was large.
                                       
                                        At least 2 large tornadoes occurred in northeast Wyoming about 30 miles south-southwest of Gillette. The first
                                        one occurred 6 miles west of Wright in mostly open country and lasted for 8 to 10 minutes, tearing the roof off a
                                        hangar at the Durham Buffalo Ranch. The same storm produced another tornado about 10 miles northeast of
                                        Wright near the Hilight Road and Breene Roads. Another large tornado was photographed near the Pumpkin Buttes
                                        and lasted 10 to 15 minutes. This tornado was also over open country and only hit
a private repeater antanae and
                                        some power poles.


                                         Tornadoes also occurred west of the Big Horn mountains in north central Wyoming. These storms moved into
                                         southern Montana and produced more tornadoes. A tornado was also witnessed at Ellsworth AFB in south Dakota.                             
                                                                                  

                                                        Laramie county tornado pictures
                                                    
                                                   

                                                        Laramie1
    Laramie2   Laramie3  Laramie4   Laramie 5    coutesy Ian Wittmeyer, Stephen Hodanish and  David Blanchard
                                                                                                         
                                                       Campbell1
    Campbell2  Pumpkin Buttes tornado  courtesy  Dalene and Mike Moore (cooperative observer in Campbell county)

                                                       
                                       
  Campbell3       Tornado west of Wright  courtesy Gillette News Record,  photo taken by  Dale Muhlbauer -- Gillette resident
                                   




Meteorological Discussion   
     
           
Evening Before
                      
            
                                       

                                        A deep 500mb trough was developing across the western United States on May 23. Ahead of this feature, a
                                        700mb warm plume was located over the central and southern Rockies with 700mb temperatures from 12 to 15C.                    
                                        A weak frontal boundary was located across southern Nebraska. North of this boundary, upslope flow was
                                        transporting moisture into the Nebraska panhandle and eastern Wyoming. This initial surge of moisture was probably
                                        quite shallow. Indeed, dewpoints started to fall after sunset into the upper 40s at Douglas and Gillette. The 00 UTC
                                        North Platte sounding showed some deep moisture, but actual mixing ratios were  not all that high. A frontal boundary
                                        was moving slowly southwest in central Wyoming. By 03 UTC, the surface cold front associated with the deep,
                                        western US through was moving through Nevada. A front was also pushing east into Montana, western Wyomimg
                                       and southern Utah.
                      
           
           

Early Morning May 24        

                                   
                                
The pacific cold front was progressing into eastern Nevada by 06 UTC. The front that was moving through Montana,
                                        western Wyoming and southern Utah was beginning to become stationary.
A surge of moisture began overnight from
                                        far southwest Nebraska and northwest Kansas into northeast Colorado.  At 06 UTC, the dewpoint at Akron was up
                                        to 56F. This was probably deeper moisture return.

                                       
The 09 UTC surface chart shows the pacific cold front in eastern Nevada. Low level moisture was pushing into Akron
                                        where the surface dewpoint was up to 57F, along with advection fog.


                                        By 12 UTC, respectable surface dewpoints in the upper 50sF were present all the way west to Scottsbluff and Akron.
                                        The 12 UTC North Platte sounding showed shallow moisture, but it was probably deeper to the southwest and west
                                        of North Platte.

                                        The 12 UTC 500mb chart showed  a very strong shortwave trough centered over northeast Nevada (547 dm). Two 
                                        areas of strong flow were noted, one from San Francisco into Las Vegas (55kts) and another (60-65kts) in New
                                        Mexico and Arizona.              

                                        The 700mb warm plume had shifted east into the central and southern plains overnight. This is very typical though.
                                        Boundary layer cooling over the Rockies leads to cooler 700mb temperatures by 12 UTC.  The 700mb temperatures
                                        at Dodge City and Amarillo were both 14C.

                                        Rich moisture was noted at 850mb at Norman and Topeka. Even though the 850mb dewpoint was only 11C at
                                        North Platte with fairly shallow moisture, deeper moisture was likely moving upslope into northeast Colorado and
                                        southeast Wyoming. The 300mb chart showed a broad band of strong flow 70-80kts across the southern Rockies.                             
           

Late Morning   


                                        By 15 UTC, it is obvious that moisture is surging into eastern Wyoming. One can simply look at the surface
                                        observations from Douglas, Wyoming to figure this out. Notice that the moisture near the ground increases first as
                                        a 400ft sct deck of stratus accompanies an increase in dewpoint from 48 to 53F. Then between 14 and 15 UTC,
                                        an overcast deck appears at 4000ft AGL or 9800ft ASL. This is about 710mb. I suspect that this cloud deck may have
                                        been lower than 4000ft. But nevertheless, this still indicated an increase in deep low level moisture. The dewpoint
                                        steadily climbed to 56F by noon with breezy southeast winds at Douglas. The surface dewpoint at Cheyenne also
                                        jumped into the mid to upper 50sF between 12 and 15 UTC.

                                        The pacific cold front was charging into central Utah by 15 UTC.                          

                                        The pacific cold front was surging into southwest Wyoming at 17 UTC but was not yet through Big Piney.  The
                                        1646 UTC visible satellite picture shows cumulus clouds developing in the dry air over central Wyoming. So the low
                                        level thermal profile must have been dry adiabatic up through the mid levels by 17 UTC. By 17 UTC, cumulus were
                                       developing along the edge of the low level moisture in the vicinity of Casper, WY. 

                                        By 18 UTC (11 am MST), thunderstorms were beginning to develop just northeast and east of Casper and to the
                                        northwest of Cheyenne along the eastern edge of the Laramie Ridge. The 1830 UTC picture shows even more
                                        storm development. Storms(probably high based) were also developing on the Big Horn mountains.                 

                                        The pacific front was well east of Big Piney by 18 UTC. Ahead of this the Wyoming front was beginning to move
                                        north as a warm front. But further east near Casper and between Cheyenne and Laramie this boundary was stationary.                      

           
           
Mid-day


                                        
By 19 UTC MST, storms were developing in the immediate lee of the
 Big Horns. Storms were still occurring northeast
                                        of Casper and in Platte county WY. Storms were also developing around Cody in northwest Wyoming as the 1830
                                        and 1852 UTC observations from Cody indicated a thunderstorm. The pacific front at 19 UTC was already east of
                                        Lander and Rock Springs, WY and still charging east. Dewpoints were in the mid 50sF at Sheridan and Billings.
                                        Enough instability was in place east of the Big Horns for severe storms. The 19 UTC surface chart also indicates
                                        northeast low level winds advecting low level moisture into south central Montana and almost as far southwest as Cody.


                                        
The 20 UTC surface chart showed the pacific front still charing east through Rawlins, WY. The surface dewpoint
                                        at Billings was up to 55F. The elevation at Billings is only 3100 ft though, so these values cannot be directly compared
                                        with Cheyenne and Douglas unless we calculate theta-e. Its is possible that lower 50sF dewpoints wrapped around
                                        the back side of the Bighorn mountains since the surface wind at Billings was from the northeast during the morning and
                                       early afternoon as shown on the 17 and 19 UTC surface maps.  
                   

                                        By 20 UTC (1 pm MST), the storms that developed near Casper were now about 40 miles northeast of Casper.
                                        Storms were still rumbling across Platte and northwest Laramie counties in Wyoming.

                                        The first severe report with these storms was a tornado at 2010 UTC just northwest of Wheatland in Platte county.
                                        The satellite picture 15 minutes before this tornado is here. Golfball sized hail was reported in exactly the same location
                                        at 2030 UTC(perhaps a delayed report the same storm?). 1 inch hail was reported east of Iron Mountain at 2030 UTC.
                                        A tornado was reported in western Laramie county just east of Horse Creek at 2045 UTC. Apparently this storm
                                        moved northeast and produced other tornadoes at 2055 UTC and 2105 UTC in wide open country with no damage.
                                        This storm initially developed near Buford as can be seen on the 1956 UTC visible shot.           

                                        A thunderstorm exploded in the immediate lee of the Big Horns west of Buffalo, WY between 1930 and 2020 UTC.
                                        This thunderstorm likely became severe but no severe weather was experienced and reported.
                                       
                                        The storm that developed near Cody around 1830 UTC progressed north or north-northeast and was near close to
                                        the Montana border by 20 UTC. Storms were ongoing in an arch from north of Cody to northeast of Casper to
                                        northwest of  Cheyenne at 20 UTC.
                               

                                       
The 21 UTC surface chart shows the pacific cold front surging through Rawlins, WY.
                    
                               
           
Early Afternoon

                                   

                                       
A thunderstorm erupted in southwest Kansas between 2026 UTC and 2046 UTC. By 2143 UTC this storm produced
                                        1.25" hail and a F3 tornado occurred at 2210 UTC. A second storm developed behind the first one and produced
                                        weak tornadoes at 2200 and 2203 UTC. Violent tornadoes occurred with the first storm through several counties in
                                        central Kansas. Storms also developed by 2046 UTC in far northeast Wyoming on the west edge of the Black Hills.

                                        The storm that developed near Cody apparently moved into better moisture and produced a tornado in extreme
                                        southern Montana near Warren (4400ft) at 2100 UTC. Some minor damage (F1) was reported. Another tornado
                                        occurred just north of Billings at 2200 UTC. This tornado could have been produced by the same storm although
                                        the times indicate otherwise (60 mph storm motion seems very unlikely).

                                        The 2131 UTC visible shot shows thunderstorms continuing over southern Campbell county in northeast Wyoming.
                                        No severe weather has been reported thus far with these storms. Storms were also occurring in near the Black Hills
                                        of South Dakota and over southeast Wyoming. A tornado was reported close to Ellsworth AFB at 2216 UTC.

                                        
The following illustrates the important contribution of elevated heating on the front range to theta-e. At 20 UTC, the
                                        temperature and dewpoint(T/Td) at Cheyenne and Dallas were 70F/57F and 85F/69F respectively. Despite the
                                        temperature and dewpoint being 15F/12F higher at Dallas than at Cheyenne, the theta-e was actually higher at
                                        Cheyenne. This is because the potential temperature was 106F at Cheyenne compared to 87F at Dallas. Keep in
                                        mind that a dewpoint of 57F at Cheyenne has the same amount of moisture as a 63F dewpoint at Dallas. So on
                                        a "level playing field", the dewpoint was about 6F  higher at Dallas, resulting in a mixing ratio that was 24% higher.
                                        Obviously elevated heating more than compensated for the moisture deficit at Cheyenne.  
                              


20 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) Td(F) MR(g/kg) theta(F) theta-e(K)
Cheyenne      6140 796 998.9 70 57 12.6 105.6 353.5
Mcalester 600 982 1009.3 81 69 15.7 83.8 348.5
DFW 530
988 1009.1 85 69 15.6 86.9 350.2


           
Late Afternoon



                                        At 22 UTC the cold front was surging through Casper, WY. The front was passing through Worland at 22 UTC. It
                                        The observation at Worland was time stamped 2150 UTC but was probably make prior to this. Instead of analyzing
                                        the front southwest of Worland, I put the front right over them at 22 UTC.

                                        The 2230 UTC visible shot indicated a line of cloudiness developing along or just ahead of the pacific cold front along
                                        a line from Big Horn county southeast to near Casper. However, notice that there is a storm out ahead of this cloud
                                        band. This storm may have been the one that produced a tornado 6 miles west of Wright at 2335 UTC and north of
                                        Wright at 2349 UTC. The 23 UTC and 00 UTC surface charts suggest that the tornado at 2330 UTC was out
                                        ahead of the cold front. Keep in mind that paralax errors are present in these satellite images. Another tornado was
                                        observed at 0030 UTC near North Butte in southwest Campbell county. This storm must have occurred along or
                                        immediately ahead of the pacific cold front. The frontal position from hour to hour through 00 UTC is shown here.
                                        This tornado picture was taken by a Dalene Moore. Her husband is a cooperative observer for the Rapid City NWS.                                                            
                                        Several storms were also occurring in central and north central Montana at 2230 UTC but no severe weather was
                                        reported with these storms.

                                        A picturesque rope tornado and a larger tornado occurred in southeast Wyoming around 23 UTC. The larger tornado
                                        was photographed by Dave Blanchard while the beautiful rope tornado was photographed by Ian Wittemeyer and
                                        Steven Hodanish.

                                        There were only 5 reports of severe hail in Wyoming on May 24 1990, and 4 of these were in Laramie county. No
                                        large hail was reported with the storms in northeast Wyoming. This is not surprising given that this region is desolate,
                                        with only 2 small towns over a 6,000 sq. mi. area. From January 1955(beginning of hail records) to December 1973.
                                        there were no hail reports larger than 3/4" in this huge, unpopulated area. There were a few hail reports from 1974 to
                                        1992 that were clustered near Wright along highway 59.

                                        At 00 UTC, the center of the 500mb low was just east of Pocatello, ID (550 dm). The 500mb temperature at
                                        Glasgow, MT was -14C and this is probably similar to the 500mb temperature near the northeast Wyoming tornadoes.
                                        A shortwave trough was apparently located over western Kansas. The 500mb flow over central Kansas was
                                        moderately strong (50kts) and veered to the west-southwest. The 700mb warm plume had shifted back to the eastern
                                        Rockies by 00 UTC, with 700mb temperatures of 13C at Denver and 15C at Albuquerque. The 700mb wind speeds
                                        across Kansas were strong with 35 kts from the southwest at Dodge City and Topeka. The 850mb wind at Topeka
                                        was a strong 40kts. Other charts:  400mb    300mb                                                                                                       

                                        Dewpoint depressions were not all that high in the dry air. So we can't assume like we have in other cases that the
                                        thermal profile in the dry air was dry adiabatic from the surface to 500mb. Nevertheless, it appears that the low to
                                        mid levels were close to dry adiabatic at least through 700mb.                      

                                       
18 to 20 UTC Elev(ft) Pres.(mb) SLP(mb) T(F) 700mb T 500mb T
Casper
5300 817 1000 77-81 12 -14
Rawlins 6813 787 1004 72 11-13 -14
Worland 4227 853 1010.1 78-82 10
Laramie 7270 768 1011.9 72 13-14 -12 to -13
Eagle 6540 782 75 13 -13
Aspen 7820 749 68 14 -12


                                        Let us compute the surface based CAPE for the northeast and southeast Wyoming tornado events. We will use the
                                        surface observations from Cheyenne along with the 00 UTC upper air charts to compute the CAPE available for the
                                        Laramie county storms. Then we will use surface observations from Douglas to compute the CAPE for the northeast
                                        Wyoming tornadoes. Keep in mind that the temperature was likely warmer northwest of Douglas since Douglas may                     
                                        have been influenced by cool outflow from storms to the southeast.                                 

                                        The surface based CAPE at Cheyenne was 3500 j/jg while the CAPE at Douglas was 2600 j/kg.