This is a preliminary analysis of the October 1997 blizzard. This storm brought up to 4 feet of snow to eastern Colorado and adjoining mountains with snowdrifts up to 15 feet. Special weather statements issued by the NWS offices in Pueblo and Denver summarize the snowfall totals. A few additional reports for southeast and south central Colorado are listed here. The METAR observations for Pueblo and Colorado Springs are listed.
The analysis will utilize satellite imagery, surface analysis, and upper air analysis. Data for the upper air analysis are the initial RUC. In a few instances, the initial RUC data were not available and the three hour forecast from the previous RUC run was used. Data for the surface analysis came from the surface RUC (RSAS). The satellite images are from GOES-8. Data are presented in 3 hour intervals from 1200 UTC on 24 October to 1200 UTC on 26 October. Because of missing data, the times on the plots may not match exactly, but the greatest difference in time is only 1 hour. The plots were generated using GARP, which was developed at COMET.
This study is not meant to be a detailed analysis of the storm, but it does clearly present some of the fundamental elements of the storm. I view the storm as having two phases. Phase 1 lasted until about 1200 UTC on 25 October. This phase is characterized by a cutoff low slowly moving south near the Colorado/Utah border. Cold air is surging south over the eastern plains producing strong north to northwest winds. The second phase occurred after about 1200 UTC on 25 October. During this phase the upper low redeveloped over northern New Mexico and a strong surface low developed over southern Kansas. After the discussion of the storm, plots for various fields with satellite imagery are given at 3 hour increments.
At 1200 UTC, 24 October a 500 mb cut off low was over Northern Utah dividing southward. Cold air at the surface was poised to move southward into Colorado. IR cloud field shows some cooling of cloud tops over central Colorado. A plot of the 300 mb isotach and dynamic tropopause shows the cutoff low had a 120 knot jet maximum on its west side. The cutoff low was forecast to continue moving south. The pressure of the dynamic tropopause (where potential vorticity equals 2 PVU) shows a lowering of the tropopause associated with the diving cutoff low.
At 1500 UTC the 500 mb cutoff moved to the Colorado/Utah border with continued cooling of cloud tops over central Colorado. Surface cold air continued to progress southward toward Colorado. A broad area of lower pressure remained over eastern New Mexico and the Texas/Oklahoma Panhandle.
At 1800 UTC the upper low continued its slow southward progress with cold surface air continuing to move southward over the eastern plains. The area of colder cloud tops (as seen on the IR image) moved east and extended further south. At this time rainfall began over southern Colorado. Rain was observed up to 7000 feet MSL.
At 2100 UTC the cutoff low continued to progress southward and cold low level air funnelled south across the eastern plains. The area of colder cloud tops continued to move east across southern Colorado. The snow level dropped to around 5000 feet over southern Colorado.
At 0000 UTC 25 October the cold air continued racing southward over the eastern plains with wind gusts to 40 knots fairly common. Peak gusts reached 50 knots. The strong surface winds were associated with the cold air plunging southward over eastern Colorado. By this time rain had changed to snow near the mountains, including Colorado Springs and Pueblo. By 0600 UTC the rain changed to snow over the entire southeast plains. The IR cloud fields show a continued blossoming of the clouds on the east and northeast side of the upper low. A broad area of surface low pressure remained over eastern New Mexico and the Texas/Oklahoma Panhandles.
The jet stream and dynamic tropopause plot shows the jet streak started to "turn the corner" around the upper low. Winds over 100 knots remained on the west side of the upper low. The dynamic tropopause shows one area of lower tropopause over southwest Colorado associated with the cutoff low. A second area of lower tropopause was over Lake Mead (extreme southern Nevada). The water vapor image shows an area of drying associated with this second disturbance.
The clouds over eastern Colorado were associated with strong isentropic lift. The 304K surface and 300K surface show upglide associated with most of the cooling cloud tops, especially over the northern three-quarters of eastern Colorado. (Besides the "pressure advection term", another term in the vertical motion equation on isentropic surfaces is associated with the movement of isentropic surfaces. In this case the isentropic surfaces remained at the same pressure or "lifted" with time. Isentropic surface rising with time would enhance the rising motion calculated purely from the advection of pressure on an isentropic surface).
At 0300 UTC on 25 October the cold air continued to plunge south along the eastern plains with the 500 mb cutoff slowly drifting south. The IR image shows continued "blossoming" of the cloud pattern over eastern Colorado. This blossoming cloud field continued to be associated with strong isentropic lift east and north of the upper low as seen by the 304K surface and 300K surface.
At 0600 UTC 25 October the 500 mb low moved to the four corners region. The cloud field continued to increase east and north of the upper low. A broad area of lower pressure remained over the Texas Panhandle. Cold air had raced into eastern Colorado. The tropopause and jet stream show the disturbance, which was over Lake Mead at 0000 UTC, advancing into eastern Arizona. The water vapor image shows a dry slot advancing toward southeast Colorado.
At 0900 UTC 25 October the colder cloud tops have moved over northern Colorado. The influence of the dry slot was beginning to affect southeast Colorado. The precipitation coverage decreased and the winds on the eastern plains started to weaken slightly. A secondary area of colder cloud tops is evident over the Texas/Oklahoma Panhandle. A broad area of low pressure remained over northern Texas, eastern New Mexico, and Oklahoma. The inverted trough in southwest Kansas represented the eastern boundary of the cold arctic air. A disturbance, as indicated by the jet streak and lower tropopause, continued to move eastward across central Arizona and New Mexico. Drying is evident on the water vapor image.
At 1200 UTC the "second stage" of the blizzard begins. The upper low appears to have moved to north central New Mexico. The tropopause and jet stream plot indicates the upper low over the four corners weakened. A new upper low developed over northern New Mexico, associated with the jet streak/disturbance moving across New Mexico. A broad area of low pressure remained over northern Texas and Oklahoma. The IR loop shows blossoming of the cold cloud tops over southeast Colorado.
By 1500 UTC a surface low developed over western Oklahoma and southwest Kansas. The cloud tops have continued to cool over southeast Colorado. A strong pressure gradient between the low over Kansas and high over Wyoming intensified the surface winds. Isentropic plots indicate strong isentropic upglide over southeast Colorado.
The strong surface low developed as the upper disturbance reached the baroclinic zone over western Kansas and western Oklahoma. Plots of 350-400 mb potential vorticity, surface equivalent potential temperature, and sea level pressure shows that the cyclone developed where the PV anomaly reached the low level theta-e ridge (1200 UTC and 1500 UTC).
(Author's note: GARP/NTRANS/GEMPAK does not have the ability to apply the cowbell filer to model data (see the article in March 1996 BAMS by Stan Barnes). As a result, Q-vector plots very likely are noisy. I'm certain there are strong QG signals in this situation as well. Of course, the atmosphere is the same whether you examine it using PV or QG.)
At 1800 UTC the blizzard continued. The surface low remained nearly stationary over southern Kansas and the 500 mb low slowly moved east. Strong winds continued over the southeast plains resulting from the strong pressure gradient. Winds gusted to over 50 knots over the plains. Isentropic analyses showed continued strong upglide over southeast Colorado.
At 2100 UTC the surface low remained nearly stationary with strong winds on the eastern plains. Precipitation continued over much of the eastern plains.
At 0000 UTC 26 October the surface low remained over southern Kansas, as the upper low drifted eastward. Moisture continued to be wrapped around the north and west side of the upper low.
At 0300 UTC the surface low remained over southern Kansas maintaining strong winds especially over the southeast plains. As the upper low continued to drift east, the region of colder cloud tops gradually moved east. A good match between isentropic upglide and colder cloud tops continued.
At 0600 UTC the surface low weakened as the upper low moved to over the surface low. Strong winds remained over the eastern portions of the plains and precipitation over southeast Colorado decreased.
At 0900 UTC the surface low redeveloped further east and the surface pressure gradient over eastern Colorado weakened.
At 1200 UTC the pressure gradient over Colorado was weak.