CRH SSD

March 1998

CENTRAL REGION TECHNICAL ATTACHMENT 98-01

 

Study of an Unusual Snow Event in the Wind River Basin of West Central Wyoming
 
Robert A. Bruce
National Weather Service
Grand Junction, Colorado

 

 I.  INTRODUCTION

Riverton (RIW), at 4956' MSL elevation, is located near the center of the Wind River Basin at the confluence of the Little Wind and Wind Rivers. RIW is slightly higher than Shoshone, 22 miles to the northeast at 4817' MSL. Upslope (mechanical lifting) along that direction (northeast toward the southwest) is minimal. Lander (LND), 22 miles southwest of RIW is at 5586' MSL and is in the foothills leading up to the Continental Divide. The Wind River Mountains just southwest of LND, on a northwest to southeast orientation, rise to an elevation of over 12,000 feet (Figure 1).

Figure 1. Terrain Map - Wind River Basin and Wind River Mountains.

This, of course, can produce a much greater upslope lift (Mass 1988) and the mountains and foothills receive heavy snow during moist, northeast, forced, upslope conditions. LND averages just over 100 inches of snow per year while RIW only averages about 30 inches, roughly a 3 to 1 ratio.

During the period January 15-17, 1995, a weather system moved through the area and left a foot of snow on the ground at RIW, but only five inches at LND. This heavier snowfall at RIW is a reversal of that typically observed, and the purpose of this paper is to document the case and determine, if possible, what may have caused the unusual snowfall distribution.

II.    SYNOPTIC SITUATION

On the morning of January 15, 1995, a weak ridge axis lay just east of the Rocky Mountain states. A jet streak was analyzed on the 1200 UTC, 300 hPa chart, with a trough along the west coast, to northern Colorado. An area of divergence at 300 hPa had formed over north central Wyoming at the left front quadrant of the jet streak (Figure 2).

Figure 2. 300 hPa heights (decameters), wind (knots), divergence (10-6/sec) from initial NGM, 1200 UTC 15 January 1995.

This location is often associated with upward vertical motion in the mid levels and convergence at the lower levels of the atmosphere (Bluestein 1993). This forcing may have been the main mechanism for the surface low that developed over central Wyoming that morning.

A weak positive vorticity lobe at 500 hPa extended from central California into southwest Wyoming. Cold air-advection and winds perpendicular to the mountains may have contributed to the flattening of the upper level ridge (Knight and Williams 1992) over eastern Wyoming. There was abundant moisture (1 to 2 degrees C dew point spread) at 500 to 700 hPa during the early phase of this developing system and increasing moisture at the lower levels. Cold air-advection from the west at the mid to upper levels contributed to the 1000-500 hPa thickness decreasing over western Wyoming.

The mean sea level pressure (MSLP) surface low at 1200 UTC had formed in the center of Wyoming (Figure 3), in close agreement with the upper-level divergence maximum. Surface observations showed that initial surface winds were light and variable at both locations. Snow began to fall before sunrise at both LND and RIW on January 15. Snow was light and mixed with rain at times during the morning at both locations but turned to light snow by early afternoon. During the day, the upslope was confined to the lowest levels near the surface, with relative humidity in the S982 (bottom sigma layer of NGM) at 70 to 75 percent.

Figure 3. Surface analysis, 1200 UTC 15 January 1995.

By 0000 UTC on January 16, the divergence area at 300 hPa had moved to the southeast and was located over southeast Wyoming and northeast Colorado. The surface and 850 hPa lows also moved to the southeast. The east slopes of the Wind River Mountains were in a weak, moist upslope flow. The 0000 UTC upper air sounding at LND showed a dew point depression of less than or equal to 1 degree C from the surface all the way up to 583 hPa. The synoptic-scale scenario, with large-scale weak ascent, pointed toward significant snowfall at LND and the foothills west and southwest of LND, with lighter snow expected near RIW.

 III.  OBSERVATIONS

Snow continued all night (from sunset on January 15 to sunrise on January 16) at both locations. Snow continued falling steadily around RIW, but became very light at LND with visibilities as high as 20 miles at times; very unusual for this type of system. Surface winds at RIW stayed light from the north to northwest which provided minimal terrain effect. By contrast, winds at LND had switched by January 15 at 1800 UTC to the southwest and although light (5 knots or less) implied a weak downslope; also unusual. By local sunrise on January 16 (1442 UTC), parts of RIW reported seven inches of snow on the ground while LND had only two inches. The usual snow distribution was greatly reversed. Isentropic up-glide was neutral west of LND on January 16 at 0000 UTC, but just east of LND weak to moderate up-glide was analyzed. This would have contributed to greater lifting of moisture over RIW (Moore 1992).

On January 16 at 1200 UTC, the 300 hPa jet was progressing east and there was very weak divergence indicated over west central Wyoming. A weak 500 hPa low was just northwest of LND with weak to neutral flow and dynamics over west central Wyoming. The 700 hPa low was in southeast Wyoming and the winds were very light. Vertical velocities (omega) at 700 hPa were near zero. Precipitable water values were less than 3/10 of an inch. However, a 1200 UTC cross-section from RIW, to the foothills near LND, showed consistency between LND and RIW (Figure 4) with >90% RH.

Figure 4. Cross-section with wind vectors and percent RH; 1200 UTC 16 January 1995. Latitude/Longitude are along the bottom (Riverton is along the right side, Lander is near the center).

The winds at 850 hPa were nearly parallel to the mountains. This weakened the upslope flow near LND. The 1200 UTC surface analysis the morning of January 16 showed the surface low in northeast Colorado, which often indicates low-level upslope flow over west central Wyoming.

There was the interesting development of a quasi-stationary front (or trough) that extended from the MSLP low center in Colorado, up through west central Wyoming between LND and RIW, into western Montana (Figure 5). This position put LND in apparent weak low level downslope flow. The upper air soundings at LND from 16/0000 UTC through 17/0000 UTC showed north to northwest winds in the low levels indicating little, if any, upslope flow. The air was nearly saturated with dew point depressions on January 16 at 1200 UTC between 1 and 2 degrees C from the surface to about 550 hPa.

Figure 5. Surface analysis 1200 UTC 16 January 1995.

Snow depths, in the vicinity of RIW, late in the day on January 16 ranged from eight inches to over a foot. Snowfall at LND on January 16 reached a maximum snow depth of five inches late in the day.

By January 17 at 0000 UTC, the 500 hPa trough had moved east of Wyoming, along with the 700 hPa and 850 hPa low centers. The MSLP low and frontal system were entering southwestern Minnesota. Relative humidity was still >90% at 500 hPa over the area of interest, but had dropped to near 80% in the lower levels. Shortly after 0000 UTC on January 17, the snow ended.

 IV.  SUMMARY AND CONCLUSION

This study documents a unique snowfall situation in west central Wyoming where the usual upslope terrain forcing was apparently not the dominant mechanism. Snow amount at RIW, at an elevation 630' lower and 22 miles northeast of LND, more than doubled the snow amount at LND.

From January 15 at 1200 UTC to January 16, 1995 at 0900 UTC, the surface low tracked into southern Wyoming. This track is usually favorable for significant snowfall over the east-facing slopes of the Wind River Mountains, including RIW and LND. Deep moisture also accompanied the evolution of this event in west central Wyoming.

However, by 1200 UTC on January 16, a surface boundary (trough or front) developed just northeast of LND (Figure 4). This effectively cut off the upslope flow in that region and generated occasional weak downslope winds during a period of favorable large scale ascent. Soundings at LND showed a lack of northeasterly (upslope) flow throughout the event in the low levels. The exact mechanism for heavy snow at RIW was difficult to determine, specifically due to the lack of observed data on the mesoscale. Large-scale conditions were favorable for general area-wide precipitation and no evidence for localized downslope at RIW was found. Frontogenesis just west of RIW and associated convection may have been significant factors in the localization of heavy snowfall areas.

In the future, with the installation of the WSR-88D radar at RIW, improving satellite imagery, and the stationing of full time, on-site forecasters, enhanced analyses of snowfall cases in central Wyoming will be possible.

 V.  ACKNOWLEDGMENTS

The author would like to thank Dr. Douglas Wesley (Comet, Boulder, CO) and Dr. Michael Meyers (SOO, Grand Junction, CO) for their helpful reviews of this paper and assistance in preparing the final manuscript.

 VI.  REFERENCES

Bluestein, Howard B., 1993: Synoptic-Dynamic Meteorology in Mid-Latitudes, Volume II, Observation and Theory of Weather Systems, Oxford University Press, New York, NY, 594p.

Knight, David J. and R.T. Williams, 1992: Baroclinic Flow over Mesoscale Mountains. Preprints, Fifth Conference on Mesoscale Processes, Atlanta, AMS (Boston), 36-37.

Mass, Clifford F., 1988: Topographic Influence on Weather Systems. Postprints, Colloquium on Synoptic Meteorology, National Center for Atmospheric Research (NCAR), Boulder, CO, 224-236.

Moore, J.T., 1992: Isentropic Analysis and Interpretation: Operational Applications to Synoptic and Mesoscale Forecast Problems. St. Louis University, St. Louis, MO, 88pp.

 


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