The Routt Divide Blowdown

John S. Snook (Colorado Research Associates, Boulder, CO)
Micheal P. Meyers (NOAA/National Weather Service, Grand Junction, CO)
Gregory S. Poulos (Colorado Research Associates, Boulder, CO)
Douglas A. Wesley (UCAR, COMET, Boulder, CO)

    OVERVIEW

    A wide variety of significant weather covering a large region was observed during the Rocky Mountain winter storm of 23-25 October 1997. Two particularly hazardous areas included, 1) blizzard conditions and heavy snowfall from Wyoming to southern New Mexico along the Front Range of the Rocky Mountains, and 2) extreme winds exceeding hurricane force at several locations west of the mountains. Observations showed total snowfall amounts exceeding 100 cm (40 inches) in many Colorado mountain locations east of the Continental Divide. Meanwhile, on the west side of the Divide, snowfall amounts were light, but high winds caused numerous weather related problems. Wind gusts at 3800 m (12,500 feet) exceeded 45 m s-1 (100 mph) for 5 hours at the Arapahoe Basin ski area just west of the Continental Divide, and a peak gust of 51 m s-1 (114 mph) from the east was recorded (Table 1). Very cold mountain top temperatures around -20 C (-4 F) created extreme wind chill temperatures as low as -50 C (-60 F). Wind gusts were possibly even higher in the Routt National Forest and Mount Zirkel Wilderness areas, northeast of Steamboat Springs, where a region of severe wind destroyed over 20,000 acres of "old growth forest." According to the United States National Forest Service (USFS), this was the largest known forest blowdown ever recorded in the Rocky Mountain region with an areal extent of several miles wide and 20 miles long (see a USFS map of the damage area). In addition, fallen trees were stacked up to 30 feet in some locations, requiring nearly two days for rescue operations in the devastated forest (Wesley et al. 1998). The destruction had the appearance of a Colorado Front Range downslope windstorm, but it was unusual in that the strong winds were easterly and the damage was located to the west of the mountain barrier. Further observational details of this storm are described by Wesley et al. (1998) and more blowdown information is available from the USFS. The NOAA Forecast Systems Laboratory (FSL) has been demonstrating a data assimilation system called LAPS (Local Analysis and Prediction System). LAPS is designed to function in a local forecast office using affordable computer hardware. All available data sources are collected and used to generate local area analyses of the atmosphere. The analyses are then available as initialization to local area forecast models. The National Centers for Environmental Prediction (NCEP) suite of numerical prediction models provided excellent regional scale guidance for the area and timing of heavy snow (Poulos et al. 1998). These national domain models, however, are not configured to provide detailed forecasts of mesoscale phenomena such as the snowfall variability and the local areas of extreme winds observed in the October 1997 Rocky Mountain storm. LAPS analyses and forecast products are designed to provide additional mesoscale prediction guidance and understanding of local weather events to the local forecast office. The utilization of LAPS helps to define the unique aspects that caused the forest blowdown event.

    LOCAL AREA MODEL FORECAST

    High-resolution, local-area model forecasts are designed to provide improved mesoscale detail during highly variable mesoscale weather events. Peak wind gusts were observed during the morning of 25 October between 1100 and 1700 UTC (Wesley et al. 1998). The predictive component of LAPS is designed to utilize any available mesoscale forecast model. Typically, LAPS analyses are used to initialize the forecast model, and a larger domain model (usually NCEP's Eta model) is used as forecast lateral boundary conditions. The archived LAPS analyses are not available at this time for this study. As a substitute, model initializations were accomplished using the 60-km Rapid Update Cycle (RUC) analyses (Benjamin et al. 1991). Plans are to retrieve the LAPS analyses for the October 1997 case and to re-run the local area forecasts.

    An 18 h forecast, using the the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University (Pielke et al. 1992), was initialized with 25 October 0600 UTC RUC analyses and used the NCEP operational 48 km Eta model as forecast lateral boundary conditions. A nonhydrostatic version of the model is implemented with a full microphysics option. RAMS was configured with a two-way interactive double grid domain, in which the outside domain used a 61 x 61 15-km horizontal grid covering nearly all of Colorado and Wyoming and portions of surrounding states. The inner nest domain used a 71 x 71 5-km grid that covered the regions of observed high winds (Fig. 1).

    The topography used by the RAMS model is depicted in Figure 1. The large area of forest destruction occurred to the north-northeast of Steamboat Springs in the Routt National Forest and Mount Zirkel Wilderness areas located just west of the Continental Divide (Wesley et al. 1998). The Arapahoe Basin ski area, which reported a peak wind gust of 51 m s-1, is located approximately 80 km west of Denver (Fig. 1). The 6 h wind forecast from RAMS (Fig. 2) indicates three areas of high winds exceeding 17.5 m s-1: 1) north of Steamboat Springs and west of the Continental Divide, 2) along the Continental Divide in the vicinity of the Arapahoe Basin ski area and 3) west of the Continental Divide in Grand County (approximately halfway between Steamboat Springs and Denver). RAMS shows a north-south elongated region of high winds that corresponds well with the area of forest destruction; however, the magnitude is underforecast based on the amount of forest destruction. The underforecast is partially due to the model predicting a sustained wind, and it is unrealistic to expect the model to capture the strength of peak gusts using these model grid resolutions.

    RAMS also appears to have successfully predicted the area of high winds along the Continental Divide west of Denver as observed at the Arapahoe Basin ski area, but again the magnitude has been underforecast. Although no high winds or forest destruction were observed west of the Continental Divide in Grand County, it is reasonable to expect that high winds did occur in this region but they were not observed in this sparsely populated area which is mostly above tree line.

    Predictions of high winds downwind of the mountain barrier suggest that mountain wave activity was an important component. A west to east vertical cross section through the region of strongest winds north of Steamboat Springs is depicted in Figure 3 for the 6 h RAMS forecast. The forecast shows a very stable lower troposphere and a relatively unstable middle and upper troposphere. A well-developed mountain wave is indicated within the stable layer with winds exceeding 25 m s-1 on the lee of the Continental Divide and a relative wind speed minima of less than 10 m s-1 in the upper portion of the wave. The near vertical isentropes and weak winds to the lee of the mountain barrier suggest evidence of a wave induced critical layer located along the stability gradient. Strongest winds are indicated beneath this layer and are likely the result of vertically propagating gravity waves reflecting off the critical layer.

    CONCLUSIONS

    Results from the high-resolution, local-area model forecast highlight several of the unusual features that came together resulting in a rare event of old growth forest blowdown. Linear mountain wave theory suggests a direct correlation between mountain barrier height and strength of barrier induced downslope winds. The barrier height in the vicinity of the Mount Zirkel range is relatively low by Rocky Mountain standards, with an average height of about 1000 m which is roughly half that of the Colorado Front Range (Fig. 1). The lower barrier height also allows forest growth which is non-existent over the higher terrain of the Front Range. The fact that old growth forest exists in the areas northeast of Steamboat Springs is testimony to the rare nature of this event.

    The indication of very strong downslope winds over a relatively low barrier suggests the importance of nonlinear effects in this event. Forecast model output corroborates this suggestion as evidenced by the wave induced critical layer and enhanced wind speeds beneath this layer. Observations and forecast model results indicate the unusual juxtaposition of two features. 1) Strong synoptically driven easterly flow and 2) very cold low tropospheric air contributing to a stability profile that favors the enhancement of mountain wave development by nonlinear effects. Although both of these features do occur with some regularity over Colorado during the cold season, the strength of both features at the same time was unusual. Typically, strong easterly winds over the mountain barrier result from a deep cyclonic system, as was the case for the blowdown event. These cyclonic systems, however, are generally not accompanied with an extremely cold boundary layer. Very cold boundary layers are more typically observed with shallow anticyclonic events which normally generate weaker easterly flow over the mountain barrier.

    The likely explanation for this rare event is the combination of a deep, very cold boundary layer and strong, cyclonic easterly flow over a relatively low mountain barrier which created the proper conditions to generate a severe downslope wind storm that destroyed many acres of old growth forest. High resolution, local area model forecasts provide an important component in formulating conceptual models of highly variable mesoscale events including the rare Rocky Mountain blowdown event. The case study demonstrates the potential capability of operationally predicting these rare events using LAPS, which incorporates these local area models, combined with all other operational products.

    ACKNOWLEDGEMENTS

    Drs. Roger Pielke and William Cotton of Colorado State University and Dr. Craig Tremback of Mission Research Corporation are acknowledged for their continued permission to use RAMS for this project. RAMS was developed under the support of the National Science Foundation and the Army Research Office. Alan Henceroth provided observations from the Arapahoe Basin ski area.

    REFERENCES

    Benjamin, S. G., K.A. Brewster, R. Brummer, B.F. Jewett, T.W. Schlatter, T.L. Smith, and P.A. Stamus, 1991: An isentropic three-hourly data assimilation system using ACARS aircraft data. Mon. Wea. Rev., 119, 888-906.

    Pielke, R.A., W.R. Cotton, R.L. Walko, C.J. Tremback, W.A. Lyons, L.D. Grasso, M.E. Nicholls, M.D. Moran, D.A. Wesley, T.J. Lee, and J.H. Copeland, 1992: A comprehensive meteorological modeling system - RAMS. Meteor. Atmos. Phys., 49, 69-91.

    Poulos, G. S., D. A. Wesley, J. S. Snook, and M. P. Meyers, 1998: Modeling small-scale spatial heterogeneity of snowfall in complex terrain: The October 1997 Front Range blizzard. Preprints, Eighth Conference on Mountain Meteorology, 3-7 August 1998, Flagstaff, AZ, Amer. Meteor. Soc., 37-40.

    Wesley, D. A., G. S. Poulos, M. P. Meyers, J. S. Snook, and A. Judson, 1998: Observations and forcing mechanisms during the October 1997 Front Range blizzard and forest destruction. Preprints, Eighth Conference on Mountain Meteorology, 3-7 August 1998, Flagstaff, AZ, Amer. Meteor. Soc., 25-30.

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