The destruction of thousands of acres of old growth forest during the blowdown event discussed here is but one of several catastrophic results of the October 1997 Rocky Mountain Winter Storm. Deep snowdrifts and zero visibility left transportation and commerce paralyzed, including the closure of the Denver International Airport. Subfreezing temperatures combined with the huge snowdrifts caused thousands of livestock deaths.
Here we present an overview of the winter storm itself, discuss the advantages of using local-domain models such as LAPS to complement national-domain models in predicting severe weather events, highlight the performance of the high-resolution models used in this case, and conclude with future research needs.
Figure 1. A photo showing a swath of flattened trees in the Routt National Forest after the 25 October 1997 storm. Photo courtesy U.S. Forest Service.
Overview of the Winter StormAn extraordinary characteristic of the Rocky Mountain winter storm of 23 – 25 October 1997 was the variety of significant weather covering a very large region. The storm's two most powerful forces included blizzard conditions and heavy snowfall from Wyoming to southern New Mexico along the Front Range of the Rocky Mountains, and extreme winds exceeding hurricane force at several locations west of the mountains.
Observations showed total snowfall amounts exceeding 100 cm (39 inches) in many Colorado mountain locations east of the Continental Divide. Snow accumulation variability of as much as 25 inches was recorded within small spatial areas (a 5-mile region, for instance), especially between the mountains and the plains.
On the west side of the Divide, snowfall amounts were light, but high winds caused numerous serious weather related problems. Meteorological records confirmed that wind gusts at 3800 m (12,500 ft) 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. Very cold mountain top temperatures around -20oC created extreme wind chill temperatures as low as -50oC (-60oF). Wind gusts were possibly even higher in the Routt National Forest (Figure 1) 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 U.S. National Forest Service, this was the largest known forest blowdown ever recorded in the Rocky Mountain region. The fallen trees were stacked 30 ft high in some locations, requiring nearly two days for emergency operations to rescue some trapped hunters from the wilderness. The blowdown also raised environmental and economic concerns regarding fire hazard, beetle infestation, spruce/fir ecosystem, timber salvage, and restoration of recreational areas.
In many respects the destruction had the appearance of a Colorado Front Range downslope windstorm, but it was unique in that the strong winds were easterly and the location of the damage was west of the mountain barrier.
National-Domain Versus Local-Domain ModelsThe National Centers for Environmental Prediction (NCEP) suite of numerical prediction models provided excellent regional-scale guidance for the area and timing of heavy snow in this study. 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.
FSL has been demonstrating the Local Analysis and Prediction System (LAPS), a data assimilation system designed to function in a local forecast office using affordable computer hardware. LAPS collects all available data sources and uses them to generate local-area analyses of the atmosphere. The analyses are then available as initialization to local-area forecast models.
The LAPS analyses and forecast products are designed to provide forecasters at the local forecast office additional mesoscale prediction guidance for increased understanding of local weather events. They can be very useful in defining the unique aspects that cause such events as the forest blowdown. High-resolution, local-area model forecasts especially provide improved mesoscale detail during highly variable mesoscale weather events.
Modeling the Blowdown EventThe predictive component of LAPS is designed to utilize any available mesoscale forecast model. LAPS analyses are typically 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 for the October 1997 case were not available for this study, but they will be retrieved and re-run at a later date.
RUC – As a substitute for LAPS, model initializations were accomplished using the 60-km Rapid Update Cycle (RUC) analyses, an operational version of the Mesoscale Analysis and Prediction System (MAPS) developed at FSL.
RAMS – An 18-hour forecast using the the Regional Atmospheric Modeling System (RAMS), developed at Colorado State University, was initialized with 25 October 0600 UTC RUC analyses, and the NCEP operational 48-km Eta model was used as forecast lateral boundary conditions. A nonhydrostatic version of the model was 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 (Figure 2).
The topography used by the RAMS model is depicted in Figure 3. 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. The Arapahoe Basin ski area, which reported a peak wind gust of 51 m s - 1, is located approximately 80 km west of Denver.
The 6-hour wind forecast from RAMS (Figure 2) indicates three areas of high winds exceeding 17.5 m s -1: north of Steamboat Springs and west of the Continental Divide, along the Continental Divide in the vicinity of the Arapahoe Basin ski area, and west of the Continental Divide in Grand County (about 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. A factor that affected the underforecast is the model's prediction of a sustained wind. It is unrealistic to expect the model to capture the strength of peak gusts using coarser model grid resolutions.
Figure 2. Six-hour RAMS forecast of low-level wind speed (color shading, knots). Wind barbs (one full barb = 5 knots) are plotted every other grid point. Wind speed contour interval is 10 knots and minimum contour level is 30 knots.
Figure 3. RAMS model topography (m) derived from a 30-s dataset for the 5-km inner grid. Contour interval is 300 m. Station locations are indicated for Steamboat Springs (SBS), the Arapahoe Basin ski area (ABS), and Denver (DEN).
Figure 4. West-to-east vertical cross section of 6-hour RAMS forecast potential temperature (K, black contours) and wind speed (knots, color shading and white contours). Wind barbs (one full barb = 5 knots) are plotted every third grid point. Cross section is positioned through the area of strongest winds located to the northeast of Steamboat Springs.
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. Again, though, 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 4 for the 6-hour 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. The strongest winds (indicated beneath this layer) are likely the result of vertically propagating gravity waves reflecting off the critical layer.
ConclusionsResults from the high-resolution, local-area model forecast highlight some of the unusual features that – combined – resulted 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 (Figure 2). The lower barrier height also allows forest growth which is nonexistent 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 the 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 occur with some regularity over Colorado during the cold season, the fact that both conditions were so strong and occurred simultaneously was indeed 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 that normally generate weaker easterly flow over the mountain barrier.
The likely explanation for this rare event is that the combination of a deep, very cold boundary layer and a strong cyclonic easterly flow over a relatively low mountain barrier created the perfect conditions to generate a severe downslope wind storm that destroyed many acres of old growth forest.
Future Research NeedsHigh-resolution, local-area model forecasts provide an important component in formulating conceptual models of highly variable mesoscale events, including the event discussed here. 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.
AcknowledgmentsDrs. 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. Lynne Thurston provided valuable information from the U.S. Forest Service.
[A complete list of references relating to this topic is available from the author.]
(Dr. John S. Snook is a researcher in the Local Analysis and Prediction Branch, headed by Dr. John A. McGinley. His e-mail is here.)