Case 3: Western Montana
It’s early February and you’re forecasting for a search and rescue (SAR) operation that’s looking for a snowmobiler lost in a remote area of the mountains of western Montana. The SAR crew is to be flown into the mountains early in the morning, during a break between storm systems. The weather is forecast to worsen by late morning. There are known avalanche paths in the area and elevations are between 6,500 and 8,500 feet (2000 and 2600 m).
All that we know about the weather is that snow has been on the ground down to 6,500 feet (2000 m) since mid-November. At the end of November, there was one day with a brief, light rain/mist up to around 8,000 feet (2450 m), which was followed by several days of cold temperatures and very low-density snow in early December.
The area had a week-long spell of cold and dry weather in mid-December, but we don’t know if it was windy. Smaller storms hit the area from the latter half of December through the end of January. Each lasted from two to four days, producing 2 to 4 inches (5 to 10 cm) of snow at a time. There were one-to-three day clearing periods between storms.
Temperatures during and between the storms varied widely, from single digits to the upper 20s F (-15°C to -3°C). Winds were unknown, but climatology suggests that they were probably from the southwest, west, and northwest during the storm periods.
FORECAST FOR WESTERN MONTANA MOUNTAIN WEATHER at 8,500 feet (2600 m):
Precipitation:
- 12 to 16 inches (30 to 40 cm) of new snow at 8,000 feet (2450 m)
- Water content: From 1.00 to 1.50 inches (25 to 38 mm)
- Average new snow densities: Around 9%
- The heaviest snowfall period will be during the 12- to 24-hr forecast period, with snowfall rates approaching 1 inch per hour (2.5 cm/hr) during that period
Wind:
- Starts out westerly at 25 to 35 mph (11-16 m/s) through the first 24 hours
- Switches to northwesterly at 15 mph (7 m/s) for the last 12 hours
Temperatures:
- Start out between 25° and 30°F (-4° to -1°C) for the first 24 hours
- Slowly cool down to between 20 and 25°F (-7° to -4°C) for the last 12 hours
Select the Questions tab and go through the questions, then read the Synopsis.
1. What type of avalanche climate does the area have? (Choose the best answer.)
The correct answer is B.
The elevations, geographic location, and mix of frequent storms and dry periods make this an intermountain avalanche climate.
2. At the end of November, the light rain/mist followed by cold temperatures formed a thin crust on top of the pack up to 8,000 feet (2450 m). What type of crust was it? (Choose the best answer.)
The correct answer is C.
The light rain/mist followed by cold temperatures would have created an ice crust.
3. What type of layer likely formed on the snow surface in mid-December? Recall that we didn’t have any wind information then. (Choose the best answer.)
The correct answer is C.
The low-density snow that fell in early December would not have bonded with the crusty snow surface. The one-week cold spell that followed would have probably turned the snow into near-surface facets. Surface hoar may also have formed but we cannot say for sure without any wind information.
4. Was this an upside-down storm? (Choose the best answer.)
The correct answer is B.
The period started out warm and ended cold, with higher-density snow early in the storm followed by lower-density snow later on. Therefore, lighter snow overlay heavier snow, which is a right-side-up situation.
5. Is the snow loading forecasted for the next 24 to 36 hours sufficient to increase the potential for avalanches during the storm? (Choose the best answer.)
The correct answer is A.
The new snow will meet or exceed almost all critical thresholds: 12 inches (30 cm) of new snow in 24 hours; one inch (25 mm) of water in 24 hours; snowfall rates of one inch per hour (2.5 cm/hr); more than 6 inches (15 cm) of 9% or greater new snow density; wind speeds in the critical range; and a consistent wind direction for first 24 hour period.
6. Which aspect should have the highest potential for avalanching during the forecast period? (Choose the best answer.)
The correct answer is B.
Westerly winds at 25 to 35 mph (11-16 m/s) will primarily load east-facing slopes. Northwest winds at 15 mph (7 m/s) for the last 12 hours of the forecast period are not sufficient to transport significant amounts of snow to southeast-facing slopes, especially with new snow densities around 9%.
7. What layer in the snowpack is most likely to fail, possibly producing larger, more dangerous avalanches given a sufficient trigger? (Choose the best answer.)
The correct answer is C.
We don’t know how deep the individual layers are, but most of the snowpack since late November has sat atop the ice crust and near-surface facet layer, which developed in late November and early December. Layers of surface hoar could also be interspersed with layers of near-surface facets higher up in the pack, which formed between the smaller storms from late December thru January. However, we do not know for sure. Remember to read the synopsis before moving on to the next case.
The elevations and geographic location make this an intermountain avalanche climate even though we do not know the overall depth of the snowpack. The fluctuations in temperature produced a lot of variability in the snowpack, which is also typical of intermountain avalanche climates.
The most suspect weak layer is probably the near-surface faceted layer that formed in mid-December. It would sit upon a fairly good sliding surface (the ice crust that formed in late November after the rain/mist event).
The lower-density snow that originally fell on the ice crust would probably not have bonded to it very well. The subsequent cold spell would have made that layer even weaker as it turned to near-surface facets. This was all buried deeper in the snowpack by the late December and January snowfalls.
The potential for avalanches (especially on wind-loaded, east-facing slope aspects) would increase with the amount of snow and water and the wind loading. The new snow would probably avalanche but the really dangerous slab avalanches would come from the faceted snow layer above the ice crust. This layer could persist throughout the winter.
Surface hoar layers probably formed between storms when it was clear, especially on open slopes protected from the wind. Without snowpit test results to verify the depth and weakness/strength of these layers, our pre-season weather information leads us to treat the facets on the crust as the most likely weak layer to fail with any avalanches that might occur.