Case 2: Rocky Mountains
You’re planning a backcountry hut-to-hut ski trip from December 23 to 26 in the Rocky Mountains of Colorado. You’ll be skiing between elevations of about 10,000 feet (3050 m) to over 11,500 feet (3500 m). The surrounding peaks are higher, with tops ranging from 12,500 to 14,000 feet (3800 to 4300 m). Your route will take you through avalanche terrain and you’ll be crossing many avalanche paths that could produce D2 and D3 avalanches. You’ll also need to ski beneath several D4 paths on your approach to the huts.
In mid-October, the area got between 2 and 3 feet (0.6 to 1 m) of snow at approximately 11,000 feet (3350 m). From mid-October through mid-November, conditions were very dry (under high pressure) and temperatures at higher elevations were relatively low, especially at night. Temperature rose during the day but still remained below freezing.
Several smaller storm systems occurred during the latter half of November. Each storm brought 2 to 3 inches (5 to 8 cm) of snow, accompanied by high winds from the southwest. On December 1, the settled snow depth at 11,000 feet (3350 m) was 30 inches (76 cm).
From December 1 through 20, no new snow was recorded; skies were generally clear; winds were moderate (30 mph or 13 m/s) to high at times (gusts over 100 mph or 45 m/s); and temperatures were very cold (0°F to 10°F or -18°C to -12°C) between 10,000 and 12,000 feet (3050 to 3650 m).
On December 21, temperatures began warming into the teens Fahrenheit (-12°C to -7°C) between 10,000 and 12,000 feet (3050 to 3650 m) and clouds increased.
On December 22, the area got 4 inches (10 cm) of new snow at 11,000 feet (3350 m); the SWE was 0.20 inches (5 mm); the snow density was 5%; and ridge-top winds were westerly to southwesterly, averaging 20 mph (9 m/s).
It’s now December 22 and you’re getting ready to leave on your trip. Here’s the forecast.
December 23:
- At 11,000 feet (3350 m)
- Approximately 8 inches (20 cm) of new snow expected over the entire day (from midnight to midnight)
- Snow water equivalent (SWE): 0.40 inches (10 mm)
- New snow density: 5%
- Temperatures: In the teens Fahrenheit (-12°C to -7°C)
- At 13,000 feet (4000 m): Southerly to southwesterly winds at 25 mph (11 m/s)
December 24:
- At 11,000 feet (3350 m)
- 12 inches (30 cm) of snow expected from noon to midnight
- SWE: Up to 0.80 inches (20 mm)
- New snow density: 6 to 7%
- Temperatures: Around 20°F (-7°C)
- At 13,000 feet (4000 m): Westerly winds at 40 mph (18 m/s)
December 25:
- At 11,000 feet (3350 m)
- Another 16 inches (40 cm) of snow from midnight to 3 PM
- SWE: 1.25 inches (32 mm)
- New snow density: Almost 8%
- Temperatures: Between 20 and 25°F (-7°C and -4°C)
- At 13,000 feet (4000 m): Southwesterly winds at 40 to 50 mph (18 to 22 m/s)
December 26:
- At 11,000 feet (3350 m)
- No new snow
- Temperatures: Teens to single digits °F (-15°C to -9°C)
- At 13,000 feet (4000 m): Northwesterly winds at 20 mph (9 m/s)
Click the Questions tab and answer the questions, then access the Synopsis and review the case.
1. What type of avalanche climate does the area have? (Choose the best answer.)
The correct answer is A.
This is a cold, alpine region at or above timberline. It has a relatively shallow snowpack, cold temperatures, and extended periods of drought, all characteristic of a continental avalanche climate.
2. How would you characterize the early season snowpack in the mountains above 11,000 feet (3350 m)? (Choose the best answer.)
The correct answer is A.
The settled snow depth was only 30 inches (76 cm) on December 1. There were two extended periods of very dry weather: from mid-October to mid-November and during the first three weeks of December. During these periods, weak layers could have formed in the snowpack.
3. In general, what would you expect the snowpack structure to look like just prior to December 25? (Choose the best answer.)
The correct answer is B.
With a shallow snowpack and long periods of dry, cold weather, depth hoar probably formed. Even if it did not, other weak layers, such as near-surface facets or surface hoar, probably formed.
4. Was there sufficient loading on December 23 to significantly increase the potential for avalanches on all of the mountains’ slopes? (Choose the best answer.)
The correct answer is B.
Twelve inches (30 cm) of new snow over 48 hours, SWE of 0.60 or 15 mm, and new snow density of 5% probably do not add enough load to increase avalanche potential on all slopes. But, there’s been sufficient loading on wind-loaded aspects and in gullies (north to northeast in this case) for avalanche potential to have increased.
5. Which aspect would have some potential for smaller slab avalanches late on December 23? (Choose the best answer.)
The correct answer is C.
Soft slabs could develop on north- and northeast-facing slopes steeper than 35 degrees with 25 mph (11 m/s) winds from the south and southwest.
6. Based on the forecast through December 26, when might more widespread natural avalanche activity occur on slopes with angles between 30 and 45 degrees? (Choose the best answer.)
The correct answer is B.
The 28 inches (71 cm) of new snow expected from noon on December 24 to 3 PM on December 25 is enough to increase avalanche potential on December 25. In addition, the snow that day is expected to be heavier (higher density) than the previous day’s snow, further increasing avalanche potential.
7. Near the end of the December 25 storm, how deep would you expect the crowns of any avalanches on wind-loaded slopes to be? (Choose the best answer.)
The correct answer is C.
From December 22 to 25, the area’s total storm snowfall was 42 inches (107 cm), with a SWE of over 4 inches (100 mm). If wind loading added more snow, any slabs on leeward slopes could easily exceed 4 feet (1.2 m) in depth, even with some snow settlement over the three-day period. Access the Synopsis to review the case.
This is a typical setup for developing a weak layer of depth hoar. The area has a high-altitude, continental avalanche climate and it was early in the season. There were less than three feet (1 m) of snow on the ground and temperatures were very cold for an extended period of time. The depth hoar would probably be a persistent weak layer throughout the winter.
Then the upside-down storm created an unstable snowpack within the new snow layers. The following factors compounded the situation:
- Wind speeds increased through the snowiest time period
- There was a lot of wind loading
- The new snow sat on top of weak-faceted snow or depth hoar
Under these conditions, it would probably have been prudent to cancel the hut trip!