Avalanche Weather Forecasting

Air temperature not only determines the type of snow crystals that fall from the sky, but the metamorphic processes that occur within the snowpack.

You need to track:

• Daily temperature values (highs and lows) and range
• Temperature trend (increasing or decreasing) and rate and duration of change

Daily temperature values: Use daily high and low temperatures to determine freezing levels in the mountains as well as snow and rain levels during storms. Note the following.

• Crusts may form if temperatures rise above freezing during the day and drop below freezing at night; the crust can create a potential sliding layer for the next storm
• When temperatures remain above freezing for more than 24 hours, the upper snowpack becomes saturated from melt water and may cause wet snow avalanching
• When temperatures stay well below freezing for long periods of time and high and low temperatures never rise above 15°F (-10°C) day and night, weak layers may develop on the surface or near the ground in a shallow snowpack; this can create a layer that fails with the next load of new snow

Key points:

• Avalanches may occur when air temperatures rise above freezing and stay there
• Weak layers often develop when temperatures get very cold and remain that way

Temperature trend: As temperatures increase, the snowpack undergoes settlement, deforming and becoming denser. Settlement on an incline results in creep, a slow downhill motion.

• If air temperatures increase rapidly (more than 15°F or 8°C) in less than 12 hours, the rate of creep increases, which can lead to avalanching. This is most critical when temperatures are near or above freezing because the rate of creep increases exponentially with rising temperature.
• If temperatures increase slowly over several days, settlement rates are slower. If air temperatures never rise above freezing for very long, the snowpack deforms slowly and creep rates are slower.
• As temperatures decrease, settlement and creep rates also decrease.
• Prolonged periods of cold temperatures promote the metamorphic processes that form weak layers.
• Warm days and cold nights strengthen the pack, especially overnight and during early morning hours.
• Several days of warmer temperatures followed by colder temperatures also strengthen the pack.

Key points:

• Rapid rises in temperature can destabilize a pack, while slow rises stabilize it (especially if a cooling trend follows)
• Long periods of cold temperatures build weaker snow, which isn’t a problem until the next significant snowfall

Summary table:

Select Operational Information at the top.

Data: You can get temperature data from thermometers at precipitation and wind instrumentation sites. During clear conditions, surface temperatures can also be estimated from high-resolution infrared channels onboard weather satellites.

Gather temperature information across your forecast area for the previous 24-hour period, noting:

• Maximums and minimums
• Temperatures at different elevations
• Temperatures near avalanche starting zones
• 24-hour and longer-term temperature trends

If it snowed, determine the freezing level and if it was at or above avalanche starting zones.

Temperature trend:

• Increasing temperatures: Notice when air temperatures warm by about 15°F (10°C) or more during the day, especially when they’re near the freezing mark; the pack may settle too rapidly, causing avalanching; the faster temperatures rise, the more unstable the snowpack will be
• Melt-freeze: Note when a warm-up is followed by a cooling or days with melt-freeze cycles, since this will help stabilize the pack

Warm temperatures: Monitor temperatures, especially near avalanche starting zones, to determine if they will remain above freezing for more than 24 hours. The longer this goes on, the more likely wet slab avalanches will develop.

If there’s a storm, compare the freezing level with the avalanche starting zone elevations. Rain at a starting zone will quickly destabilize the snowpack.

Cold temperatures: Evaluate whether weak layers of surface hoar, near-surface facets, or depth hoar could have formed.

• Surface hoar forms during clear, cold, calm weather, mainly at night with wind speeds less than 10 mph (8 kts or 5 m/s) and air temperatures at or below 14°F (-10°C)
• Near-surface facets form with clear, cold weather, mainly at night even with wind speeds above 10 mph (8 kts or 5 m/s) and air temperatures at or below 14°F (-10°C)
• Depth hoar forms from low air temperatures, usually at or below 14°F (-10°C), mainly at night over a shallow snowpack (one that’s less than about 3 feet or 1 m deep)

Also monitor extended periods of clear and cold weather. The longer the weather remains clear and cold, the weaker the weak layers will become.

Summary table: