Avalanche Weather Forecasting

• Introduction
  • Glory Bowl Avalanche Incident
  • Avalanche Impacts
  • Avalanche Weather Forecasts
  • About the Module
• Basic Avalanche Information
  • Introduction
  • Avalanche Climate
  • Types of Avalanches
  • Avalanche Paths
  • Avalanche Terrain
  • Avalanche Triggers
  • Avalanche Sizes
  • Avalanche Development
  • Sliding Surfaces
  • Stable vs. Unstable Snowpacks
  • Snow Bonding
• Avalanche Weather Forecast Process
  • Overview
    • The Forecast Process
    • Forecasting Guide and Data Form
  • Pre-forecast Preparation
    • Overview
    • Avalanche Climate
    • Avalanche Terrain
    • Weather History
    • Recent Avalanches
  • Current Weather
    • Overview
    • Precipitation
    • Wind
    • Temperature
    • Cloud Cover & Solar Radiation
  • Future Weather
    • Overview
    • Precipitation
    • Wind
    • Temperature
    • Cloud Cover & Solar Radiation
  • Making the Avalanche Weather Forecast
• Forecast Exercises
  • Introduction
  • Case 1
  • Case 2
  • Case 3
  • Case 4
  • Case 5
• Summary and Resources
  • Summary
  • Resources

Making the Avalanche Weather Forecast

Now it’s time to pull together all of the information that you’ve analyzed (and hopefully tracked on the Avalanche Weather Forecast Data Sheet).

• Check the findings from your pre-forecast preparations. Do they indicate that the area is prone to avalanches? Has the season’s weather set the stage for their formation?
• Review your weather data for the last 24 hours. Is the weather increasing, decreasing, or not changing avalanche potential?
• Analyze your 24-hour forecasts and determine their impact on avalanche potential
• Look at the combined data and identify the trend. Does the potential for avalanche formation appear to be increasing, decreasing, or remaining the same based on weather conditions?

It’s much easier to forecast the trend if you know what the avalanche potential was prior to this point in time. If not, be conservative and assume that it’s increasing unless most indicators imply otherwise.

Finally, be on the lookout for high-risk situations that make an area particularly prone to avalanches.

• Heavy, dense snowfall of 6 inches (15 cm) or more produces avalanches, especially on steeper terrain with slopes greater than 40 degrees
• 12 inches (30 cm) of new snow produce sluffing on steeper terrain at a minimum
• 12 inches (30 cm) or more of new snow, especially with higher densities, may overload an already weak snowpack and produce deeper slab releases
• New slabs created with 6 inches (15 cm) or more of new snow and winds from a consistent direction for 6 hours or more averaging 20 to 60 mph (~17 to 52 kts or 10 to 30 m/s) pose high risks
• Rain on snow almost always produces avalanching, with heavier rain producing larger avalanches
• Storms beginning with cold temperatures and low density snow and ending with warm temperatures and higher-density snow often initiate direct action avalanches
• Rapidly warming temperatures during the day increase snowpack settlement rates and can cause avalanches without any new snow loading
• Warm surface temperatures (at or above freezing) combined with intense solar radiation produce wet snow avalanches
• Water percolates deeper into the snowpack if continuous above-freezing temperatures last for more than 24 hours; the longer the situation continues, the deeper the avalanches may be (even reaching the ground)
• One day of clear, cold, calm weather followed by a significant snowfall can bury a layer of surface hoar, posing risks to the future stability of the pack
• Longer spells of dry, relatively cold weather followed by a significant snowfall often will bury near-surface faceted snow or depth hoar and may pose a risk if more loading occurs