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

Weather History

Avalanche forecasting begins with a good understanding of past weather, from the very beginning of the “winter” season when the first snow stays put in the mountains.

If you’re in a formal avalanche forecasting situation and will be forecasting for, say, a highway department, ski area, or helicopter skiing operation, ideally you’ll want the full complement of daily historical weather data from the start of the winter season. This includes:

• Precipitation (daily snowfall and its water content or SWE)
• Maximum and minimum temperatures at forecast-area elevations
• Winds near ridge-top level or at all forecast-area elevations
• Average cloud cover each day over the forecast area

You can get daily weather observations from ski areas, transportation agencies, and automated weather sites. Some historical information can be obtained from climate station records, satellite data, and SNOTEL sites. (SNOTEL is an extensive, automated system for collecting snowpack and related climatic data in the Western United States and Alaska run by the U.S. Natural Resources Conservation Service.)

Display the data in a graphical format so you can roughly reconstruct snowpack depths and structure.

If detailed historical data are not available, try to find out about the region’s general weather. Has it been wetter or drier than climatic norms? Warmer or colder? Warmer and wetter conditions can create deep mountain snowpack that may be strong and consolidated. Colder and drier than normal-early season weather tends to produce shallow and perhaps weaker snowpack.

Try to get snowpack observations or snow pit profiles from someone trained to perform snowpack assessments. The data can help you figure out:

• If the current snowpack is stable or unstable. If it’s unstable, the potential for avalanches may already be high; if it’s stable, the potential will be low to start with.
• If the snow surface is hard and crusty or soft and powdery, if it’s very rough looking or smooth and planar; and if it’s relatively warm or cold. New snow bonds well to rough, soft, or warm snow surfaces but not to smooth, hard, or cold snow surfaces.

Summary table: