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

Summary

Basic information:

• An avalanche is a mass of snow that moves rapidly down a steep mountain slope
• The main weather parameters that impact avalanche formation are precipitation, wind, air temperature, and cloud cover

Avalanche weather forecasts:

• Contain detailed, area-specific weather information for mountainous areas that indicate the likely impact of current and upcoming weather on avalanche potential
• Are created by weather forecasters for avalanche forecasters who integrate information about the weather, snowpack, and terrain to create a comprehensive avalanche hazard forecast

Avalanche climate:

• The average winter weather patterns that cause certain kinds of avalanche conditions to develop
• Three primary types: maritime (lie within coastal mountain ranges), continental (located in higher elevation, interior mountain ranges), and intermountain (transitional zones, usually found between coastal and interior mountain ranges)

Slab avalanches:

• Occur when a cohesive layer of snow slides down a slope
• Can consist of snow from a single storm or from multiple layers of snow from several storms
• Cause the most fatalities and do the most property damage

Parts of a slab:

• A fracture line at the upper limit on the slope
• Flanks or continuations of the fracture lines down both sides of the slab
• A stauchwall (bottom or lower limit of the slab); is often obliterated by the avalanche
• A bed surface upon which the avalanche slides; is usually smooth and planar

Soft slab avalanches:

• Form when winds are relatively light and/or the snow has relatively low density
• Break up easily and become more powdery as they run downhill

Hard slab avalanches:

• Form when winds are relatively strong and/or the snow is of higher density
• Maintain large blocks of snow as they descend to the bottom of the slope

Avalanche path: A fixed area within which avalanches travel

• Starting zone: Uppermost part of the avalanche path
• Track: Area within which a particular avalanche travels
• Runout zone: Where debris accumulates

Most common slope angle for avalanche formation: 30 to 45 degrees

Direct-action avalanches occur during a storm or just after it’s ended whereas delayed-action avalanches occur more than 24 hours after a storm has stopped.

Basic components of a slab avalanche:

• Slab:
  • A consolidated mass of snow put into motion as a unit when the avalanche releases
  • Usually forms when wind transports new snow from a windward to leeward slope
  • Usually forms during storms but can develop between storms given sufficient wind
• Weak layer:
  • Unconsolidated or poorly bonded snow that can easily collapse under stress
  • Three main types: Surface hoar, depth hoar, near-surface facets
  • Generally forms between storm periods with clear skies and cold temperatures
  • Most avalanches occur when a weak layer fails, putting the slab above it in motion
  • Can remain in the pack and cause delayed-action avalanches weeks or even months after forming
• Sliding surface under a weak layer:
  • A hard or slick snow surface that provides a relatively smooth surface upon which an avalanche can move
  • Three main types of crusts: Ice, rain, melt-freeze
  • Problems arise when a good sliding surface is buried by new snowfall

Unstable snowpack:

• Has a slab on a weak layer that’s on top of a good sliding surface
• Contains weak and strong snow layers

Good bonding between layers is likely if:

• The snow surface is relatively warm (air temperatures near or above freezing) just before new snow falls; then new snow of almost any temperature or crystal type will bond with it
• There’s a crust on the surface and new snow occurs at relatively warm temperatures
• Weak snow and warmer temperatures precede a warmer snowfall or rain event

Poor bonding between layers is likely if:

• The new snow surface is relatively cold and consists of a weak snow type
• The new snow surface consists of preserved, colder snow crystals, such as stellars and dendrites
• The new snow surface is a crust and air temperatures remain cold as new snow begins falling

Avalanche weather forecast process:

• Pre-forecast preparation: Gather basic information about your area (its avalanche climate, any areas prone to avalanches, the weather history from the start of the season, snowpack observations about the snow surface and stability of the pack, and reports of recent avalanche activity)
• Assessing current weather: Evaluate the weather over the last 24 hours or so
• Forecasting future weather: Make a weather forecast for the important avalanche weather parameters (precipitation, wind, temperature, and cloud cover)
• Making the avalanche weather forecast: Evaluate the impact of the current and forecasted weather on your area’s avalanche potential and determine if it is increasing, decreasing, or remaining the same

For information on the avalanche weather forecast process, access the Avalanche Weather Forecast Guide and the Avalanche Weather Forecast Data Form (instructions)

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