Reading Avalanche Forecasts

What is an Avalanche Forecast?

Avalanche forecasts are daily assessments of avalanche danger produced by professional forecasters. They synthesize weather data, snowpack observations, and recent avalanche activity to predict avalanche conditions for the coming day(s).

Modern forecasts go beyond simple danger ratings - they identify specific avalanche problems, describe where hazards exist (elevation and aspect), and provide detailed information to help you make terrain decisions.

The North American Avalanche Danger Scale

The danger scale rates avalanche danger from 1 (Low) to 5 (Extreme). Understanding what each rating means is essential for planning.

Critical note: Most avalanche fatalities occur on days rated Considerable (3). This is because people still go out and misjudge the hazard. Low ratings don't mean zero risk.

Avalanche Problems

Modern forecasts identify specific "avalanche problems" - patterns of instability in the snowpack. Each problem has characteristic behavior and requires different management strategies.

1. Storm Slab

• What it is: New snow accumulation bonding poorly to old snow surface
• When it occurs: During and shortly after storms (24-48 hours)
• Where to find it: Recently loaded slopes, often widespread
• How to manage: Wait for stabilization, avoid steep terrain during/after storms
• Duration: Hours to days (short-lived)

2. Wind Slab

• What it is: Wind-transported snow forming slabs on lee slopes
• When it occurs: During and after wind events
• Where to find it: Lee slopes (downwind), behind ridges, in gullies
• How to manage: Identify wind direction, avoid fresh wind-loaded slopes
• Duration: Hours to days
• Signs: Pillows, cornices, hollow sounds, shooting cracks

3. Persistent Slab

• What it is: Slab over persistent weak layer (facets, surface hoar, crusts)
• When it occurs: Can persist for weeks or months
• Where to find it: Specific elevations/aspects where weak layer exists
• How to manage: Most challenging - conservative terrain choices essential
• Duration: Weeks to months (long-lived)
• Character: Tricky - may feel stable then fail unexpectedly

4. Deep Slab

• What it is: Very large avalanches breaking deep in snowpack
• When it occurs: Mid to late season, specific weak layers
• Where to find it: Often high elevation, specific aspects
• How to manage: Extreme caution - hard to assess, massive consequences
• Duration: Weeks to entire season
• Character: Low probability but enormous size and destructive force

5. Wet Avalanches

• What it is: Water weakening bonds in snowpack
• When it occurs: Warm temperatures, rain, intense sun
• Where to find it: Sun-exposed slopes, any aspect with warming
• How to manage: Travel early (frozen conditions), retreat when snow warms
• Duration: Daily cycle (freeze/thaw) or persistent with warm weather
• Signs: Pinwheels, wet loose avalanches, heavy wet snow

6. Loose Snow (Dry or Wet)

• What it is: Unconsolidated snow sluffing
• When it occurs: New snow (dry) or warming (wet)
• Where to find it: Steep terrain, particularly convex slopes
• How to manage: Generally smaller, but dangerous in terrain traps
• Character: Point release, often manageable but watch for terrain traps

7. Cornice Fall

• What it is: Overhanging snow masses collapsing
• When it occurs: Warming, wind, weight loading
• Where to find it: Ridge crests, wind-loaded features
• How to manage: Stay back from cornices (above and below), avoid travel beneath
• Character: Can trigger avalanches on slope below

Understanding Elevation Bands

Forecasts divide terrain into elevation bands because avalanche conditions often vary with elevation:

• Below Treeline: Typically lowest danger, denser forest provides anchoring
• Near Treeline: Transition zone, can have elevated danger
• Above Treeline: Often highest danger, more wind loading, colder temps

Different elevation bands can have different danger ratings and different avalanche problems. Always check all three zones.

Aspects of Concern

Forecasts specify which slope aspects (compass directions) are most hazardous. This is critical for route selection.

Why Aspect Matters:

• Sun exposure: South-facing warm faster (wet avalanches), north-facing stay cold (persistent problems)
• Wind loading: Lee slopes (downwind of ridges) collect wind-transported snow
• Temperature gradient: Shaded slopes can develop persistent weak layers

Aspect Rose Diagram

Forecasts often show a circular diagram (rose) with shaded sections indicating aspects of concern. Learn to read these and match them to your planned route.

Likelihood and Size

Likelihood (Probability)

• Unlikely: Specific terrain features or unusual conditions required
• Possible: Could happen on identified terrain
• Likely: Expect avalanches in identified terrain
• Very Likely / Certain: Avalanches expected or occurring

Size (Destructive Potential)

• Small (D1): Relatively harmless (unlikely to bury person)
• Medium (D2): Could bury, injure, or kill a person
• Large (D3): Could bury and destroy a car, damage truck, destroy small building
• Very Large (D4): Could destroy railway car, large truck, several buildings
• Historic (D5): Largest known, could destroy village or forest

Risk = Likelihood × Size × Consequences: Even "unlikely" avalanches can be fatal. Low likelihood doesn't mean acceptable risk if consequences are deadly.

Applying Forecasts to Your Plans

Step 1: Read the Full Forecast

• Don't just look at the danger rating
• Read the entire discussion and reasoning
• Understand the avalanche problems identified
• Note elevation bands and aspects of concern
• Check the confidence level (how certain is forecast?)

Step 2: Match Forecast to Your Route

• Identify elevations you'll travel through
• Determine slope aspects on your route
• Note maximum slope angles you'll encounter
• Identify avalanche terrain exposure

Step 3: Make a Plan

• Choose routes that avoid problematic elevations/aspects when possible
• Identify safe zones and regrouping points
• Plan timing (early for wet avalanche problems)
• Prepare alternative routes (Plan B, C)
• Set decision points where you'll reassess

Step 4: Observe and Reassess in Field

• Does actual snowpack match forecast?
• Any avalanche activity observed?
• Red flags (whumpfing, cracking, etc.)?
• Weather different than predicted?
• Adjust plan based on observations

Regional Avalanche Centers

North America

• Avalanche.org: Links to all North American centers
• Northwest Avalanche Center (NWAC): Washington, Oregon - nwac.us
• Colorado Avalanche Information Center (CAIC): avalanche.state.co.us
• Utah Avalanche Center (UAC): utahavalanchecenter.org
• Avalanche Canada: British Columbia, western Canada - avalanche.ca
• Chugach National Forest Avalanche Center: Alaska - cnfaic.org

Europe

• EAWS: European Avalanche Warning Services - avalanches.org
• Météo France: France - meteofrance.com
• SLF: Switzerland - slf.ch / whiterisk.ch
• Avalanche.report: Tyrol, South Tyrol, Trentino - avalanche.report

Common Forecast Interpretation Mistakes

• Rating shopping: Looking for low ratings to justify your plan
• Ignoring aspects/elevations: Focusing only on danger rating number
• Overconfidence at Moderate: Most accidents happen at Moderate/Considerable
• Not reading full discussion: Missing critical details and reasoning
• Assuming uniformity: Conditions vary locally from regional forecast
• Old forecast: Using yesterday's forecast instead of today's update

Beyond the Forecast: Field Observations

Forecasts are regional assessments. You must make site-specific observations:

• Recent avalanche activity
• Cracking, collapsing (whumpfing)
• Weather observations (wind, temperature, precipitation)
• Snowpack tests (if trained)
• Terrain choices and group management

Remember: Forecasts predict probability, not certainty. Your observations and conservative decision-making are essential layers of safety.