Read it like a forecaster.
Every COAC forecast follows the same structure. Learn the parts once — and what's going on inside the snow — and you can read any day in about five minutes.
One slope, all at once.
Most fatal avalanches are slabs, and most are triggered by the people caught in them or someone in their group. A slab doesn't trickle — the whole layer breaks free together.
- A cohesive slab of snow rests on a weaker layer.
- A rider's weight collapses the weak layer — sometimes with a loud "whumpf".
- The fracture races through the weak layer, often far beyond the rider.
- The slab breaks free at the crown and runs downhill as blocks and debris.
Top to bottom, most important first.
Bottom line
The forecaster's summary in a sentence or two. If you read nothing else, read this.
Danger rating
A 1–5 rating for each elevation band, today and tomorrow.
Avalanche problems
What kind of avalanche to expect, where, how likely and how big.
Discussion
Recent weather, snowpack and field observations behind the ratings.
Weather
Temperature, wind and snowfall forecast for the mountains.
North American Avalanche Danger Scale
Five levels. The official scale explains the travel advice, likelihood and size for each one.
- 1Low
- 2Moderate
- 3Considerable
- 4High
- 5Extreme
Danger is rated for three bands.
Each zone has its own limits for the bands, set by its forecasters. Read the rating for the elevation you'll actually travel through — it can change on the way up.
- Above treeline: open alpine terrain, exposed to wind. Where wind slabs and cornices usually form.
- Near treeline: the transition — scattered trees, open glades and wind-affected snow.
- Below treeline: forested slopes and openings. Trees give shelter but don't anchor every slope.
Where the problem lives.
Each avalanche problem in the forecast comes with a rose. It packs elevation and aspect into one small picture, as the forecaster published it.
- Picture the mountain from above, with the summit in the middle.
- The rings step down in elevation: above, near and below treeline.
- The eight wedges are the directions slopes face.
- Shaded cells show where the forecaster says the problem is.
Nine types, each managed differently.
Each forecast names the problems for the day. Knowing which one you're dealing with tells you where to look and what to avoid.
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Loose
Loose Dry
Cohesionless dry snow sliding from a point and fanning out. Usually small, but dangerous above cliffs or terrain traps.
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Slab
Storm Slab
A cohesive slab of new snow from a recent storm. Typically settles out within a few days of the storm ending.
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Slab
Wind Slab
Snow drifted by wind onto leeward slopes. Look for pillowed snow, cornices and shooting cracks below ridgelines.
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Slab
Persistent Slab
A slab over a buried weak layer that can stay reactive for weeks. Can be triggered from below or a distance.
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Slab
Deep Persistent Slab
A thick slab over a weak layer near the ground. Hard to trigger, unpredictable and often very large.
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Loose
Loose Wet
Wet, heavy surface snow released by sun or warmth. Timing matters — be off steep sunny slopes before it warms.
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Slab
Wet Slab
A slab released when water weakens a layer within the snowpack. Most common during spring warm-ups or rain.
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Other
Cornice
An overhanging mass of wind-drifted snow on a ridge. Can break far back from the edge and trigger slopes below.
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Other
Glide
The whole snowpack sliding on smooth ground. Watch for glide cracks and avoid spending time below them.
Wind moves snow.
Wind can load a slope many times faster than snowfall alone. It strips snow from one side of a ridge and drops it on the other — so a slope can be loaded on a day with no new snow at all.
- Wind picks up loose snow on the windward side and carries it over the ridge.
- It drops on the lee side, below ridgelines and in gullies, building a stiff slab and a cornice above.
- Shooting cracks and smooth, pillowed snow are signs you're on a wind slab.
Buried, not gone.
Some weak layers last for weeks or months under new snow. They're hard to see, hard to test, and the reason a forecast can stay cautious long after the last storm.
- In a clear, cold spell, weak feathery or sugary crystals grow at the snow surface.
- Storms bury them. The weak layer survives under the new snow, storm after storm.
- It can be triggered from flat ground or from below a slope — at a distance.
Starts at a point, spreads into a fan.
Loose snow has no cohesion. Loose dry avalanches run in cold, new snow; loose wet avalanches follow sun and warmth — rollerballs and pinwheels are the warning.
- On a warming slope, rollerballs and pinwheels show the surface is getting wet.
- A loose avalanche starts at a single point and picks up snow as it goes.
- It fans out into a teardrop. Usually small, it can still push you into trees, gullies or over cliffs.
They break farther back than you think.
A cornice is an overhang of wind-drifted snow on a ridge. They often break well back from the edge, and a falling cornice can trigger the slope below.
- Wind builds an overhanging lip on the lee side of a ridge.
- It can break far back from the edge — often where it's still flat underfoot.
- The falling blocks can trigger a slab on the slope below.
How big it could be.
Forecasts give each problem a size range. Size is about destructive potential — what the avalanche could do to a person, a car or a forest.
- Small: relatively harmless unless it carries you over a cliff or into a terrain trap.
- Large: can bury, injure or kill a person.
- Very large: can bury and destroy a car or a small building.
- Historic: near the maximum size the path can produce.
Steepness matters.
Slab avalanches are most common on slopes of about 30 to 45 degrees. The steepest part of a slope is often steeper than it looks from below.
- Most slab avalanches start on slopes between about 30° and 45°.
- Measure it — an inclinometer or the slope layer on the map, not a guess.
- Lower-angle slopes are still reachable: avalanches from steeper slopes above or beside can run onto them.