How to read an avalanche forecast before you leave the resort

An avalanche forecast rates the danger in a mountain zone on a five-level scale (Low, Moderate, Considerable, High and Extreme), usually for three elevation bands, and then names the avalanche problems: what kind of avalanche, on which slopes, how likely and how big. Read the danger for the elevation you'll be in, then the problems, then the bottom line. Level 3, Considerable, deserves careful attention even when conditions do not look extreme. Avalanche.org notes that historically the most avalanche deaths have happened at Considerable, a level where avalanches can be smaller, less widespread or less likely than at High, which can make the danger less obvious.

A forecast describes backcountry terrain across a whole zone. It is not a judgment about the slope in front of you, it doesn't replace avalanche training, and it can't dig anyone out; that takes rescue gear and a partner who has practiced with theirs.

Avalanche center forecasts are written for terrain outside ski area boundaries. Inside the boundaries, ski patrol manages avalanche hazard on open terrain. If you leave the resort through a gate or skin up from a trailhead, you are in the terrain the forecast describes.

The five danger levels, in the scale's own words

This is the North American Public Avalanche Danger Scale that US and Canadian forecasts use:

Level	Danger	Travel advice	Likelihood of avalanches	Avalanche size and distribution
1	Low	Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.	Natural and human-triggered avalanches unlikely.	Small avalanches in isolated areas or extreme terrain.
2	Moderate	Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.	Natural avalanches unlikely; human-triggered avalanches possible.	Small avalanches in specific areas; or large avalanches in isolated areas.
3	Considerable	Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.	Natural avalanches possible; human-triggered avalanches likely.	Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
4	High	Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.	Natural avalanches likely; human-triggered avalanches very likely.	Large avalanches in many areas; or very large avalanches in specific areas.
5	Extreme	Avoid all avalanche terrain.	Natural and human-triggered avalanches certain.	Large to very large avalanches in many areas.

The scale carries one more line, printed beneath the rows in Parks Canada's copy, and it applies at every level: "Safe backcountry travel requires training and experience."

The numbers look evenly spaced, but the danger rises exponentially from one level to the next. The middle of the scale is where judgment is hardest. Moderate limits the hazard to specific features, yet it can cover low-probability, high-consequence days when "erratic yet very large avalanches are possible." At Considerable, many slopes should be avoided, and the simplest default is low-angle slopes that aren't connected to steeper terrain above.

The fatality fact in the opening is a count, not a rate, and the count has a context. The Colorado Avalanche Information Center compared its forecasts with accidents over the five winters through 2018. It rated the danger Moderate on 57% of days, Considerable on 19% and High on 3%. Forty percent of the people killed died on Considerable days, and another 40% on Moderate days. The same paper notes that from 1996 to 2006, half of Colorado's avalanche deaths came at Moderate. Neither figure says how many people were out, and a study of skier days at Rogers Pass in British Columbia found people significantly more likely to go out at Moderate or Considerable than at High. The Colorado paper does estimate a rate, per forecast zone per day rather than per person, from 19 days with a fatal avalanche. The chance of a fatal avalanche in a Colorado forecast zone on a given day was about 1 in 500 at Moderate, 1 in 190 at Considerable and 1 in 47 at High. So each step from Low to High made a fatal day more likely, and the deaths gather in the middle because that's where most of the days are and where people go out. Considerable and Moderate are where most of the deaths happen: common days, when the danger is real but less obvious.

What "possible" and "likely" mean in a forecast

The words in a forecast have defined meanings, and avalanche.org pins them down with a thought experiment: someone wandering randomly through avalanche terrain all day with no avalanche awareness. Under its likelihood definitions, "possible" means a handful of problem slopes are out there. That person comes home without an avalanche encounter more often than not, but both outcomes happen. "Likely" means the odds favor encountering or triggering one, and they would trigger several of the steep slopes they crossed. On a "likely" day, if you ride steep slopes inside the problem's terrain, expect some of them to release.

Size gets the same treatment. A small avalanche (D1) is unlikely to bury a person on an open slope, and avalanche.org calls it relatively harmless unless it drags you into trees, rocks or a gully. A large one (D2) can bury, injure or kill a person. Picture the first floor of a large house filled with about 6 feet of debris, roughly 100 tons of snow, from a slide that runs the length of a football field.

Read it in this order

1. Check the date and time. Most forecasts cover 24 to 48 hours. Make sure you're reading today's.
2. Find your zone. Zones cover large areas. Confirm the one that contains your route, especially near a boundary between zones.
3. Read the danger by elevation. Most US forecasts rate below treeline, near treeline and above treeline separately (names vary a little by center). The rating where you'll actually be is the one that matters.
4. Read the avalanche problems. Problem types run from dry loose and storm slab through persistent slab to wet slab and glide. Each comes with a rose, a compass divided into eight aspects and three elevation bands with the affected slopes shaded, plus a likelihood and an expected size.
5. Read the bottom line and the discussion. The forecaster's summary says what changed, what to avoid and why.
6. Look at recent observations. Recent avalanches, collapsing ("whumpfs"), shooting cracks, rapid warming, heavy snow or rain, and wind loading are the red flags, and most avalanches happen when at least one is present.

The rebuilt SkiBuds app, coming soon, will show a resort's official zone rating from the public avalanche forecast map at Avalanche.org, and only when the bulletin is current and the resort sits inside one zone rather than on a boundary; otherwise it will show no rating and say why. Every card will link to the center's full bulletin and call itself a zone summary, not a resort or route safety assessment, because steps 3 to 5 happen in that bulletin.

A worked example: one made-up forecast for Stevens Pass

Say it's a Saturday in February and the Northwest Avalanche Center's forecast for the Stevens Pass zone reads like this. It is invented to show the parts; no real day is described.

• Danger: Considerable above treeline, Considerable near treeline, Moderate below treeline.
• Problem 1, wind slab: north through east aspects, near and above treeline. Likely. Small to large.
• Problem 2, persistent slab: north and northeast aspects, above treeline. Possible. Large.
• Bottom line: strong southwest winds drifted 8 to 14 inches of new snow onto north- and east-facing slopes. Avoid steep wind-loaded slopes near ridgelines. A buried weak layer on high north aspects could produce large avalanches.

Your plan was a tour to a north-facing bowl near treeline. The date is today's and the route sits inside the Stevens Pass zone, so move to the danger. Near treeline it's Considerable; the Moderate rating below treeline doesn't apply to the bowl. Then the problems. The bowl faces north near treeline, which puts it squarely inside the wind slab rose, rated likely, up to large. By the definitions above, that's a day when a random wanderer triggers several steep slopes, and a large slide can bury a person.

The persistent slab sits higher, and it can reach you from outside its own slopes. Persistent slabs can be triggered remotely, from flatter ground below, above or beside a steep slope, and tracks on a slope are not a sign it is stable. One photo on that page shows a persistent slab released by the fifth skier down a bowl.

So the bowl is out. A plan that fits this forecast is low-angle terrain, under about 30 degrees, below treeline or on aspects outside both roses, with nothing steeper above it. If you hear a whumpf or see a shooting crack on the way, that is a sign of instability, not a diagnosis of which avalanche problem is present. Avoid exposing the group to steep slopes or their runouts, and change the route or turn back.

A rule to carry: match the problem to the slope

Before you ride, cross or travel below any slope steeper than about 30 degrees, ask two questions. Does its aspect and elevation fall inside a shaded problem rose? Is the danger at that elevation Considerable or higher? If either answer is yes, choose low-angle terrain that isn't connected to steeper slopes above it. At High or Extreme, stay out of avalanche terrain entirely, including the runouts below it. And if you see a red flag the forecast didn't mention, believe the snow in front of you.

Two "no" answers are not clearance to enter a slope. Even when no specific avalanche problem is listed, Avalanche.org's normal-caution guidance still calls for rescue gear, one-at-a-time travel and attention to signs of instability. Those precautions do not make an unstable slope safe. If your group is uncertain about the terrain or conditions, choose a route outside avalanche terrain or turn back.

The 30-degree line matches where slides start: in a Swiss dataset of more than 1,000 avalanches that avalanche.org cites, 96% released on slopes between 30 and 50 degrees. But the rule has other limits. In rare instances slides release on slopes between 25 and 30 degrees, and avalanches from above can run into flat terrain. Telling 28 degrees from 35 by eye is hard, and avalanche.org calls an inclinometer "the most cost-effective safety device you can own": lay a ski pole or probe down the fall line and set the inclinometer against it, or use one built into a compass, beacon, pole or phone app. Measuring slopes well, reading the snowpack and finding a buried partner are still skills a course teaches and a forecast can't.

Before you go into avalanche terrain

• Get trained. Take a recognized avalanche course before riding uncontrolled terrain.
• Carry rescue gear and know how to use it: a transceiver, shovel and probe for everyone, and practice with them.
• Never go alone. Ride one at a time on exposed slopes and keep each other in sight. How to find a backcountry ski partner covers where to find one and what to ask.
• Make a plan that fits the forecast, and agree in advance on what would make you turn around.

In the United States, find your local avalanche center through Avalanche.org. In Canada, Avalanche Canada publishes forecasts. Safety on SkiBuds and on the mountain covers the rest of a safe day out, and Chinook winds and snow explains how rapid warming changes the snowpack.

On the made-up Stevens Pass Saturday, five minutes with the full bulletin turned a north-facing bowl into a lap of low-angle trees. A good forecast read often ends that way, with a smaller plan than the one you drove up with, and the place to make it is the kitchen table, with the forecast open and your partner on the phone, before anyone is standing at the top of the bowl.