Avalanche Risk in the US: How FEMA Scores It and How to Prepare

Written by Dvir · Data: FEMA National Risk Index v1.20 · Updated August 4, 2026

Avalanches are the National Risk Index’s most specialized hazard: they threaten a narrow slice of mountain America, and unlike nearly every other hazard on this site, the victims mostly choose their exposure. Around 25–30 people die in US avalanches in a typical winter — almost all of them backcountry skiers, snowboarders, snowmobilers, and climbers who triggered the slide themselves. In the NRI, avalanches (hazard code AVLN) account for roughly $420 million in national expected annual loss across 684 mountain counties.

What counts as avalanche risk

An avalanche is a mass of snow releasing and accelerating down a slope — the dangerous kind is the slab avalanche, where a cohesive plate of wind- or storm-deposited snow fractures on a buried weak layer and takes the slope with it, reaching 60–80 mph within seconds. The recipe: a slope steep enough to slide (roughly 30–45 degrees — prime skiing terrain, not coincidentally), a weak layer in the snowpack, and a trigger. In 90% of fatal accidents, the trigger is the victim or someone in their party.

Risk to fixed infrastructure — homes, highways like Little Cottonwood Canyon or US-550, ski areas — exists but is heavily managed with explosives, closures, and snow sheds. The unmanaged risk is the backcountry, and it has grown as lightweight gear and snowmobile power put far more people on avalanche terrain.

How the NRI measures it

FEMA combines each county’s exposure (population and property in avalanche terrain), annualized avalanche frequency, and historic loss ratios — dominated by the valuation of fatalities — into an Expected Annual Loss scored 0–100 against all counties. Because deaths drive the dollars, the ranking tracks where recreation meets serious snowpack, not just where the most snow falls.

Where the risk concentrates

Teton County, WY (Jackson Hole) leads the nation — score 100, ~$23M expected annual loss, and roughly two significant avalanche events a year in the data. Alaska takes three of the top five slots: Matanuska-Susitna Borough (the Talkeetna and Chugach ranges’ snowmobile country), Kenai Peninsula Borough, and the Copper River area, with Park County, MT (the Absarokas, north of Yellowstone) rounding out the group. Colorado — which leads the nation in avalanche deaths most winters — blankets the high-rating list with its San Juan, Elk, and Front Range counties, alongside Utah’s Wasatch and Washington’s Cascades.

How to read your county’s score

If your county scores high, avalanche risk is a real feature of local winters — but it is overwhelmingly avoidable risk. For residents: know whether your home, access road, or commute crosses mapped runout zones (mountain counties publish these), and respect highway closures — they exist because control work is imminent. For anyone who travels the winter backcountry, the county score is beside the point: your risk is determined by terrain choice, snowpack, and training, every single day.

What actually reduces the risk

  • Check the forecast — every time. Regional avalanche centers (avalanche.org) publish daily danger ratings by elevation and aspect. Most fatal accidents happen on days the danger was rated Considerable or higher — the ratings work if you obey them.
  • Get trained. An avalanche course (AIARE Level 1 or equivalent) teaches the terrain recognition and decision frameworks that actually prevent burials. Slope angle is learnable: under 30 degrees, slab avalanches essentially don’t start.
  • Carry the trio, know the trio: transceiver, probe, shovel — on your body, practiced regularly. A buried victim’s survival drops below 50% after 15 minutes; rescue comes from your own party or not at all. Airbag packs add meaningful survival margin.
  • Travel protocol: one at a time on suspect slopes, watch your partner from safe ground, never assume tracks mean stability — a slope can hold a dozen crossings and fail on the thirteenth.
  • Snowmobilers are now half of US fatalities: high-marking on steep faces is the signature fatal maneuver, and machine burials are deep. Same forecast, same gear, same training apply.
  • In-bounds at ski areas is a different world — avalanche risk inside open, controlled runs is very low. The rope line is the risk boundary; ducking it puts you in uncontrolled snowpack, legally and physically alone.

From mining disasters to recreational deaths

American avalanche history divides into two eras, and the transition explains everything about the modern risk profile.

The Wellington disaster, Washington, March 1, 1910. After days of heavy snow, an avalanche off Windy Mountain swept two stalled Great Northern trains off the tracks near Stevens Pass and into a ravine, killing 96 people — the deadliest avalanche in US history. It remains the benchmark for avalanche risk to fixed infrastructure, and it drove the construction of snow sheds and the eventual relocation of the rail line into a longer tunnel.

The mining-era slides of the San Juan Mountains. Through the late 19th and early 20th centuries, avalanches repeatedly destroyed mining camps and killed workers around Silverton, Telluride, and Ouray. Colorado’s mountain communities lost more people to avalanches in that period than in the entire recreational era since. Those events, and the highway closures on routes like US-550’s Red Mountain Pass, produced the first organized American avalanche control programs.

Alta, Utah, and the birth of avalanche forecasting. Beginning in the late 1930s and 1940s, the Forest Service’s work at Alta — particularly under Monty Atwater, who pioneered the use of artillery for avalanche control — created American avalanche science. The techniques developed there became the basis of ski area control work nationwide.

The modern era is recreational. As mining declined and highway and ski area control matured, avalanche fatalities shifted almost entirely to backcountry travelers. The Colorado Avalanche Information Center, founded in the 1970s and now the model for the national network of avalanche centers, tracks every US fatality. Its accident database shows the modern pattern with unusual clarity: roughly 25 to 30 deaths in a typical winter, over 90% triggered by the victim or a member of their party, and an increasing share involving snowmobilers.

March 2023, Colorado. A single season’s accident reports illustrate the recurring themes — experienced parties on Considerable-rated days, groups exposing multiple members at once, and rescues that succeeded or failed on whether companions had transceivers and knew how to use them.

Season, snowpack, and the geography

  • Season runs November through May, with fatalities peaking in January through March when the snowpack is deepest and persistent weak layers are most developed. Late-season wet avalanches in April and May are a distinct and often-underestimated problem.
  • Continental snowpacks — Colorado, Utah, Montana, Wyoming — are the most treacherous. Cold, dry, relatively thin early-season snow forms persistent weak layers (depth hoar, surface hoar, faceted crystals) that can lurk for months and fail catastrophically long after the storm that buried them. Colorado leads the nation in avalanche deaths most winters for exactly this reason.
  • Maritime snowpacks — the Cascades and Sierra Nevada — are warmer, wetter, and denser. They stabilize faster, and the danger is concentrated during and immediately after storms.
  • Intermountain snowpacks — the Wasatch, Tetons, and much of Idaho and western Montana — sit between the two and can produce both failure modes in the same season.
  • Alaska combines enormous terrain, deep snow, and a heavy snowmachine culture, and carries some of the highest expected losses in the index.
  • The dangerous slope angle is 30 to 45 degrees, with most fatal slab avalanches starting between roughly 35 and 40 degrees. That is not a coincidence — it is also the most appealing skiing and riding terrain that exists. Terrain traps (gullies, creek beds, cliff bands, tree stands) turn survivable slides into fatal ones by increasing burial depth and trauma.
  • The trigger is almost always human. In roughly nine out of ten fatal accidents, the victim or a partner triggered the slide. Natural avalanches kill people mainly on highways and at fixed structures, which are heavily managed.

Gear, training, and the decisions that matter

Avalanche risk is unusual in this index because it is almost entirely avoidable through decisions, and the equipment is the last resort rather than the plan.

  • Read the forecast, every day you go out. Avalanche.org aggregates the daily bulletins from every US avalanche center, rated by elevation and aspect on the five-level North American danger scale. The overwhelming majority of fatal accidents occur on days rated Considerable or higher. The ratings work; people override them.
  • Take a course. AIARE Level 1 or an equivalent avalanche course (typically three days) teaches terrain recognition, snowpack assessment, trip planning, and companion rescue. The single most valuable skill it teaches is reading slope angle — because below 30 degrees, slab avalanches essentially do not start, and terrain selection is the only control that works reliably.
  • Carry the trio and practice with it: transceiver, probe, shovel. Worn on the body, not in a pack that gets stripped off. A digital transceiver is useless if you have not practiced a search within the last month. Survival odds for a completely buried victim fall steeply after about 15 minutes, and rescue comes from your own party or not at all — organized rescue arrives long after the window has closed.
  • An avalanche airbag pack measurably improves survival by keeping victims closer to the surface, and is worth the cost for anyone regularly in avalanche terrain. It does not help with trauma, and it is not a license to take worse terrain.
  • Travel protocol. One person at a time on any suspect slope, with the others watching from genuinely safe ground. Regroup in protected spots, not below the slope you just crossed. Never assume existing tracks mean stability — a slab can hold a dozen crossings and fail on the thirteenth, when someone finds the thin spot where the weak layer is closest to the surface.
  • Group dynamics kill. The documented pattern in accident reports is social, not technical: large groups, deference to the most confident member, reluctance to voice doubt, commitment to a planned objective, and the familiarity of terrain skied many times before. Agree in advance that anyone can call off a slope without having to justify it.
  • Snowmobilers now account for a large share of US fatalities. High-marking — driving as far up a steep face as the machine will climb — is the signature fatal maneuver, and machine-triggered burials tend to be deeper. The same forecast, gear, and training apply; the higher speeds and greater range simply mean a rider can reach dangerous terrain much faster.
  • In-bounds is a different world. Avalanche risk inside open, controlled terrain at a ski area is very low, because control teams do explosive work every morning. The rope line is the boundary between managed and unmanaged snowpack. Ducking it puts you in uncontrolled terrain, alone, often without gear, and frequently without anyone knowing where you are.

Residents, infrastructure, and insurance

  • Know whether you live in a runout zone. Mountain counties in Colorado, Utah, Washington, and Alaska publish avalanche hazard mapping, and many have zoning that restricts building in mapped paths. If you are buying in a mountain valley, ask specifically — a house in a runout zone can go decades without incident and then be destroyed.
  • Respect highway closures. Passes close for control work, and the explosives are about to fly. Driving around a closure gate is how motorists get caught in a path that would otherwise have been made safe.
  • Homeowners insurance generally covers avalanche impact as a form of falling-object or weight-of-snow damage, but coverage varies and a slide that pushes a structure off its foundation may be argued as excluded earth movement if it carries soil and rock. Ask your carrier directly and get the answer in writing if you are in a mapped path.
  • Search and rescue costs. Most US states do not bill for rescue, but some do in limited circumstances, and helicopter evacuation and medical transport are billed regardless. A rescue-card program (several states sell inexpensive ones) or an appropriate insurance rider is worth the small cost for frequent backcountry travelers.
  • Tell someone your plan. A written trip plan left with a person who will notice you are late is the cheapest safety measure in this entire guide.

Before, during, and after

Before. Check the forecast and read the whole bulletin, not just the danger rose — the discussion of which problem type (persistent slab, wind slab, wet loose) is active tells you which terrain to avoid. Plan a route with terrain options that let you downgrade the objective without abandoning the day. Confirm everyone has the trio and has practiced. Do a beacon check at the trailhead, every time.

During. Watch for the red flags the forecasters emphasize: recent avalanche activity on similar slopes and aspects (the single best indicator there is), cracking or whumpfing underfoot, heavy snowfall of a foot or more in 24 hours, strong wind loading and cornice formation, and rapid warming or rain on snow. Any of these means step back to lower-angle terrain. If you are caught in a slide: try to escape to the side, jettison poles and pack straps if you can, fight to stay on the surface, and as it slows, get an arm in front of your face to make an air pocket and thrust the other hand upward. Snow sets like concrete the instant it stops.

If your partner is buried: this is a companion rescue and the clock is brutal. Mark the last-seen point, switch all transceivers to search, do the search, probe to confirm, and dig aggressively from downhill of the probe strike. Call for help, but do not stop searching to make the call — the difference between a live recovery and a body recovery is usually measured in single-digit minutes.

After. Get medical evaluation even for victims who appear fine; hypothermia and trauma can be masked by adrenaline. Report the accident to your regional avalanche center — the accident reports are how the entire American avalanche community learns, and they are written without blame precisely so people will file them.

Sources

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