Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Feb 21st, 2023–Feb 22nd, 2023

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells, Dogtooth, East Purcell, St. Mary, Bull.

Up to 45 cm of recent snow and strong northerly winds have created dangerous storm slabs that will be reactive to human triggers at all elevations on Wednesday. The recent snow is overloading a weak snowpack and very large human triggered avalanches are especially likely in the alpine.

Make conservative terrain choices and avoid overhead hazard.

Confidence

High

Avalanche Summary

Several very large (up to size 3.5) naturally triggered deep persistent slab avalanches failing on weak facets near the bottom of the snowpack were reported on Monday. These avalanches all occurred in the alpine and were likely triggered by wind loading.

Additionally, several cornice failures triggered persistent slab avalanches around 40 cm deep on alpine features in the dogtooth range near Golden.

Last Thursday, a size 3.5 avalanche failed on a deep persistent weakness, resulting in 2 fatalities west of Golden. See MIN post.

Snowpack Summary

30-45 cm of new snow has buried a layer of surface hoar that formed in sheltered areas and a sun crust that exists on steep south aspects. Strong northwesterly winds have created wind slabs on lee features in open terrain.

A variety of persistent layers still exists in the middle snowpack and continue to see avalanches occur on them periodically. Don't let these layers surprise you.

The lower snowpack contains a widespread layer of large, weak facets that is typically 80 to 150 cm deep. This layer continues to periodically produce very large and destructive avalanches; especially during periods of rapid loading from snow and/ or wind.

Snowpack depths at treeline range between 80 and 200 cm, with the shallowest snowpacks found on the eastern edge of the Purcells.

Weather Summary

Tuesday night

Cloudy / Light, east ridgetop wind / Low temperature at treeline around -28 C.

Wednesday

Mix of sun and cloud / Light, southeast ridgetop wind / High temperature at treeline around -19 C.

Thursday

Mostly sunny / Strong, east ridgetop wind / High temperature at treeline around -21 C.

Friday

Sunny / Strong, west ridgetop wind / High temperature at treeline around -17 C.

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Travel in alpine terrain is not recommended.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Cornice failure may trigger large avalanches.
  • Avoid rock outcroppings, convexities, and anywhere the snowpack is thin and/or variable.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.