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Avalanche Forecast

Archived

Feb 12th, 2018–Feb 13th, 2018

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

Regions

Lizard-Flathead.

Wind slabs are the main concern at higher elevations - they may sit on several critical weak layers. Give cornices a wide berth too, and be aware of overhead hazard as you travel.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

We're looking at cool temperatures and some gusty west through north winds for the next few days. Overnight lows will be near -20 Celsius at tree line. TUESDAY: A mix of sun and cloud. Ridge wind becoming strong from the west. Temperature -7. Freezing level 1100m in the afternoon.WEDNESDAY: Flurries (5 -15cm accumulation). Ridge wind moderate gusting strong from the north / east. Temperature -7. Freezing level valley bottom.THURSDAY: Sunny with cloudy periods. Ridge wind gusting moderate from the west. Temperature -10. Freezing level valley bottom.

Avalanche Summary

On Saturday, explosive control produced wind slab results up to size 2 on north facing slopes near 2000m. A week ago, a snow cat remotely (from a distance) triggered a size 3-3.5 persistent slab avalanche that stepped down from 80-250 cm and ran to ground in the lower start zone on a south aspect at 2100 m. Also a week ago, the east facing Mt. Corrigan slidepath produced a very large, natural avalanche. The avalanche is estimated to be a size 4.0, and it took out mature timber in the path as it overran the Flathead FSR south of Corbin.

Snowpack Summary

New snow totals from last week vary from 5-20cm above 1700m, thanks to redistribution from variable winds. Wind slabs up to 20cm thick have formed on down wind (lee) features higher up. Below 1700m, rain last Wednesday formed a thick crust and capped the snow pack. That said, avalanche hazard remains elevated below tree line due to the possibility of large avalanches running full path from higher elevations. For the past month there has been regular avalanche activity on multiple weak layers in the mid and lower snowpack. A widespread weak layer from mid December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack.The take home message is that several weak layers are still lurking in the snowpack and deserve a lot of respect. Conservative terrain use is recommended while avoiding all overhead hazard.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.