Avalanche Forecast

Lizard Range and Flathead

Date issued

Feb 1, 2018 - 5:21 PM

Valid until

Feb 2, 2018 - 2:00 PM

Prepared by

shorton

Snow forecasts are uncertain and the snowpack is untrustworthy. Conservative decision making is critical.

Regular season
Thursday - February 1, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Friday - February 2, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Saturday - February 3, 2018
Alpine

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.

Treeline

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.

Below Treeline

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.

Avalanche Problem 1: Storm Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

Fresh storm slabs are building and will be most touchy at higher elevations and in wind-affected terrain.

Travel and Terrain Advice
  • Choose low-angle, well supported terrain without convexities.
  • Be alert to conditions that change with elevation.
  • Minimize exposure during periods of heavy loading from new snow and wind.

Avalanche Problem 2: Deep Persistent Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

Large avalanches are running on deeply buried weak layers. Avoid exposure to overhead avalanche terrain, including cornices which have been a common trigger.

Travel and Terrain Advice
  • Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.
  • Good day to make conservative terrain choices.
  • If triggered the storm/wind slabs may step down to deeper layers resulting in large avalanches.

Avalanche Summary

On Thursday, the fresh low density snow was reactive to ski cutting and produce small soft slab avalanches (20 cm thick) and dry loose avalanches in steep terrain. Some natural slab activity was observed in alpine terrain.

Large deep persistent slab avalanches have been reported on a regular basis throughout January. On Tuesday, numerous large avalanches (size 2-3.5) were triggered naturally as well as with explosives. Many of the natural avalanches were triggered by cornices falling and releasing slabs on the December and November layers, producing 150-250 cm thick crowns. See the avalanche photos in this Mountain Information Network report for an example. The snowpack is still weak and additional stress (such as a cornice, person, or new snow) could trigger destructive persistent slab avalanches.

Snowpack Summary

Expect waves of fresh snow over the next few days as storm snow accumulates. So far, about 40 cm of low density snow sits above a breakable crust up to 1700 m. Stay aware of the condition of the new snow. Mostly low density powder is expected, but could become heavy if it warms up. Also watch for slab development in wind affected areas.

For the past month we have seen regular avalanche activity on multiple deep weak layers. A layer of surface hoar on sheltered aspects (especially prominent from 1400-1900 m) and sun crust on solar aspects was buried mid-January and is now 60-80 cm deep. Another surface hoar layer from early-January is buried 90-110 cm deep. A widespread weak layer from mid-December is composed of facets, crusts, and surface hoar and is now buried 120-160 cm deep. Finally, a rain crust with sugary facets buried late-November is near the bottom of the snowpack and is up to 200-250 cm deep.

The bottom line is the snowpack structure in this region is weak. Human triggering is most likely on the shallower weak layers, but any avalanche has the potential to step down to deeper layers and become much larger.

Weather Forecast

Clashing fronts make the next few days difficult to forecast. A Pacific warm front and an Arctic cold front are converging over the Rockies, creating the potential for surprise accumulations of low density snow. Timing, location, and intensity are highly uncertain (and so are the snowfall estimates).

FRIDAY: The first wave of snow starts Thursday night with 10-20 cm then another 5-15 cm possible throughout the day on Friday/ moderate west wind / alpine temperatures around -6 C.

SATURDAY: Continued flurries with 5-20 cm possible / moderate west wind / alpine temperatures around -5 C.

SUNDAY: Ongoing flurries with some possible sunny breaks / strong northwest wind / alpine temperatures around -3 C.

Danger Ratings Explained

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.

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