Avalanche Forecast

Issued: Jan 3rd, 2015 3:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs and Persistent Slabs.

Peter, Avalanche Canada

Sea To Sky.

Heavy snow followed by warming temperatures and rain will push avalanche danger to HIGH by Monday and tip off a natural avalanche cycle.

Summary

Confidence

Fair - Freezing levels are uncertain on Monday

Weather Forecast

Synopsis: A strong Pacific frontal system will impact the Coast beginning on Sunday morning. Precipitation should start as snow at low elevations as the moist airmass clashes with the entrenched Arctic air. On Sunday we could see 20-40 cm of snow with ridge winds increasing to moderate or strong from the SW. On Monday temperatures are expected to climb, but how warm it will get is still somewhat uncertain. The freezing level could jump to around 2000 m. Heavy precipitation will continue with another 40-60 mm (or cm at higher elevations) possible. Tuesday looks even warmer with freezing levels rising as high as 2500 m. Moderate precipitation should continue.

Avalanche Summary

On Friday there was one report of a couple intentionally triggered size 1 wind slabs from 35 degree north-facing slopes at treeline. Avalanche activity will increase over the next few days as snow accumulates, winds increase, and temperatures rise. We could see natural storm slab avalanches step down to deeper weaknesses creating very large avalanches in some areas.

Snowpack Summary

Forecast snowfall will cover a highly variable snow surface that includes wind affected surfaces (crust, hard and soft slabs), a freezing rain crust or sun crust, and surface hoar or sugary facets in sheltered shady terrain. How well the new snow bonds to this surface is the question. Below this we also have to contend with the mid-December surface hoar or crust/facet weak layers. These are buried between 40 and 80 cm deep on average. The surface hoar layer may be found in sheltered shady areas at and below treeline, while the crust/facet layer could be more widespread but also more variable. Reports suggest is could be between 0 and 120 cm deep depending on exposure to wind. Heavy loading in the coming days could trigger these weaknesses creating very large avalanches.

Problems

Storm Slabs

Storm and wind slabs (exposed N-E aspects) will build throughout the day on Sunday. The new snow may not bond well to the old snow surface. Human triggering is likely with a natural avalanche cycle expected on Monday.

Use ridges or ribs to avoid pockets of wind loaded snow.>Choose conservative lines and watch for clues of instability.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

Heavy loading from new snow, wind, and/or rain could overload persistent weaknesses in the upper 100 cm of the snowpack. The resulting avalanche would be very large and destructive.

Whumpfing, shooting cracks and recent avalanches are all strong inicators of unstable snowpack.>Avoid open slopes and steep convex rolls at and below treeline where buried surface hoar may be preserved.>

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

2 - 5

Valid until: Jan 4th, 2015 2:00PM