Avalanche Forecast

Issued: Dec 12th, 2015 8:41AM

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

ghelgeson, Avalanche Canada

Purcells.

The trees are NOT a safe haven right now as a persistent weak layer at lower elevations remains sensitive to human triggers. Snow and wind from the weekend storm will likely form touchy storm slabs in wind exposed terrain at upper elevations too.

Summary

Confidence

Low - Timing, track, or intensity of incoming weather system is uncertain on Sunday

Weather Forecast

A tightly wound and rapidly deepening low pressure centre is expected to track just south of Vancouver Island Saturday afternoon. The northern edge of this system should track through the Purcells Saturday night with lingering convection generating continued snowfall through the day Sunday. A ridge is expected to take over the pattern Monday afternoon and should persist through Wednesday. SATURDAY NIGHT: 2 to 10cm of snow, freezing level near valley bottom, moderate to strong SW winds. SUNDAY: 1 to 5cm of snow, freezing level rising to 1000m, moderate to strong SW winds. MONDAY: 1 to 5cm of snow, freezing level near valley bottom, light W/NW winds. TUESDAY: No snow expected, freezing level at valley bottom, light variable winds.

Avalanche Summary

On Friday two significant observations were reported: The first was a size 3.5 natural avalanche that was observed on a SE facing feature at 2800m running to ground in some places. The second was a report of a skier triggered size 2.5 avalanche that was accidentally triggered on an east facing feature at 1500m in the northern Purcells. This avalanche failed on the early December Persistent Weak Layer. Control work in the central and southern Purcells Thursday produced avalanches to size 3.5 on a variety of aspects, some of which ran full path.

Snowpack Summary

A string of storms between December 2nd and December 11th produced 35 - 80cm of storm snow in the Purcells which is settling out at all elevations. You may find a brittle crust about 10 to 20cm below the snow surface as high as 2000m that was formed by rain and warm temperatures last Tuesday. Below all the new snow lies the early December persistent weak layer. This weak layer manifests as an old sun crust on due south facing features in the alpine, large grained surface hoar below 1800m and small facets in isolated pockets. The surface hoar is the major player and it's been most reactive below treeline between 1400 and 1800m. Recent winds out of the south and southwest have formed wind slabs on lee features that are mainly confined to the alpine, but you may find the odd fresh wind slab at treeline too. The mid and lower portions of the snowpack are thought to be well settled.

Problems

Storm Slabs

New snow and wind is expected to create fresh storm slabs in wind exposed zones. Human triggered avalanches are most likely immediately lee of ridge crest and on mid slope terrain features like convexities, wind loaded chutes and rock outcroppings.

The new snow will require at least a day to settle and stabilize.>Avoid all avalanche terrain during periods of heavy loading from new snow and/or wind.>Avoid freshly wind loaded features and use ridges or ribs to travel around these problem areas.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

Limited alpine visibility may push riders below treeline where a reactive layer of buried surface hoar has been most sensitive to human triggering between 1400 and 1800m. Savvy folks will seek out well supported simple terrain at treeline Sunday.

Avoid open slopes, convex rolls, burns and cutblocks below treeline where buried surface hoar is best preserved and most volatile.>Avoid exposure to terrain traps where the consequences of avalanches will be the most serious.>Storm slabs in motion on south facing alpine features have the potential to step down to a deeply buried crust resulting in very large destructive avalanches.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

3 - 6

Valid until: Dec 13th, 2015 2:00PM