Friday December 14, 2018

Friday December 14, 2018
Sunday December 16, 2018
Saturday December 15, 2018
Confidence: Low - Limited weather data, multiple wx stations not operating/available. Minimal field observations (esp obs in the alpine)

Main Concerns

Cornice Fall, Wet Slab, Loose Wet - view Avalanche Problems for detailed information.

Travel & Terrain Advice

Avoid exposure to all avalanche terrain during the warm temperature spikes accompanied by rain events over this forecast period. The thin/shallow early season snowpack will quickly lose stability as temps climb and rain saturates the snow.

In all but the alpine, the snow levels are still low, so be wary and on the look out for open creeks/holes and rocks/stumps etc lurking just below the surface.

Due to limited obs, spend a good amount of time doing your own snowpack observations and if unsure/unableā€¦ as usual avoid avalanche terrain.

Avoid wind loaded slopes where trigger-able slabs maybe (NW-NW aspects).

Avalanche Summary

No avalanche data/observations are currently available due to minimal field time (as it is the beginning of the season teams have not gotten out and about) and there has been limited visibility. During this forecast period, one can certainly expect to find evidence of a widespread loose wet avalanche cycle in the alpine and at treeline as a result of the big rain event (at all elevations) that happened Thursday Dec 13.

Avalanche Problems

Cornice Fall

High winds have developed moderate sized cornices. As temps and freezing levels sore high, rain saturates the upper snowpack and/or as new snow accumulations increase, we may see these cornices fail. They will be found on NW to NE aspects in the alpine and on some open ridge zones at treeline. They will have a likely chance of triggering during the warm wet spells. They have the potential to produce up to size 2 avalanches so avoid exposure above and below these features.

Wet Slab

Deep pockets of wind transported snow lay on top of a near basal(ground) crust in the alpine and treeline. As rain and more snow loads these zones there is the potential for slab activity. Expect to find these slabs on NW-NE aspects and on cross loaded features where wind accumulations are great. While triggering is only possible-likely (both natural and human) these slabs could produce up to size 2.5-3 avalanches in specific spots. The thick crust will have a poor bond to smooth ground features (rock slabs, heather slopes, glacier ice), has a weak layer mid crust (in some locations,) and its upper surface may also act as a sliding surface.

Loose Wet

With fluctuating freezing levels and rising temps, snowfall may change to rain. This rain (potentially heavy at times) will quickly destabilise and weaken the snowpack. Expect loose wet avalanche activity to be widespread (especially near steep rocky terrain) at all elevations and aspects during these rain events. These avalanches will be size 1-2 below treeline and potentially up to size 2-2.5 in the alpine where accumulations are greater. Natural and human triggering is very likely to almost certain if and when the warm ups happen.

Snowpack Summary

A thin early season snowpack down low (below treeline). Zones of deep snow are found in wind loaded lee pockets in the alpine and treeline. Rain has saturated shallow areas right down to the ground and the majority of lower below treeline terrain is below threshold (to shallow to have avalanches of significant size). High winds have resulted in some cornice development in the alpine and treeline.

Snowpack Details

Surfacewet snow from Thursday nights rain event with new moist snow arriving Friday
Upperrain saturate snow with a few thin melt freeze crusts
Midrain saturated snow from previous snow events
Lowerlarge crust from early season snow with a potential failure layer (of still dry snow treeline and above)mid crust, all bonded poorly to ground.

Past Weather

A large rain event Thursday night has saturated the thin early season snowpack. High winds during prior snow events has transported significant amounts of snow in the alpine and treeline elevation bands.

Weather Forecast

Multiple rain and snow events as temps and freezing levels fluctuate with steady moderate to strong SW to SE winds.

Friday - Heavy rain and snow with strong winds during the day at all elevations. Temps and freezing levels drop overnight resulting in light to moderate snowfall.

Saturday - Light-moderate snow fall in the morning with cold temps (-2 to -5). Snowfall increasing to moderate mid day then heavy overnight. Winds remaining moderate to strong SW to SE. Small reprieve between two big warm weather events

Sunday - Heavy snowfall continues into the morning potentially changing to moderate to heavy rain around noon. Winds remain moderate to strong.

Posted on Friday December 14, 2018 by Bill Phipps

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