Saturday December 19, 2020

Monday December 21, 2020
Sunday December 20, 2020
Saturday December 19, 2020
Confidence: High - Weather models in agreement, Moderate field data available, insufficient Weather station data.

Main Concerns

Wet Slab, Wind Slab, Storm Slab - view Avalanche Problems for detailed information.

Travel & Terrain Advice

Human triggered avalanches are VERY LIKELY this weekend on slopes steeper than 30 degrees. Be aware as you travel on or adjacent to slopes steeper than 30 degrees as precipitation, wind and potential Critical Warming remain a factor this weekend contributing to snowpack instability.

Careful route finding and snow-pack evaluation are paramount during periods of sustained moderate to strong snowfall this weekend as well as a fluctuating air temperature (Critical Warming) and strong/extreme winds.

Avoid traveling both above and below cornice features.

Careful and cautious route finding when entering into or over convex rolls or steep features.

Human Triggered triggered avalanches are “very likely” and “likely” (depending on elevation band) by skiers/snowmobilers/snowshoers on Saturday, Sunday and possibly Monday if forecasted precipitation, critical warming and winds arrive as scheduled. Finding lower angled terrain (generally less than 30 degrees) should be considered an appropriate choice given the increasing avalanche risk over the course of the next 3 days.

Mt Washington Alpine Resort: Please no ski touring on Mt Washington as explosives control work and heavy equipment are on the mountain in several locations

Avalanche Summary

Mt Washington reported several wind slab avalanches (up to size 1.5) occurring as a result of ski cut/avalanche control work on the mountain.

Avalanche Problems

Wet Slab

Moderate Rain events at lower elevation. Over the next several days (particularly Saturday) Rain could arrive up to 1900M of elevation (especially later in the day). This additional rain load could destabilize the upper snowpack with additional rain loading of fresh snow. In addition to Rain and snow events forecasted for Saturday, the increased daytime warming (especially later in the day), coupled with rain on surface snow may cause snow to become more easily triggered. This avalanche problem when triggered is likely to remain small, but on isolated terrain features that are steep, these features will promote a small avalanche to gain mass and result in a large avalanche up to size 2. Expect this avalanche problem to become more prevalent once the incoming rain arrives in addition to rising air temperature (particularly Saturday and into Sunday).

Location: On all aspects and found at Below Treeline and up to the Treeline elevation band on Saturday into Sunday depending on extent of forecasted air temperature rise in the afternoon of Saturday Dec 19th.

Possibility: Triggering of this avalanche problem is LIKELY from light loads such as skiers. Natural avalanches are possible.

Size: If triggered expect these avalanches to be small, and on isolated terrain features large, size 2.

Wind Slab

New snow from incoming storm system will rapidly fall on a variety of surfaces. Expect this new avalanche problem to be initially very touchy to light triggers such as skiers or sledders as strong amounts of precipitation and strong to extreme winds arrive on Saturday into Sunday.

Location: All aspects and at all elevations. New Storm snow may fall as rain at the Below Treeline Elevation Band.

Possibility: Triggering of this avalanche problem is Likely at Alpine to Treeline from light loads such as skiers. Natural avalanches are possible to likely.

Size: If triggered expect these avalanches to be small (Size 1) -and large (up to size 2) OR avalanches could appear on isolated terrain features (very large, size 3).

Storm Slab

Moderate precipitation in the form of storm snow coupled with strong-extreme winds on Saturday to Sunday could promote wind slab instabilities (particularly in leeward or down wind areas) on Vancouver Island. Expect all Alpine/Treeline environments in leeward/down wind terrain to be likely areas to trigger a wind slab avalanche over the next several days.

Location: Leeward and down wind aspects at Alpine and Treeline elevation bands.

Possibility: Triggering of this avalanche problem is LIKELY (depending on Elevation) from light loads such as skiers or sledders. Natural avalanches are unlikely to likely (depending on Elevation).

Size: If triggered expect these avalanches to be up to size 2 OR avalanches could appear on isolated terrain features (very large, size 3).

Snowpack Summary

Limited observations at this time however snowpack depth decreases substantially lower than the 1200M elevation band. Recent snowpack summaries show a depth of 120cm of Snow at an elevation of 1400M in the Mount Washington area.

Several melt freeze crust have developed on all aspects at the Treeline and Alpine elevation bands on Vancouver Island. The upper snowpack melt freeze layer is not yet completely frozen (in certain areas) and reports show that this layer is still isothermal (0 degrees) and slushy (in certain areas). In zones where freezing temperatures have remained consistent, the upper snowpack melt freeze crust is frozen completely, a facet layer is developing and is showing stubborn results on testing. Storm and windslab layers remain touchy and explosives control work at Mt Washington have produced avalanches up to size 1.5 as a result of new snow and wind slab sensitivity.

Snowpack Details

SurfaceStorm Snow, Wet Slab and Wind Slab instabilities will exist at Treeline and Alpine Environments this weekend
UpperWell bonded and dense moisture laden snowpack that contains a melt freeze crust that is showing consolidation with new snow above
MidWell settled
LowerWell Settled

Past Weather

Large amounts of snow and rain have fallen in the past 24hrs and the forecast indicates additional precipitation in the form of rain and snow for Saturday and Saturday evening. Alpine and Treeline elevation bands will prevail in terms of gaining the most amount of new snow without rainfall, however the freezing level will rise considerably (from 850M to 1800M in the span of several hours in the evening of Saturday Dec 19. In addition to moderate precipitation (rain/snow) Strong to Extreme winds on Saturday will exacerbate leeward loading on downwind slopes creating new wind slabs.

Weather Forecast

Saturday: 15cm Snow and 5mm Rain, Winds Strong to Extreme from the SW, Freezing levels 850m throughout day rising to 1850M in early evening, Temps @1500m -2 (rising to +4 at 7p).

Sunday: 6cm Snow, Winds Strong from the South shift to Moderate SW winds in afternoon, Freezing levels to 1,050m, Temps @1500m -2.

Monday: Trace amounts of snow possible, Winds Moderate from the NW, Freezing levels 500M, Temps @1500m -5.

Posted on Saturday December 19, 2020 by Ryan Shelly

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