Monday December 24, 2018

Wednesday December 26, 2018
Tuesday December 25, 2018
Monday December 24, 2018
Confidence: Moderate - Good weather data for all regions but few snowpack obs for the north and west zones of the forecast region.

Main Concerns

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

Travel & Terrain Advice

Active layers still linger in the snowpack so extra caution, conservative decision making and good snowpack data gathering are highly recommended. Avoid wind loaded pockets and slopes at all elevations especially on NW-NE asp and be wary of crossloaded features. Large cornices have developed at treeline and in the alpine so give these monsters a wide berth both from above and on slopes below. Thanks to Steve Janes for his report on avalanche activity up at Mt Arrowsmith, its a good reminder that climbers need to be aware of avalanche hazard as well and avoid terrain when the bulletin suggests that significant hazard exists.

Avalanche Summary

Well its been an exciting few days. Avalanche activity both natural and explosive (Mt Washington Patrol) has been widespread from size 1 to size 2 through out the entire forecast region. There have been limited obs for the alpine but we can expect a natural cycle up to size 3-3.5 to have commenced during the past couple of storms. Conditions still exist so be very cautious out there over the next few days. Sunday Dec 23 widespread settlements (whumphs) were still being felt as storm layers failed (thanks Sam Lam for the reports from Paradise Meadows).

Avalanche Problems

Cornice Fall

New snow and high winds have begun to build up our island cornices. These overhangs now exist in the alpine and on some treeline ridge tops lingering over NW to NE slopes. Triggering may be possible by natural and human causes and if they fail may lead to size 1-2 avalanches. As failed cornices come with significant weight, they may even cause a step down effect on deeper weak layers on the slopes below (from within our mid and lower snowpack). Stepping down would result in an even larger avalanche (sz 2.5-3), possibly triggering our Dec 22 crust layer and in a worse case scenario (although unlikely) the weak layer just above the ground from early season (some 150-300 cm deep).

Storm Slab

Significant new snow has fallen in the past three days. Due to variable winds and temperatures, multiple weak layers exist in the new upper snowpack and they will be slow to heal (become more stable) over the next three days. Wind redistribution of the new storm snow to NW and NE slopes and crossloaded features creates even higher hazard due to the existence of wind slabs. These widespread danger layers will be found at all elevations and will exist on all aspects (esp NW to NE). They have the potential to cause avalanches from size 1-2, may possibly trigger naturally and are likely to trigger with human activity. The additional weight of these avalanches also comes with the threat of stepping down to mid snowpack layers (Dec 22 crust), which would increase the potential avalanche size up to 2-3.

Snowpack Summary

Significant new snowfall has burst onto the island in all our forecast regions. Variable winds have redistributed storm snow into even deeper pockets on lee slopes. Weakness exist within the new storm snow and in layers from previous reports.

Snowpack Details

SurfaceNew snow fall has been redistributed by strong SE-SW winds at treeline and above. Warm temps below treeline Sunday created a moist surface that may result in a thin crust as temps drop.
Upper40-80 cm of new snow fell over the weekend. Variable winds from the SW and SE have left a weakness in the storm snow approx 20-40 cm down.
MidA crust buried on Dec 22 still lingers in treeline and below elevation bands with weak dry snow below it.
LowerMainly well settled and bridged, but a weakness near the ground from early season still exist (limited information has been gathered on its potential)

Past Weather

Over the past three days 40-80 cm of new snow has fallen (larger amounts found in Strathcona and Mt Washington, less up north). Winds were strong from the SE and SW during these events which caused significant redistribution of the snow. Temps ranged from 0 to -8 also resulting in irregularities in the upper snowpack.

Weather Forecast

Things finally slow down a bit for Christmas with only a small storm arriving later in the evening Christmas into early Boxing day morning. Time for some holiday adventures but be cautious out there.

Monday Dec 24 - a trace (north) to light snowfall (up to 6 cm east and west) will fall through out the day. Winds will be light SE rising and shifting to moderate NW. Temps ranging from -2 to -4 in the hills. Freezing levels 900 m dropping to 600 m in the north and down to near sea level in the east and west of the forecast region.

Tuesday Christmas Dec 25 - a mix of sun and cloud during the day with snow starting later on and increasing over night. Winds moderate NW switch to strong SE as the storm system arrives. Temps range from -2 to -6. Freezing levels around 500 m to 800 m.

Wednesday Boxing day Dec 26 - New snow overnight ( 10-15 cm approx) tapers off in the morning. Winds SE and strong overnight drop and swing to light W. Temps hover around -4. Freezing levels 600 m to 900 m.

Posted on Monday December 24, 2018 by Bill Phipps

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