Friday February 26, 2021

Sunday February 28, 2021
Saturday February 27, 2021
Friday February 26, 2021
Confidence: Moderate - Widespread extreme wind event island wide so significant new snow being transported to lees, very few public reports, warm up forecast for Sunday but that's three days way at time of writing (Thursday night).

Main Concerns

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

Travel & Terrain Advice

Avoid wind loaded zones/pockets on the lee side of ridges, in bowls, and on cross loaded features. Give these new windslabs time to heal and bond to the slippery sliding surface (the crust) they have landed on. Avoid travel on wind loaded aspects in the alpine and be very wary of alpine like features in the treeline (like the bowls around Mt Cain… A lot of snow fell up north!)

Keep an eye on the temperatures Sunday as freezing levels rise, especially at below treeline elevations. Watch for hints like trees shedding their snow, and pinwheeling. Solar aspects may be the most prone to loose wet avalanche activity.

Avoid exposure to cornices, both from above and below, as they will certainly have grown with the latest winds. The warming Sunday might also make them more sensitive to failure.

Avalanche Summary

Mt Washington patrol avalanche control Thursday: couple naturally triggered wind slabs to size 2 N aspects treeline down 30-40 cm, few ski cut size 1.5 on N to E aspects treeline down 10-20 cm, and many ski cut size 1 avalanches both wind slab and loosed dry on W-N-E aspects treeline and below treeline down 10-15 cm. All running down on the old crust layer.

Avalanche Problems

Cornice Fall

Strong winds and low density, movable new snow will certainly have added to the size of our cornices. The warming Sunday may make them even a bit more sensitive to triggering.

Location: The alpine and treeline ridges on N to E to SE aspects.

Likelihood to triggering: Possible with natural or human activity.

Size: 1 to 2.

Loose Wet

The warm up forecast for Sunday could potentially trigger loose wet avalanche activity at lower mountain elevations.

Location: Below treeline on all aspects, especially solar facing ones.

Likelihood of triggering: likely with human traffic and possible with natural events.

Size: size 1 to 1.5

Wind Slab

Touchy/ reactive Windslabs that formed with Thursday’s extreme winds from the S and W, will continue to grow and extreme winds from the NW will add to the aspects the slabs are found on. It will take these slabs time to heal and bond to the slippery sliding crust layer they landed on.

Location: All elevations (especially the alpine and treeline) on N to E to SE aspects. Lee of ridgetops, in bowls, depressions and crossloaded features.

Likelihood of triggering: Very likely with human activity and likely with naturally occurring events.

Size: size 1 in many locations, 2 in specific prone terrain features (bowls, lee of ridges..) and potentially even size 3 in isolated large avalanche paths.

Snowpack Summary

New snow fell Wednesday night into early Thursday morning with anywhere from 6-30 cm. 6-10 cm for the east and southern zones, around 15-20 cm for the west and Strathcona, 20-30 cm for the north. Extreme winds stripped this new snow off windwards down to the old crust and built significant windslabs on lees, loading the crust. The mid and lower snowpack is well settled.

Snowpack Details

SurfaceWind affected surfaces in all but the most well sheltered pockets (where new snow remains preserved)
UpperA crust with variable support to skis and unsupportive to sleds and foot traffic. Some dry loose snow under.
MidWell settled with a dormant persistent weak layer and old crusts
LowerWell settled

Past Weather

The new snow that fell Wednesday night into Thursday morning was redistributed by strong to extreme winds from the S-W.

Weather Forecast

The strong to extreme winds continue, transitioning from south and west to northwest overnight Thursday and though the day Friday. Some new snow will fall with the biggest accumulations for northern areas. Sunday sees temps climb as the freezing levels goes to near 1500 m island wide.

Friday: 1 to 15 cm (largest amounts for the north island), winds strong to extreme NW, temps for 1500 m -4 to -8, freezing levels 400 to 950 m.

Saturday: no new snow to a trace of new, winds moderate to light NW to SW, temps for 1500 m -3 to -9, freezing levels 0 to 800 m

Sunday: 5 to 10 cm (with some rain or a rain snow mix for low elevations), winds strong to moderate S to SW, temps for 1500 m 0 to -5, freezing levels 600 to 1500 m.

Posted on Friday February 26, 2021 by Bill Phipps

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