Wednesday February 24, 2021

Friday February 26, 2021
Thursday February 25, 2021
Wednesday February 24, 2021
Confidence: High - Similar surface conditions island wide. Weather models in agreement. Moderate to large amounts of snow with very high winds.

Main Concerns

Storm Slab - view Avalanche Problems for detailed information.

Travel & Terrain Advice

Avoid wind loaded lee avalanche slopes and cross loaded features for the next few days and give the storm snow time to settle and bond to the firm slippery surface it will land on.

Stay out of ALL avalanche terrain for elevations rated HIGH during this forecast period. Large quantities of snow with high to extreme winds all landing on a firm sliding layer add up to huge avalanche potential.

Avalanche Summary

No new reports from the public of avalanche involvements.

Mt Washington patrol no new avalanche activity observed.

Would expect a widespread natural avalanche cycle in the alpine/ treeline from last Sunday night in to Monday morning’s rain/wet snow event but few observations done or reported. Thanks Joe for your reports of this cycle from the Mt Cain region of multiple avalanches up to size 1 to 2.5 and potentially even a size three.

Avalanche Problems

Storm Slab

Large amounts of new snow (potentially up to 65 cm for some locations!) will begin to fall Wednesday night into Thursday morning. This new snow will be accompanied by strong to extreme winds ranging from the S to W and then W to NW as the storm front moves its way through. This will very likely result in a widespread storm slab problem that will be reactive to human travel.

Location: All elevations and aspects but especially the alpine and treeline bands on NE to SE aspects. Wind loaded lee aspects near ridgetops and crossloaded features will be hot spots for reactive slabs.

Likelihood of triggering: Very likely to human traffic and likely with natural triggering.

Size: 1 to 3

Snowpack Summary

Light to moderate amounts of new snow has fallen since the rain/warm up event last Sunday night into Monday morning. While snow amounts for the Mt Washington zone have been minimal, Strathcona Park and Mt Cain have received around 20 cm. Strong to moderate SW winds have transported the new snow in the alpine and treeline elevation bands creating wind slabs on lees and cross loaded features. The new snow has landed on a crust/firm snow surface that formed after the rain/ warm up event from Sunday Monday. A well settled upper/mid snowpack has basically eliminated the concerns of the persistent weak layers (PWL) that resulted in so many human triggered avalanches a few weeks back. These PWLs are now down approx 100 to 130 cm and are well bridged/ sheltered from the snow above.

Snowpack Details

SurfaceA trace to 20 cm of new snow, wind blown at treeline and alpine elevation bands.
UpperFrim snow/crust with variable support to ski traffic, over some dry older snow
MidWell settled snow over the old PWL layers
LowerWell settled

Past Weather

Sunday night in to Monday morning saw a rain event that saturated the upper snow up to approx 1300 m, with wet snow falling at higher elevations. Cold temps Monday re froze surfaces prior to variable amounts of new snow falling on the firm/crust surface. Moderate to strong SW winds have moved the new snow in terrain at treeline and alpine elevations.

Weather Forecast

Winds will start to pick up Wednesday as they usher in a big snow event set to arrive Wednesday night into early Thursday morning. The storm will taper off slowly with continued snowfall rates (moderate to light) by late Friday. The new snow will be accompanied by strong to extreme winds ranging from SSW to NW. Temps will peak during the storm (-2) then taper down after to near -6 -7.

Wednesday: no new snow, winds light SE to strong S to SE, temps for 1500 m -4 to -7, freezing level 0 to 800 m.

Thursday: 11 to 40 cm of new snow starting to fall Wednesday evening (heaviest amounts for the west side and north island), winds strong to extreme S to W, temps for 1500 m -1.5 to -5, freezing level 1000 dropping to 650 m.

Friday: 3 to 25 cm (heaviest amounts for the north island), winds extreme to strong NW, temps for 1500 m -4 to -8, freezing level 1000 to 400 m.

Posted on Wednesday February 24, 2021 by Bill Phipps

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