Monday December 21, 2020

Wednesday December 23, 2020
Tuesday December 22, 2020
Monday December 21, 2020
Confidence: High - Many days of field data and weather models in agreement

Main Concerns

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

Travel & Terrain Advice

-Give wind loaded south east slopes in the alpine and treeline time to settle and bond after the new snow falls in the AM Monday. This snow will likely have a poor bond to the rain crust that formed with the bit of rain Sunday night.

-Cornice growth in the Alpine on all aspects will present a danger. Avoid exposure both on top of and below these recent addition to our season.

-Warming on Wednesday may make weak layers in the snowpack more sensitive to triggers. Keep an eye on how warm it is getting where you are, and adjust plans as temps get into the positive numbers.

-Avoid exposure to steep slopes in forest openings and/or near water sources as a now buried weak layer may linger there.

Avalanche Summary

Avalanche control work at Mt Washington produced a few size ones with explosive control and ski cutting on Saturday. No new avalanches were reported by the resort on Sunday.

Natural loose wet avalanche activity was noted to have taken place late Friday night when rain saturated the new snow that fell during the day up to size 1.5.

A few natural storm avalanches up to size two were also reported from activity sometime during the last three days.

Avalanche Problems

Loose Wet

Wednesdays warm up has the potential to create loose wet sloughs as temps climb to near +4 at 1500 m. Minimal cloud cover will allow the sun to heat up the snow surface and may increase the sensitivity of weak layers in the snowpack.

Location: Southern slopes that are steep, shallow and rocky, at all elevation bands especially warmer treeline and below treeline elevations.

Possibility of triggering: Possible natural activity and likely to trigger with human activity.

Potential size: size one to two

Persistent Slab

The existence of a very weak snow crystal layer has been noted in the snowpack approx 20 cm down on top of the second rain crust. The extent of this surface hoar layer is not known island wide, but it does exist in the Forbidden Plateau zone. This layer has been shown to fail easily in snowpack tests.

Location: Surface hoar often forms in openings in the forest especially near water sources like creeks and ponds. This buried layer will exist at treeline and below treeline elevation bands.

Possibility of triggering: Possible to natural and human activity. May become more reactive with snow loading Monday or with warming Wednesday.

Potential size: size one to two.

Wind Slab

The new snow that will fall Monday AM will be easily moved by strong to very strong atypical north west winds. The transport of this cold light snow will create wind slabs on south east slopes and these slabs will most likely have a poor bond to the rain crust surface they will land on.

Location: South east slopes in the alpine and treeline. Especially bowls and gulley features that tend to trap/fill with snow.

Possibility of triggering: We are likely to see naturally triggered avalanches and will very likely trigger slides with human activity.

Potential size: size one to size two. Remember size two has the potential to injure bury or kill a person.

Snowpack Summary

40 to 50 cm of new snow has fallen recently with short lived rain events after each new addition to the snow pack. Great storm riding was had during the day Friday and Saturday prior to the wet evening warmups. New snow has climbed to a level that surface anchors like trees and rocks are no longer going to prevent avalanches from running. From this point on we need to factor this into our decisions. Typical total snow heights are now around 50 -90 cm below treeline, 90 to 170 cm at treeline and above 200 in the alpine.

*A layer of very weak snow crystal has been noted in several snow pits around the Forbidden Plateau area approx 20 cm down on the rain crust that formed after Fridays storm. Tests have produced significant easy (scary) results. Big thanks to our local avalanche course providers and Tim Plante for sending in reports.

Snowpack Details

SurfaceA thin rain crust tops the new snow that fell Saturday
UpperAnother thin rain crust and below that the snow that fell Friday (potential weak layer above this crust)
MidMoist well settled snow and a old crust from the early season.
LowerWell settled

Past Weather

From zero to hero… Our snowpack has gone from thin very spring like conditions to a winter wonderland in a few short days. Multiple storms with strong to extreme SW to SE winds have dumped 40 -50 cm of new snow in the past four days. Unfortunately Friday and Saturdays snow loads were each capped by short lived rain events to well above the 1600 m mark.

Weather Forecast

Another blast of winter will hit from Monday morning until midday bringing more new snow. Atypically the wind will be strong from the NW with this event (reverse wind loading slopes) but the cold air should produce some very nice light dry powder. Things calm down Tuesday and temps remain cold until a warming trend pulls in Wednesday.

Monday: An average of 10 cm of new snow will fall in most areas Monday morning-midday, with the exception of around 30 cm near Arrowsmith and only a trace for Mt Cain, temps for 1500 m will range from -2 to -7, winds will be strong to very strong NW, with freezing levels from 400 to 800 m.

Tuesday: no new snowfall, temps for 1500 m will range from -1 to -4, winds will be light to moderate from the NW, with freezing levels from sea level to 800 m.

Wednesday: no new snowfall, temps for 1500 m will climb from 0 to +4, winds will be light to moderate from the more typical SW, and freezing levels will jump all the way from sea level to 1700 m.

Posted on Monday December 21, 2020 by Bill Phipps

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