Avalanche Forecast
Issued: Dec 23rd, 2020 1:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Storm Slabs.
Bill Phipps,
VIAC
Mt Washington Patrol reported a few avalanches with ski cutting control work up to size 1 loosed dry sloughs on Monday and no new activity on Tuesday.
No other reports of avalanche activity were reported.
We would like to remind folks to send in their snow, avalanche and weather info directly to this bulletin. Please send your snow stories to forecaster@islandavalanchebulletin.com as your info greatly helps us write more accurate bulletins for everyone's safety. It does not have to be technical snow terms... just write in your own words what you saw and how it all went out there. The great news is that, if you report to us directly, you will often get contacted by our on duty forecaster to discuss your info so its a great two way learning experience.
Summary
Past Weather
Monday saw an average of 10 -15 cm of new snow fall for all but the northern hills. Strong to extreme winds redistributed this snow from NW slopes to SE (atypical for the island). Tuesday was a mainly sunny with cold temperatures and light to moderate winds still from the NW.
Weather Forecast
Little to no new snow for Wednesday and Thursday with temperatures unfortunately rising into the positive numbers. Mainly cloudy with light to moderate winds Wednesday, increasing to moderate to strong Thursday from the south. The next blast of precipitation comes in warm on Christmas day (warm is how we like our Christmas dinner but not our storms). With varying precip amounts and varying in what form it will fall in, from 10 to 40 mm of rain or if that's all snow 10 to 40 cm of snow. Wednesday: 0 to a trace of new snow, light to moderate winds from the S to SE, temps for 1500 m -3 to +3, freezing level sea level to 700 m. Thursday: 0 cm of new snow, moderate rising to strong S to SE winds, temps for 1500 m +4 dropping to -1, freezing levels as high as 2500 m. Friday: 10 to 40 mm/cm of precip (mm if rain cm if snow), winds strong to extreme from the S to SE, temps for 1500 m +1 to -1, freezing levels 1800 to 900 m.
Terrain Advice
Watch out as temperature will rise Wednesday and Thursday. The jump in warmth has the potential to create loose wet avalanches and may also make the weak layers in our current snowpack more sensitive to triggering (likely to slide). Avoid steep solar slopes during warm times and avoid exposure above terrain traps, which these wet slides can push one into (creeks, gullies, bands of trees, cliffs). Terrain traps can increase the depth of burial and/or chance of injury. A warm storm is headed our way on Christmas day. The type of precipitation that will fall may vary based on temps at elevation. It may be rain, mixed rain and snow or all snow. Forecast weather models vary on what elevation the snowline will sit at for different locations on the island, so keep an eye on this when choosing were you will go to play. The south island appears to be warmer up high than the north so there is a greater chance of rain in the hills down there vs snow.
Snowpack Summary
An average of 10 to 15 cm of new snow fell on Monday with only the north island (Mt Cain) reporting less (2 cm). The new snow came with very strong winds from the NW that really affected the snow surface conditions. Surface conditions vary from light low density powder in very wind sheltered pockets, to very bumpy wind slab features, to zones where all the new snow has been stripped off down to the old rain crust from days back.
Snowpack Details
- Surface: Wind ravaged with features that look like something out of a science fiction novel. lol
- Upper: 10-15 cm of new snow mainly wind pressed where it came to rest over a thin decaying rain crust
- Mid: 10-20 cm of old snow over another beefier rain crust capping another 20-50 cm of well settled snow
- Lower: An old crust (showing signs of decay) over well settled snow.
Confidence
High - Weather models in agreement and good field data.
Problems
Loose Wet
As temperatures increase Wednesday and Thursday be aware of the potential for loose wet avalanches (maybe even wet slab activity). Warm temps will make our snowpack more sensitive to triggering so avoid exposure to steep solar affected slopes especially ones that are shallow, thin and rocky in nature.
Location: All aspects (especially southern) at treeline and below treeline
Possibility of triggering/sensitivity: Possible with natural triggers, likely to human activity.
Size: one to two. But remember, though slow in nature, small wet sloughs have a lot of power to push us to where we may not want to go (into terrain traps).
Likelihood
Expected Size
Wind Slabs
As temperatures increase Wednesday we may see the wind slabs that developed after Mondays storm become more sensitive to triggering.
Location: Alpine and treeline on SE slopes.
Possibility of triggering/sensitivity: Possible to natural triggers, likely to human activity.
Size: one to two.
Likelihood
Expected Size
Storm Slabs
Friday (Christmas Day!!!): A new storm will arrive bringing hopefully snow but potentially a mix of snow/rain to the hills. This new snow will most likely bond well to the old surface it lands on but it still has the potential to slide.
Locations: All aspects in the alpine and treeline especially NW as winds will be strong from the SE.
Possibility of triggering/sensitivity: Possible to natural triggers and likely to human activity
Size: one to two.
Likelihood
Expected Size
Valid until: Dec 24th, 2020 1:00AM
