Wednesday February 17, 2021
Persistent Slab, Wind Slab
Travel & Terrain Advice
Avoid or be careful when entering wind loaded zones near ridge tops (especially open bowls and gullies) as wind slabs have and will continue to form over the next few days. Winds will come from multiple directions and load all aspects vs just the typical flow from the SE.
Watch steep small bits of terrain associated with openings in the forest, as buried weak layers (surface hoar and facets) are now loaded with new snow and more is coming. Avoid exposure to terrain traps in these openings, as even small slides associated with them can magnify the potential for injury in these bits of small terrain.
Multiple recent avalanche accidents in the states with many injuries and deaths have resulted from a well know touchy persistent buried weak layer. When interviewing survivors many mentioned their mental state and decision making processes (risk tolerance) were to some degree affected by Covid (either making them more numb to the warning signs due to their desire to get out, or a concern over not getting another chance to get out if restrictions to travel tighten). Be sure to do a good and thorough analysis of your decision making process, factor in weather and snowpack and don’t forget the human factors.
Numerous reports of size one avalanches mainly associated with wind slabs on NE to NW aspects at tree line elevations both naturally and human triggered.
Widespread reports of settlements (whoomphing) at below treeline and treeline elevations on all aspects.
One report of possible skier triggered avalanche size 1.5 from Sunday Feb 14 in the west bowl of Mt Cain on the feature called Sliders. No reports of injuries or lost gear at this time.
This is a tricky one…. The two widespread facet weak crystal layers at all elevations and the buried surface hoar weak layer found specifically below treeline, all still linger in our snowpack, just below a crust that has been rotting away (due to the cold temps). Evidence of this has been noted by wide spread reports of settlements (whoomphs) island wide. The capping crust that was once supportive and bridged these lower weak layers is eroding at varying rates based on ones location, aspect and elevation. Ie the crust is almost non existent on higher cold north zones but still intact on low souths. So the big question is will one be able to trigger these weak layers, how much weight will it take and will the new snow load (and especially the new snow load plus the addition of wind blown snow) tip the scales and make human triggering even more likely? The only way to tell for sure is to take extra time to study the snowpack in the avalanche terrain you plan to venture into. Make a plan that allows for time to dig pits and do tests. Don’t know how to dig and correctly interpret pit results… then the best option might be to avoid avalanche terrain and give this snow time to settle out. Don’t worry its the island it wont take too long and there is some very nice snow still preserved on low angle terrain out of the wind.
Location: Widespread on all aspects and at all elevations. Triggering maybe more likely on colder aspects where the crust above has eroded at a faster rate.
Likelihood of triggering: While triggering these unreactive layers is unlikely with human activity at this time (Tuesday Wednesday), this problem has the potential to possibly trigger/fail with the added weight of us and the new snow forecast for Thursday into Friday and will certainly be more of a concern as we move forward.
Size: If we can trigger these layers we can expect the size of avalanche to range from size 1 to size 2.5
The moderate to strong winds from both the SE and NW over the past few days will continue to flow during this forecast period. These variable direction winds have and will continue to create a wind slab avalanche problem. Low density snow from the cold storms last weekend combined with more new snow falling Thursday and Friday will increase the depth and size of these slabs.
Location: These slabs will be specifically distributed in both the alpine and treeline elevation bands, and will sit just below the ridgelines and on the edges of cross loaded terrain features. Expect to find wind slabs in lee bowls and gullies especially where the direct windward side of the ridge has large open collection zones (know as a large fetch). Note these slabs will be on both south east and north west aspects.
Likelihood of triggering: Expect these slabs to be reactive to human activity in the likely to very likely range, while naturally occurring slab activity may be limited to likely to possible.
Size: these slabs may be in the size one to size two range which means the bigger collection zones have the potential to injure bury or kill a person.
From Sat Feb 13 until Tuesday night Feb 16 approximately 20 to 30 cm of new cold light snow fell on a old crust surface. Winds both from the SE and the NW have moved this light snow to either form wind slabs or strip zones down to the crust. Cold temperatures have eaten away at the thickness and strength of the crust that capped the two persistent weak layers in our mid snowpack to varying degrees based on location, aspect and elevation. Higher temperatures warmed the snow surface in below treeline elevations Tuesday leaving a slight crust and also saw trees shed their snow loads making travel in the forest unenjoyable.
|Surface||Variable surface conditions from wind slab to exposed crust up high. Some nice pow still lingers in pockets.|
|Upper||Low density cold snow from previous storms|
|Mid||A rotting variable thickness crust with weak snow crystals (2 facet layers and some surface hoar) below.|
The past three days have seen the temperatures climb slowly back toward our normal cool/mild island winter levels. Light to moderate amounts of snow has fallen (10-20 cm in most zones). Winds have varied from NW to SE and have been light to strong.
Wednesday sees winds changing from the NW flow that has chilled us all, back to our regular SE flow that will blow in the next winter snow storm, with new snow falling Thursday through Friday.
Wednesday - no new snow fall, winds strong NW falling to light SE, temps for 1500 m -4 to -7, freezing level 0 to 800 m.
Thursday - 5 to 15 cm of new snow, winds moderate to strong SE, temps for 1500 m -6 to -1, freezing level 400 to 800 m.
Friday - 5 to 20 cm of new snow, winds moderate to strong SSW, temps for 1500 m -4 to -5, freezing level 500 to 1000 m.
Posted on Wednesday February 17, 2021 by Bill Phipps