Monday March 4, 2019
Persistent Slab, Wind Slab, Loose Dry
Travel & Terrain Advice
Be cautious when entering terrain that has been loaded by recent winds especially in the alpine and exposed alpine like features at treeline, as wind driven snow has added additional load to buried weak layers and will increase potential avalanche magnitude, size and triggering sensitivity.
Avoid shallow zones where triggering the old weak facet crust layer will be greater. Although the layer of great concern over the past weeks has begun to heal, it will remain an issue for sometime and still deserves caution. Quick checks with an avalanche probe or flipped over ski pole can help us avoid these shallow sensitive areas. If you see rocks poking up at say a convex roll over, that’s a great hint as well that you are near a thin zone.
One skier accidentally triggered avalanche size 1-1.5 was reported from a permanently closed zone on Mt Washington on March 1st in aggressive terrain on a N asp at approx 1400 m with a slab failure from about 10-30 cm deep. The person that triggered it was unharmed but the slough did hit a skier traversing under the line at the time. No injuries or lost equipment was reported. The back country is still producing widespread easy loose dry avalanches sloughing from steep northerly terrain at all elevations up to size one.
The layer that produced multiple avalanches here on the island and main land over the past few weeks has finally begun to heal, but it still has the potential to produces avalanches in our forecast area. The crust facet layer now lies around 40 to 60 cm down but variability in the snow pack means it can be much closer to the surface in thin shallow areas. Avoid these thin areas as it will increase the triggering potential of this layer. This problem exists on all aspects and at all elevations and although it will be unlikely, it could still produce up to size 2 avalanches with human activity.
Moderate winds have transported snow in the alpine and open exposed alpine like features at treeline, creating wind slabs. The winds have mainly been from the SE (loading NW slopes) but some wind variability means that all aspects currently have the potential to hold wind slabs. These wind slabs now overlay weak layers (surface hoar) that formed between storm events last week, so the triggering potential is possible to likely via human activity with avalanches ranging from size 1 up to size 2.
Old storm snow has been slow to settle with the cool temps as of late. Expect loose dry sloughs to continue to exist up to size one on northerly aspects in the alpine and treeline. These loose dry avalanches will be easily triggered by human activity so use ski cuts and slough management to reduce the hazard if entering steep terrain.
Things are beginning to settle down after a couple weeks of uncertainly and high touchy avalanche hazard. This forecast period should see a general healing and decrease in concern of layers in the snow pack. Tests on Mt Washington have produced moderate planar results on a buried weak layer of surface hoar in the upper snowpack down approx 30 cm on a SW asp 15 degree slope 1350 m (CTM SP dwn 30 cm on SH 4.0 mm for the snow geeks out there)… and no results on the buried facet crust of doom from previous touchy times.
|Surface||loose dry snow with surface hoar crystals in specific areas up to 10 mm (thin crust on steep BTL solar asp)|
|Upper||10-40 cm of low density old storm snow with weak layers (surface hoar) that formed between storms events.|
|Mid||The nasty crust and facet layer that produced all those avalanches in the past weeks down approx 40-60 cm|
A trace to 3 cm of new snow has fallen over the past few days on the island hills. Winds have been light to moderate from variable directions (mainly SW to SE) and have been transporting old storm snow in the alpine and open exposed treeline. Cool temps have kept the old storm snow light and dry. Some steeper solar faces below treeline warmed up and resulted in a thin crust.
Mainly sunny Monday and Tuesday with increasing clouds Wednesday and light snowfall starting late in the day and overnight into Thursday. Cool temps will continue and winds will mainly be light from variable directions.
Monday - mainly sunny with 0 to 1 cm of new snow, winds will be moderate east overnight dropping to light variable during the day, temps will be -8 overnight to -5 during the day, freezing level 0 overnight rising to 800 m during the day.
Tuesday - mainly sunny with no new snow, winds light variable, temps -8 to -5, freezing level 100 to 800 m.
Wednesday - increasing cloud with light snow fall starting late in the day and overnight into Thursday (3-6 cm total), winds light variable, temps -8 to -4, freezing level 0 to 900 m.
Posted on Monday March 4, 2019 by Bill Phipps