Avalanche Forecast
Issued: Feb 28th, 2020 1:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Storm Slabs.
Bill Phipps,
VIAC
No new significant avalanche observations have been reported in the past three days. Mt Washington patrol saw some small size 1 Loose dry and Loose wet sloughs on Thursday, as the 2 cm of new snow slide easily on the old surfaces it landed on. Within the past week a group of skiers (near Mt Washington) triggered a size 1.5 avalanche on a north facing, treeline, wind loaded, convex roll that has produced many avalanches (both human and naturally triggered) in the past (ie a regular producer of avalanches over the years). This slid on the persistent weak layer above an old crust approx 20 -30 cm deep (as of Thursday). Some equipment was lost but no injuries were reported.
Summary
Past Weather
2 to 10 mm of precip has fallen in the mountains since the last bulletin. This new precip has fallen as a mix of rain and snow at all but the high alpine zones (were it was all new snow). Temperatures have ranged from -3 to +3 leaving a warm upper snowpack.
Weather Forecast
A significant snow event is forecast for Friday. Early Friday rain will change to snow and strong winds are expected from the SW to SE. Things will ease off Saturday into Sunday. Friday - 3 to 10 mm of rain changing into snow with 10 to 30 cm of snow accumulating. Winds strong SW to SE. Temps +1 dropping to -4. Freezing level 1700 dropping to 1000 m. Saturday - a trace to 4 cm of new snow. Winds Moderate NW. Temps cold at -4 to -7. Freezing level 400 to 900 m. Sunday - 1-3 cm of new snow. Winds light W to SW. Temps -4 to -7. Freezing level 0 to 800 m.
Terrain Advice
Significant new snow will fall on Friday with strong winds from the SW to SE. Avoid wind loaded features in the alpine and treeline until this snow has had time to bond to the old snow. Travel in avalanche terrain is not recommended when the avalanche danger is at high. A persistent weak layer in the upper snowpack has the potential to make small avalanches grow (step down) into larger ones. It would be smart to avoid big terrain features this weekend, even though the new snow is tempting you.
Snowpack Summary
We currently have a weak layer lingering in our upper snowpack and we have seen recent avalanches triggered by skiers on this weakness.
Snowpack Details
- Surface: Wet snow from rain and warm temps Thursday at all elevations except the high alpine.
- Upper: 20-50 cm of snow from previous storms with a few thin crust layers
- Mid: Down 20 to 50 cm is an old crust that has weak snow crystals on top of it (weakness over an easy sliding surface)
- Lower: Well settled
Confidence
High - Weather models in agreement and good field data.
Problems
Persistent Slabs
The weak snow crystal layer (facets and some buried surface hoar) on top of a smooth and easy to slide on crust layer, that was active in the past few days, will get buried even deeper with the new snowfall forecast for Friday (ie more likely to trigger and more capable of creating even larger avalanches. Currently (as of Thursday night) this layer was down approx 20 to 50 cm. Friday's storm will add an additional 10 to 30 cm on top of that (potentially 30 to 80 cm above it). This weak layer can be found on all aspect and elevations but is most present on north facing slopes at or near treeline. This layer will possibly trigger naturally or with human activity and has the potential to create very large avalanches from size 1 upto size 3.
Likelihood
Expected Size
Storm Slabs
Significant new snow will fall Friday after the rain tapers off. This new snow has the potential to create a slab avalanche problem especially where the strong SW winds will redeposit it to NE slopes. These slabs will be found on all aspects (esp NE slopes) and will likely trigger with human loading and may possibly trigger naturally. Expect to find them at all elevations (especially at treeline and in the alpine). They may create avalanches up to size 2 (possible to bury, injure or kill a person).
Likelihood
Expected Size
Valid until: Feb 29th, 2020 1:00AM
