Avalanche Forecast
Issued: Feb 27th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions are expected by Sunday afternoon especially in the near and above tree line. Careful snow evaluation and conservative decision-making should be essential on Sunday.
Summary
Detailed Forecast
A stronger cold front will cross the Northwest on Sunday. Expect increasing strong alpine winds and significantly increasing moderate to heavy rain or snow on Sunday heaviest along the west slopes. Snow levels should lower to near the pass levels in the afternoon. New snow amounts of about 5-10 inches should be seen in the near and above tree line by the end of the daylight hours.
This weather should transport snow and build new wind slab on lee slopes through the day. This should be mainly on N to E slopes in the above and near tree line bands. Watch for firmer wind transported snow and surface snow cracking from the tips of your skiis or snowmobile.
New storm slab should also build through the day in areas that rapidly accumulate snow. This avalanche problem may work its way down into the upper treeline by the end of the day. Rapid snow accumulation is generally more than an inch or 2 an hour for several or more hours.
Dangerous avalanche conditions are expected by Sunday afternoon especially in the near and above tree line. Careful snow evaluation and conservative decision-making should be essential on Sunday.
The problem of loose wet avalanches will be left off the problem list on Sunday but watch for signs of significant wet snow at the lowest elevations.
Snowpack Discussion
Weather and Snowpack
The last storm cycle occurred late last week when about 1-3 feet of snow fell from February 17th-20th. Some cornices and wind slabs formed during this period, with many triggered wind slabs reported last weekend.
Strong E-SE crest level winds in many areas Monday night and Tuesday redistributed surface snow and built new local wind slabs on unusual W facing slopes. Some of these wind slabs were touchy earlier this week. The image below highlights a 1-2 foot wind slab on W aspect near Pan Face above Paradise that released Wednesday afternoon February 24th.
Wind slab formed from E winds, Tuesday, 2/23/16. Released Wednesday, 2/24/16. W aspect, crown 1-2 feet, near Pan Face, Mt Rainier, ~7000ft elevation. Photo Peter Ellis.
Springlike weather under high pressure Wednesday and Thursday caused abundant sunshine with temperatures climbing into the upper 40's to mid 50's Thursday afternoon. This fair and mild weather will have caused thick melt form surface crusts especially on solar slopes in most areas by Friday and helped stabilize wind slab from Monday night and Tuesday.
A weak front brought light amounts of rain and snow Friday night to Saturday morning.
The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
NWAC pro-observer Dallas Glass was in the Alpental Valley Thursday reporting strong early morning melt-freeze crust that rapidly turned into wet surface snow on solar aspects. No wind slabs were found with mainly an increasing threat of loose-wet avalanches.
NWAC pro-observer Lee Lazzara was near the north side of Mt Baker on Thursday in the 4300-6500 foot elevation range and found local 30 cm wind slab on ridges to be unreactive and melt form crusts on solar slopes. There was some powder surviving on non-solar slopes.
A report on the NWAC Observations page indicated numerous loose wet avalanches on Mt Herman near the Mt Baker ski area on Thursday.
Dallas was near Crystal Mountain ski area on Friday and found saturated snow on solar slopes and moist snow on non-solar slopes with pinwheels reaching very large sizes up to 4 feet in diameter before collapsing.
The Alpental pro-patrol reported reported slightly difficult to trigger loose wet avalanches on steep non-solar test slopes and bad skiing off piste on Saturday.
Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 28th, 2016 10:00AM
