Avalanche Forecast
Issued: Feb 26th, 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
Mostly small, shallow areas of new wind and storm slab will be possible in the near and above treeline on Saturday. Avalanche problems would be greater if there is more new snow than expected.
Summary
Detailed Forecast
Rain or snow should change to showers Friday night with lower snow levels. A little convergence may be seen in the central Cascades near Stevens and Snoqualmie Friday night to Saturday morning.
Light showers mainly along the west slopes should end by Saturday afternoon.
This weather should bring a couple to a few inches of snow near and above treeline along the west slopes with a cooling trend. The cooling trend with rain changing to snow mainly in the near and above treeline should help bond new snow to previous snow.
New small areas of mostly shallow wind slab will be possible on isolated lee slopes. This should be mainly N to E slopes near and above treeline. Watch for firmer wind transported snow.
Isolated older wind slab from E-SE winds Monday night and Tuesday may also linger on unusual W aspects.
Small areas of shallow storm slab will also be possible if any areas receive a few inches of rapidly accumulating snowfall.
These avalanche problems would be greater if there is more new snow than expected.
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 but have stabilized quickly under the warm weather. 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.
Increasing clouds Friday should have helped limit solar effects in most areas.
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.
Surface hoar grew on non-solar slopes over the past few days and where not destroyed by sun or expected rain could be a layer to watch the next couple days.
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.
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, West, North West.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 27th, 2016 10:00AM
