Avalanche Forecast
Issued: Feb 25th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Moderately high freezing levels and light amounts of rain or snow at slowly cooling temperatures should not cause a significant increase in danger Friday. Shallow areas of wet snow or isolated wind slabs should be the primary avalanche concern.
Summary
Detailed Forecast
Increasing high clouds overnight Thursday should still allow for stabilizing surface crust formation by early Friday.
A weak front should allow for light rain and snow at moderate freezing levels later Friday afternoon. Only a light amount of precipitation is expected during the daylight hours and this should not cause a significant increase in the danger.
Some shallow new wind slabs may form at higher elevations by late Friday on some lee slopes that receive greater precipitation.
Snowpack Discussion
Weather and Snowpack
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 has allowed for melt-freeze crust formation overnight and varying amounts of wet surface snow depending on slope aspect, during the warmest part of the day.
The last storm cycle occurred late last week when about 1-3 feet of snow fell from the 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 of this week, redistributed surface snow and built new localized wind slabs on more non-traditional west 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 ft wind slab on west aspect near Pan Face above Paradise, Mt Rainier that released Wednesday afternoon, 2/24.
Wind slab formed from east winds, Tuesday, 2/23/16. Released Wednesday, 2/24/16. West aspect, crown 1-2ft, near Pan Face, Mt Rainier, ~7000ft elevation. Photo - Peter Ellis
Last week, heavy rain and mild temperatures dominated along the west slopes forming the latest rain crust. NWAC sites in the near and below treeline recorded 2-6 inches of water mostly as rain over the 2 days ending Tuesday, February 16th.
The mid and lower snow pack along the west slopes should be a stable mix of crusts and layers of moist and rounded snow crystals.
Surface hoar has been growing on shaded slopes over the past few days and where not destroyed by sun or expected rain, could be a layer to watch for in the future.
Recent Observations
NWAC pro-observer Dallas Glass was in the Alpental Valley Thursday, 2/25 reporting strong early morning melt-freeze crust that quickly became increasing wet surface snow on solar aspects. No wind slabs were found with mainly an increasing threat of loose-wet avalanches as the warming continued.
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 West, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 26th, 2016 10:00AM
