Avalanche Forecast
Issued: Jan 16th, 2018 11:56AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Watch for wind-loaded slopes above treeline where shallow wind slabs have been reported on Tuesday. Mitigate hazard by approaching lee slopes with caution.
Summary
Detailed Forecast
No significant new precipitation in the forecast for Wednesday, but winds will increase into the moderate range as the morning progresses. Significant additional warming is anticipated during the late morning hours.
A stout crust on Tuesday morning will provide a bed surface for shallow wind slabs which formed above treeline on Tuesday. These wind slabs will remain sensitive to human trigger on Tuesday, with additional active wind loading from a moderate southerly wind before the snow levels rise. Monitor new snow and wind transport diligently throughout the day and navigate around potentially wind-loaded slopes on W-N-E aspects.
The stout icy surface crust reported in much of the terrain should soften with warming on Wednesday late morning, making travel slightly easier.
While loose wet avalanches are not expected, monitor changing surface snow conditions as warming occurs through the day.
Snowpack Discussion
Snow has fallen above 6400 feet on Monday night. Breezy W-SW winds have redistributed snow and created shallow wind slabs (see observation below).
The recent high pressure created melt-freeze surface conditions followed by rain on Monday, which is now refreezing into a 5 cm crust at Mt. Hood Meadows, providing a good bed surface for recent snowfall. The new snow has not been sticking well to this bed surface, but is shallow at mid-elevations.
Friday, a rain and freezing rain event created icy surface conditions up to at least 7000 feet.
Above treeline, W-SW winds Wednesday through Friday combined with significant snowfall have likely created a variety of snow surface conditions, including firm wind slabs in the alpine.
Observations
On Tuesday, Mt. Hood Meadows Pro Patrol reported two natural 6-12" wind slabs on NE aspects between 7000 and 7300 ft.
Observations Friday from Mt Hood Meadows reported a supportable rain and freezing rain crust to 7000 feet. The ice crust prevented winds from transporting snow near and below treeline.
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: All aspects.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Jan 17th, 2018 11:56AM
