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 continued develop of shallow wind slabs above treeline, particularly on W-N-E aspects. Mitigate hazard by approaching lee slopes with caution and observing wind transported snow, particularly as winds increase throughout the day.
Summary
Detailed Forecast
Light to occasionally moderate snowfall with higher snowfall rates in the north and near the Cascade crest is anticipated along with increasing moderate wind during the morning hours on Wednesday. Some locations in the Cascade East-North and East-Central zones may avoid the large jump in freezing levels expected further south and also west of the Cascade crest.
Watch for further development of shallow wind slabs, particularly on W-N-E aspects above treeline in all zones and extending into the near treeline terrain in the North, where snowfall amounts are largest. Monitor new snow and wind transport diligently throughout the day and avoid wind-loaded slopes.
Note that a variety of snow conditions may still be encountered on Wednesday, including breakable crusts, wet snow, new snow over refreezing surfaces, etc.
Snowpack Discussion
Light amounts of new snow fell at higher elevations and now sit above variable surface crusts that formed during melt-freeze inversion conditions (Saturday through Monday) and subsequent refreezing wet snow from rainfall that fell in some low-to-mid elevation locations on Monday night.
Last week’s storms brought 8-14 inches of snow to the east slopes of the Cascades which has been consolidated by warm temperatures.
A supportive crust (1/5) formed from a widespread freezing rain event in the central-east zone, extending to the Stevens and Snoqualmie Pass areas. This layer is not present in the northeast from Washington Pass to Holden.
Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.
Observations
North
North Cascades Heli was in the field Saturday and observed thin wind slabs, 8 inches thick on lee slopes. Roller balls were reported on sunny slopes in the afternoon, but natural loose wet avalanche was not observed.
Central
A snow professional reported from the Mission Ridge area on Sunday where he observed a 4-5' deep snowpack on a north aspect at 6400 feet containing two potential weak layers of concern. (1) A layer of 2 mm facets 45 cm above the ground lies beneath the 12/15 crust. This layer was reactive in column tests and propagation tests (see this report) . (2) Basal facets just above the ground were present but rounding and were not the weak layer of interest in numerous tests. These layers will need to be monitored as they could develop into potentially dangerous weak layers in the future under loading.
South
No recent observations
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
