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Archived

Avalanche Forecast

Mar 4th, 2017–Mar 5th, 2017
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

The avalanche danger should gradually diminish Sunday.  Avoid steep slopes recently loaded with wind transported snow. Evaluate snow and terrain carefully.

Detailed Forecast

Light to moderate snow showers at cool temperatures are expected overnight and early Sunday before tapering through the day. Winds should be light, except light to moderate in the alpine.

Only light accumulations of new snow are expected and with cold temperatures and light winds this should not affect the current avalanche danger significantly. The avalanche danger should gradually decrease on Sunday. However, cold temperatures may help maintain older wind slab layers near and above treeline.

Careful snowpack evaluation and cautious routefinding will be essential at higher elevations Sunday.

Recent moderate to strong S-SW winds will make wind slab most likely on northwest to southeast slopes, but keep an eye out on all aspects in areas of more complex terrain. Watch for firmer wind transported snow from Friday and possible deeper wind transported layers. Older wind slabs may be masked by shallow fresh snow.

Cornices won't be listed as an avalanche problem in this area, but avoid travel on ridges, where recent cornices may have formed and avoid slopes below cornices.

Snowpack Discussion

Weather and Snowpack

The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost, very strong rain crust in the snowpack. 

A series of disturbances in cool, NW flow aloft from Saturday 2/25 through Tuesday 2/28 deposited about 10 inches of snow at Hurricane. Very strong alpine winds were seen in most of the Olympics and Cascades on Tuesday.

Southwest flow aloft began to ramp up again on Thursday as the first in a new series of fronts crossed the Northwest. Strong southwest flow carried a second stronger front across the Northwest on Friday evening. At Hurricane, 24 hour new snow of 8 inches were received as of Saturday morning, with about 12-14 inches since Thursday. The latest storm snow was received with strong SSW alpine winds and moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft.

Recent Observations

NWAC observer, Matt Schonwald was at Hurricane on Friday and found that ski tests were showing rapidly increasing propagation and increasing sensitivity of the deepening storm slab. Previous winds had loaded various slope aspects while south winds on Friday were loading N aspects. 

Avalanche 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, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1