Avalanche Forecast

Issued: Dec 10th, 2013 1:24PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Summary

Detailed Forecast

Warmer and drier air moving across the Pacific Northwest Wednesday should allow for some slow settlement of recently formed wind slab in the alpine. Recent strong winds have exposed firm or hard ice crusts and produced a wide variety of snow surfaces at higher elevations.

Snowpack Discussion

The last significant rain and snowfall occurred on December 1st and 2nd when most NWAC weather stations recorded 3-5 inches of precipitation and the Mt Hood area received over 10 inches of water. Most of the precipitation fell as rain at mid and lower elevations with higher terrain receiving rain then 1-2 feet of snow. 

That last warm storm in early December helped form a very thick and strong crust layer with initially very good bonding of the new snow to the forming crust. There was little avalanche activity following this storm with colder low density snow allowing for some loose slides on some steep slopes but little slab avalanche activity was noted.  

At mid and lower terrain a still shallow snowcover is limiting the avalanche potential in many to most areas with significant terrain and vegetative anchoring remaining in place.

Over the past week the Pacific Northwest has been in the freezer, under a cold and dry northerly flow aloft. During this dry period there have been periods of strong easterly ridge top winds, mainly last Friday and Saturday, that transported loose surface snow and built some small wind slabs on some lee slopes. There were some triggered shallow wind slab releases noted, mainly near Crystal Mountain on East peak. 

The strong east winds shifted to strong west to northwest winds Sunday, continuing through Monday. This has again redistributed weak loose surface snow back to some easterly facing slopes below higher ridges building additional but isolated wind slab.

The cold dry weather has also been active above and through the surface snow with thick surface hoar growth in many areas as well as producing abundant near surface faceted snow. In addition, the cold temperatures and hence strong temperature gradient in the upper snowpack, have allowed for the initial good bond of new snow to the crust to begin weakening.   

Photos of near surface facets and surface hoar, Monday 12/09 by WSDOT avalanche forecaster John Stimberis at Snoqualmie Pass:

    

Problems

Wind Slabs

An icon showing 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, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 11th, 2013 1:24PM