Avalanche Forecast

Issued: Feb 5th, 2016 10:15AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New wind and storm slab layers should be seen on Saturday. The skiing should be improved but don't forget to make careful snowpack evaluations.

Summary

Detailed Forecast

Strong southwest winds aloft will accompany a cold front will cross the Northwest on Friday evening. West winds will follow with orographic snow showers and a good cooling trend on Saturday morning.

The cooling trend may help bond new snow to old snow surfaces where the snow starts at above freezing temperatures. With a little luck many areas along the west slopes will have about 5-10 inches of new snow by the time snow showers taper off on Saturday.

The main avalanche problem should be new wind slab on lee slopes in the near and above treeline. Watch for signs of snowpack cracking and firmer wind transported snow on lee slopes.

A secondary avalanche problem should be new storm slab in areas with less wind if there is rapid loading. The cooling trend may help limit this avalanche problem.

 

 

Snowpack Discussion

Weather and Snowpack

The last heavy rain event January 27-28th further stabilized the mid and lower snowpack and formed a strong rain crust that is being loaded by recent storms.

A strong occluded front with strong winds crossed the Northwest January 29-30th. NWAC stations along the west slopes received about 1-2 feet of snowfall.

Cool, benign weather followed Sunday to Tuesday. A sun crust formed on many solar slopes and surface hoar was seen on many non-solar slopes.

A cold front and then a warm front crossed the Northwest on Wednesday and Thursday. NWAC stations along the west slopes for the 2 days ending Friday morning had about 9-14 inches of snowfall with 18 inches at Mt Baker and 21 inches at Paradise.

Recent Observations

The Alpental pro-patrol Thursday morning reported some natural and ski triggered 6-8" storm slab. By the afternoon some triggered small loose wet avalanches were seen below about 4000 feet.

Deeper, larger avalanches were reported from Paradise Thursday morning. The ranger reported shooting cracks and certain, sensitive, reactive human triggered 15 inch storm slabs on 40-50 degree south facing test slopes that ran onto 35 degree slopes.

NWAC pro-observer Lee Lazzara was near Mt Baker on Thursday mainly near and below treeline and found 65-90 cm of storm snow on the crust buried January 29th. He noted minor a minor storm layer at 20 cm, solar effects and rollerballs.

A skier on the NWAC observations page reported 2 skier triggered wind or storm slab avalanches on Mt Snoqualmie on Thursday.

The Alpental pro-patrol reported a quiet day today with no significant avalanches and east winds helping cool the snowpack.

 

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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 6th, 2016 10:15AM