Avalanche Forecast
Issued: Feb 4th, 2017 11:32PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Despite a cooling trend, recently formed storm slabs should still be sensitive Sunday. Colder temperatures will help preserve wind slab instabilities near and above treeline. Enjoy the new snow but choose conservative terrain and allow storm related instabilities time to heal.
Summary
Detailed Forecast
More snow is on the way Saturday night and Sunday with a cooling trend forecast. W-SW transport winds should also decrease during the day.
Despite a cooling trend, recently formed storm slabs should still be sensitive Sunday. While storm slabs are most likely to release within the most recent storm layers, various sun and rain crusts throughout the region are capable of providing bed surfaces for larger avalanches in isolated terrain. If precipitation rates become more intense than predicted in the south Cascades Sunday, be prepared for the possibility for new storm slab instabilities.
Colder temperatures will help preserve wind slab instabilities near and above treeline. We've highlighted more traditional lee easterly aspects on the elevation/aspect diagram, but be aware of cross-loaded slopes and that easterly winds earlier in the week loaded westerly aspects. Feel for firmer wind transported snow as you climb higher in the terrain.
Enjoy the new snow but choose conservative terrain and allow storm related instabilities time to heal.
Snowpack Discussion
Weather and Snowpack
Strong NE-E winds were seen this past Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.
A storm cycle began Friday with generally 1-2 feet of snow accumulating along the west slopes, with slightly lower totals at White Pass and a whopping 30 inches at Mt. Baker through 5 pm Saturday. A slow warming trend affected all areas Saturday with rain reaching 4000-4500 feet in the south Cascades. Easterly flow kept temperatures locally cooler at Pass level at Stevens and Snoqualmie Pass through the day.
Recent Observations
North
Mt. Baker pro-patrol reported easily triggered storm slabs with ski cuts and explosives on all aspects averaging 12-18" and releasing within the new storm snow Saturday morning.
Central
NWAC pro-observer Ian Nicholson was out on the lower slopes of Jim Hill east of Stevens Pass Saturday and found sensitive storm slab on short test slopes and in column tests averaging 20-30 cm down and failing within storm layers.
Stevens Pass ski patrol reported widespread shallow storm and wind slab during morning control work. Alpental pro-patrol reported similar results mid-day Saturday with storm or soft wind slabs running well during control with ski cuts and explosives.
NWAC received a report of a natural 6-12" storm slab, 150' wide, that released at 4300 ft on a SE aspect in the Kendall trees. Warming above the inversion was causing natural pinwheeling.
NWAC pro-observer Jeremy Allyn was in the Alpental Valley Saturday and in the area he traveled below treeline found new storm snow generally lacking a cohesive slab structure with the new storm snow bonding well. Wind transported snow was not observed below treeline.
South
Crystal mountain patrol reported sensitive but shallow wind slab on lee slopes below ridgelines Saturday. Pockets of sensitive storm slab could be found mid-slope up to 12" deep.
Problems
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
Expected Size
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
Expected Size
Valid until: Feb 5th, 2017 11:32PM
