Avalanche Forecast
Issued: Feb 20th, 2016 10:34AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Watch for new building wind or storm slab layers by the end of the daylight hours if you venture out on Sunday.
Summary
Detailed Forecast
A moderate front should cause increasing winds and spread increasing snow especially to the above treeline of the east slopes Sunday afternoon with a slight warming trend.
This weather should build new wind slabs on lee slopes mainly near and above treeline by the end of the day. Watch for new or previous firmer wind transported snow mainly on the lee N to SE slopes below ridges.
New storm slab is likely where more than several inches of snow rapidly accumulates Sunday by the end of the day.
A slight warming trend should help to enhance new wind and storm slab layers on Sunday.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat since several warm and wet systems have tested this layer and since it has been unreactive where it can still be identified in recent snowpits.
Dry weather with the warmest temperatures of the winter occurred February 7-10th with temperatures climbing into the 50's in most areas east of the crest.
Snow has dominated since then in the northeast zone and began to bury a crust there with about 2-3 feet of snowfall starting February 11th through today.
But rain and mild temperatures dominated in the central east and southeast zones last Sunday and Monday forming a newer crust there. Then the active and cooler pattern began to bury the crust in the central and southeast zones starting February 17th with about 6-9 inches of storm in the near and above treeline Wednesday to today.
Storm layers may linger in the recent storm snow but will have strengthened today.
Recent Observations
NWAC pro-observer Tom Curtis was at Blewett Pass on Wednesday mostly reported shallow new snow on a thick crust.
The North Cascades Mountain Guides sent a report Friday for the Washington Pass area and report that the February 11th crust is found at about 65-80 cm. Some but not all tests give hard sudden collapses with sudden propagation. Also some stiff pencil hard wind slab giving easy shears, hidden under about 10 cm of new snow, has been seen near some ridges.
The pro-patrol at Mission Ridge today reported pockets of ski triggered 6-14 inch wind slab during avalanche control on northeast to southeast slopes above about 5800 feet. These slab were releasing on the February 17th crust. Later today a skier triggered a 6-10 inch wind slab and was caught and carried but not injured in a northeast facing chute at about 6300 feet.
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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
Valid until: Feb 21st, 2016 10:34AM
