Avalanche Forecast
Issued: Apr 3rd, 2016 11:00AM
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
Winter will make a brief comeback on Monday! The avalanche hazard should increase in the afternoon as shallow storm slabs become more sensitive and with elevation as fresh wind slabs build on lee aspects.
Summary
Detailed Forecast
Winter-like conditions will make a brief comeback on Monday as a front passes through early Monday morning and cool post-frontal showers follow in westerly flow.
A cooling trend during the day Monday should generally help new snow bond to older moist snow and surface crusts. Even with the cooling trend, subtle daytime warming will make shallow storm slabs possible by the afternoon. Also, heavier showers depositing graupel layers may locally make storm slabs more sensitive to human triggering.
Persistent westerly winds will transport new snow onto lee slopes near and above treeline Monday and Monday night. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers. The wind slab problem will be elevated above treeline where more snow and stronger winds will build fresh wind slabs on lee aspects.
Loose wet avalanches will not be listed as a primary avalanche problem Monday. However, small loose wet avalanches involving new storm snow will be possible on any steeper solar slope during prolonged sunbreaks Monday.
The likelihood of cornice failure will lower Monday due to cooler weather. However, cornice releases can be unpredictable during the spring so continue to be aware of the overhead hazard and that cornices can break much further back than expected along ridges.
Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.
Snowpack Discussion
Weather and Snowpack
A large upper ridge has been over the Northwest for several days resulting in warm and dry weather. A spring avalanche cycle occurred in most areas over this stretch, likely peaking Thursday or Friday during the warmest temperatures and lightest winds. After a stormy March, the snowpack has undergone significant settlement over the last week.
Frequent March storms have built unusually large cornices along many ridges.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
Numerous professional and recreational observations focused on the spring avalanche cycle that occurred over the last several days throughout the west slopes of the Cascades.
On Saturday, a cornice failure and wet snow avalanches were observed above treeline on the Worms Flow route on Mt. St. Helens reported via the NWAC observation page.
The most recent observation comes from Dallas Glass in the Paradise area on Sunday. Dallas observed several small skier triggered loose wet avalanches mid-day on an east aspect above treeline at the 7200' level.
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: Apr 4th, 2016 11:00AM
