Avalanche Forecast
Issued: Feb 12th, 2016 10:15AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Cornices.
NWAC,
Northwest Avalanche Center
Moderate W-SW winds will transport new snow onto lee slopes near and above treeline Saturday. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects. The avalanche danger will increase late Saturday night and on Sunday as snowfall increases along with a slow warming trend.
Summary
Detailed Forecast
Light snowfall accumulations are expected Friday night through Saturday above 3500 feet along the east slopes.
Moderate W-SW winds will transport new snow onto lee slopes near and above treeline. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects.
Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.
The avalanche danger will increase rapidly late Saturday night as snowfall increases along with a slow warming trend. This unstable combination of moderate precipitation near the Cascade crest, strong alpine winds and warming may continue on Sunday again elevating the avalanche danger along the east slopes.
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 after several warm and wet systems tested this layer and it has been unreactive where it can still be identified in recent snowpit tests. A warm and wet system late last month caused a rain crust to form in most areas and elevations along the east slopes on Jan 28th.
A large upper ridge and warm air mass aloft over the West Coast from this past Sunday through Wednesday led to the warmest weather of the winter. During this stretch temperatures were generally above freezing. The warm temperatures and solar effects earlier in the week caused loose wet avalanches and overall snowpack consolidation. A warm front brought periods of very light rain and snow to the east slopes Thursday night through Friday, with snow levels oscillating around 5000-6000 ft Friday.
Recent Observations
Extensive observations in the Washington Pass area over the past few days indicated an active loose-wet avalanche cycle occurred mainly Monday afternoon. NWAC pro-observer Tom Curtis in the Blewett Pass area in the 4000-5900 foot range Tuesday and also saw several small to large wet loose avalanches on solar slopes near and above tree line.
A professional observation from the central-east zone Thursday indicated that a facet/Jan 28th rain crust combo was reactive in snowpit tests, but no avalanches were observed down to this interface. However, a skier triggered avalanche down to the faceted 1/28 crust was observed in the Washington Pass area on a NE aspect around 7000 ft Thursday. While we believe the upper snowpack to generally be well bonded to the 1/28 crust, this interface should get a decent test over the next few days and bears watching.
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
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 13th, 2016 10:15AM
