Avalanche Forecast
Issued: Dec 23rd, 2016 12:23PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Avalanche problems are expected to be somewhat varied along the west slopes on Saturday. You will need to pay attention to the observations and forecasts to best assess the avalanche danger in your area.
Summary
Detailed Forecast
An upper trough will exit the Northwest on Saturday and high pressure will begin to build offshore. Light snow showers mainly along the west slopes Saturday morning should give way to partial clearing Saturday afternoon with light west to northwest winds and cool temperatures.
Storm slab will be emphasized in the Northwest zone on Saturday. Watch for storm slab in any area where there has been more than a few inches of rapidly accumulating snowfall.
Wind slab should be possible in any areas that had wind Thursday and Friday. Watch for firmer wind transported snow mainly on northwest to southeast slopes.
The latest tests of the December 17th PWL in the Cascades don't seem to indicate a regionally reactive layer. There is still some uncertainty regarding this layer. Snow pits valid for slopes you intend to ski or ride may give some indication of the presence and reactivity of this layer. But skiing or riding on lower angle slopes is the safest bet until there is more certainty that this layer is no longer a problem. While triggering this layer seems unlikely remember that PWL's generally cause larger avalanches.
The surface crust formed following the storms early this week is strong and hard enough in some areas of the west slopes to present an out of control fall danger. Avoid steep hard slopes where there will be fall consequences if you are not confident you can manage this problem by walking or using ski or boot crampons.
Snowpack Discussion
Weather and Snowpack
Strong westerly flow directed two Pacific frontal systems across the Northwest Sunday night and again Monday night with generally 1 to 2 inches of water accumulating along the west slopes through early Tuesday morning. This caused crust layers to form in the passes and up to about 5000 feet at Baker and up to about 6000 feet at Paradise.
A sharp cooling trend Tuesday morning was followed by 2-5 inches of snow in post-frontal showers Tuesday.
A fair day was seen on Wednesday.
Another front crossed the Northwest on Thursday followed by an upper trough on Friday. This has caused about 20 inches of snow at Baker with only about 2-8 inches elsewhere along the west slopes and in the passes. South to west winds Thursday and Friday should become light by Friday night with a cooling trend Friday.
Recent Observations
Several worthwhile observations are available for the west slopes for Thursday via the NWAC Observations tab. In summary no avalanches and no signs of instability were reported. A couple of the observers at Baker and Stevens found the December 17th PWL at about 40-50 cm below the surface to be unreactive.
NWAC pro-observer Lee Lazzara was out in heavy snowfall on south slopes on Mt Herman near Mt Baker on Friday in the 4-5000 foot range and reported widespread reactive 15-30 cm storm slab releasing on a recent storm interface. He expected this to be present in all the terrain bands and on all aspects and we will have to see how soon it stabilizes. He did not find the December 17th PWL in a pit on a north slope and it was found but unreactive at about 70 cm on a south slope.
The Alpental pro-patrol on Friday reported sensitive 3-4 inch wind slab in wind loaded areas which was not bonded to the crust from early in the week. Elsewhere new snow was not cohesive and was sluffing on the crust.
A report via email for Silver Basin near Crystal Mountain on Friday indicated cracking and reactive wind slab forming on N-NE slopes.
Wind slab cracking in Silver Basin. Photo by Seth Waterfall.
A ranger at Paradise on Friday reported shallow storm slab development and cracks beginning to initiate from skis.
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, North West.
Elevations: All elevations.
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
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 24th, 2016 12:23PM
