Avalanche Forecast
Regions: Cascades - West.
Don't take this snowpack for granted just because a week ago we may have had a bomber hard crust and a very shallow snowpack. Think about using your first your early season backcountry trips to test your gear, practice rescue and transceiver techniques with your partner and maybe stay in more protective shallow angled terrain. It's a long season and there will be plenty of time to get after it safely under more favorable snowpack conditions.
Detailed Forecast
Another strong warm front is expected Tuesday. Strong southwest to west winds near ridges and increasing heavy to very heavy rain or snow at further warming is expected through the day Tuesday. Â
More newly forming or increasingly large persistent, storm or wind slab will continue to be the focus on Tuesday along the entire west slopes. Back country travel in the near or above treeline is not recommended on Tuesday.
The warming and change from snow to rain may have already caused a widespread natural avalanche cycle by the time Tuesday's storm arrives, however where still snowing this should reload new slab layers over the strong November crust.Â
Note that the west slopes still do not have a uniform snowpack, though with each storm, snowdepths are evening out by elevation band.Â
Continue to be aware of the potential for a persistent slab in many areas, produced by weather in late November, especially in the Stevens and Snoqualmie Pass areas. Persistent slab is especially dangerous because it may be harder to trigger and propagate rapidly causing larger faster moving avalanches. Also, shallower storm slab avalanches may step down to the persistent slab, releasing potentially large and dangerous avalanches.
With less snow below treeline, wet loose snow avalanches will not be in the forecast. But change your plans if you find wet snow deeper than a few inches or see signs of wet loose activity such as pin wheels or natural wet loose avalanches.
Snowpack Discussion
Weather and Snowpack:
We had a wet and wild November with about 12-25 inches of water at NWAC station along the west slopes only amounting to about 3 feet of snow in the north above 4000 feet and above 6000 feet elsewhere. This formed a strong crust in mid November all areas near and west of the crest.
In late November, strong high pressure led to strong temperature inversions and brought very cold air east to west through the Cascade passes mainly Stevens and Snoqualmie in late November. This caused widespread hoar frost and faceted snow near the November crust along the Cascade east slopes and east to west through the Cascade passes, especially in the Stevens and Snoqualmie Pass areas.
The weather so far in December has become very active with most west slope NWAC stations receiving 5-10 inches of water equivalent in the past week. The most recent heavy snowfall along the west slopes has been deposited under a significant warming trend, especially Monday. Â
This weather and snowpack scenario should put all on alert as this is a pretty classic PNW avalanche recipe!
Reports:
Monday, NWAC observer Jeff Ward traveled to the site of the avalanche accident that occurred Sunday in a closed area of Stevens Pass (Corona Bowl described below). The persistent slab overlying either buried surface hoar or facets on the mid-November crust was becoming larger and more dangerous through the day as snowfall intensity increased through the afternoon. This weather has built an increasingly unstable snowpack structure with a thickening slab of dense heavier snow over the November crust, now buried 2-4 feet or more in some areas.Â
Similar results were found in the Smith-Brook area near Stevens Pass Sunday by NWAC's Dallas Glass. Â
New NWAC observer Simon Trautman was near the Mt Baker Ski Area on Saturday and reported reactive upside down P hardness snow slab over lower density 4f snow layers. He reported shooting cracks and numerous size 1-2 natural storm slab avalanches up to 15 inches on northeast slopes and some skier triggered storm slab avalanches on south to west slopes.
We have a report of a skier burial at Stevens Pass in Corona Bowl on Sunday. A party ducked a rope and traversed to mid slope and the skier triggered a 1-2 foot x 100 foot crown and was buried with her head at about 2 feet below the surface and was dug out in about 15-20 minutes. The avalanche was on a southeast facing slope at about 5000 feet and ran on the November crust. Control at the ski area Sunday also gave widespread releases of the recent snow mainly on northwest facing slopes with crowns to 25-30 inches also running on the late November crust.
A brief email report received by the NWAC indicates that the buried hoar frost from the Thanksgiving weekend is also present to some extent at Snoqualmie Pass.
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Avalanche Problems
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: Likely
Expected Size: 1 - 1
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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1