Avalanche Forecast

The strong warm front that has been hosing rain all day Tuesday is expected to pass by early Wednesday with cooling and a period of moderate precipitation along with continued strong winds.

Further cooling and lighter showers are expected later Wednesday with strong southwest winds shifting to strong westerly winds.

This weather will cause a transition in the snowpack from wet and saturated surface layers to a draining and gradually refreezing upper snowpack. Cooling should change rain to snow by Wednesday to gradually lower elevations. New shallow snow should begin building on the refreezing snowpack. This transition should form a good bond of new snow to the forming crust.

Continued strong winds are likely to transport new snow and begin forming some wind slab layers on lee slopes below ridges by later Wednesday. 

It is likely that the problem associated with the recent persistent slab conditions overlying the November crust have abated through avalanche release or the draining rain through the upper snow layers. However, until the upper layers solidify and refreeze we can't be fully sure we can rule it out. While now significantly less likely to release, continue to be aware of the potential for a persistent slab in many areas, 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.

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.

Weather and Snowpack:

November was wet and wild (not to mention the last 48 hrs!) dropping about 12-25 inches of water as mostly rain at NWAC stations 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 in 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 allowed widespread surface hoar and faceted snow to form on or 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 8-12 inches of water equivalent in the past week, with crazy rain all day Tuesday! The most recent heavy snowfall along the west slopes has now been loaded by many inches of rain by Tuesday evening.  

This weather and snowpack scenario caused ripe avalanches conditions and there were close calls this weekend and a widespread natural avalanche cycle reported Tuesday afternoon along Shuksan Arm adjacent to the Mt Baker Ski Area. 

Reports:

Tuesday, patrol at Mt Baker Ski area observed the results of a widespread natural cycle that released Tuesday afternoon along the north slopes of Shuksan Arm. The avalanche starting zones mainly ranged from about 5500-6000 feet, with crown heights ranging from about 1-2 feet. Slides were considered large but not running quite to the valley floor. There were a few isolated climax slides to rocks as well. 

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. 

Similar results were found in the Smith-Brook area near Stevens Pass Sunday by NWAC's Dallas Glass.  

A well chronicled very close call occurred Sunday at Stevens Pass in Corona Bowl, an area on the backside that was roped closed, but accessible to backcountry skiers through a gate. A party of two ducked the rope line and traversed to mid slope, with the first skier triggering a 1-2 foot slab and carried about 200 vertical feet and fully buried. After a challenging beacon search and probe, the skier was located and dug out in about 15-20 minutes, uninjured. 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.