Avalanche Forecast

Cool weather with mostly clear skies or high clouds are expected Thursday with moderate NW winds at higher elevations. 

This weather will continue to cause a decreasing trend in the avalanche danger as recent storm snow slowly stabilizes. At higher elevations, expect wind transport and building areas of wind slab, mainly above treeline and onto unusual aspects from the mostly NNW winds.

While the overall danger is decreasing, continue to travel with caution and watch for any slab like structure or cohesion in the upper snowpack, indicating the possibility of triggered slab avalanches.   

Watch for touchy recently formed cornices and avoid travel on steep unsupported slopes.

Remember that beneath snow received during this storm cycle, early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the season's snowcover.

A strong storm cycle began Sunday and tapered Tuesday night. Storm total new snow ranges from 2-4 feet in these west slope zones and passes as of Wednesday morning. Light showers at cool temperatures with light winds Wednesday added 1-3 inches additional new snow.

One or more widespread natural avalanche cycles have occurred during the heaviest intensities, especially Tuesday. Sensitive storm slab avalanches occurred Tuesday throughout the range up to size 2 from natural, human and explosive triggers. 

These avalanches all appear to have released within storm layers with a few possibly stepping to the late November rain crust, but that was not confirmed. 

As of Wednesday, field observations indicate these previous very sensitive storm layers have settled and continue to stabilize. Cooling temperatures and light winds have left a favorable storm snow profile of increasing density with depth. 

The old snow interface of 12/15 appears to be favorable as well, from observations received as of late Wednesday.

The main take away is the current upper snowpack in most areas near and below treeline, lacks a slab structure and remains of low cohesion. Thus, there is low propensity to release slab avalanches. However, it may only take some minimal wind effect to stiffen the surface snow to change all that.

Crest level winds have shifted and strengthened from the north Wednesday, no doubt transporting the abundant new snow and building wind slab layers on a variety of aspects, especially above treeline.  

The snowpack above treeline may be very different as there are likely wind slabs lurking. Limited observations above treeline means a lower confidence on the forecast and hence travelers should approach with a high margin of safety. 

There's plenty of great snow at mid and lower elevations so eliminating any potential wind loaded terrain would be wise. 

The recent storm has formed touchy cornices along ridges, so stay clear of cornices.

Observations

North

Reports Wednesday from snow safety at Mt Baker indicate the storm related sensitivity Tuesday has settled as of Wednesday, however not disappeared. There was a 2 foot human triggered slab release that propagated about 45-75 feet wide near ridgeline. Extensive active wind transport was visible on Shuksan and higher exposed peaks above treeline midday Wednesday, as northerly winds increased at crest level. 

On Tuesday morning the Mt Baker pro patrol reported widespread triggered storm slab, up to about 12 inches, some large, and continual natural avalanches to below treeline. 

Central

Multiple backcountry reports from NWAC observers at Alpental and Crystal Mountain Wednesday confirm that storm weak layers continue to settle and stabilize. No triggered or natural avalanches were reported Wednesday with cold low cohesion storm snow well connected to the 12/15 interface. The storm snow is deep out there! generally 2-4 feet in these central zones from Stevens to Mt Rainier.

There were touchy cornices reported in the Crystal backcountry Wednesday. Dropping several cornices did not release any avalanches on slopes below.  

On Tuesday the Alpental pro patrol also reported widespread natural and very touchy 8-12 inch storm slab at all elevations due in part to a weak storm layer buried about at 8 inches. This included very long distance sympathetic releases and at least one release possibly due to vibration of the chair lift!

Touchy storm slab at Alpental on Tuesday December 19th. Photo John Stimberis

The pro patrol at Stevens Pass Resort on Monday afternoon reported no new activity on slopes that hadn't previously been controlled and the new snow was not slabby. Earlier in the morning, the patrol reported widespread ski triggered and some natural soft slabs, 18-24" deep mostly on lee aspects near ridgetops, running on the December 16th weak layers. Some skier triggered shallow slabs on lower mountain entraining wet snow to create large avalanches. 

Two professional observations from Skyline ridge came in from observations on Monday afternoon. One found a sudden collapse at 45 cm on the December 16th interface, likely due to surface hoar or near surface faceted snow. The other observer did not find this problem. The recent 1-2 feet of new snow was right-side-up snow, increasing with elevation, and lay on top of a minor crust from rain on December 17. Thanksgiving crust was down 3.5-4 feet.

Additional observations and test results on the December 16th potential weak layer are available via the NWAC observations page.

South

No recent observations.