Avalanche Forecast

Tuesday will feature a major storm system, arriving with strong to very strong S-SW free air winds and light pass-level easterlies, switching to moderate to strong westerlies behind the front with snow levels dropping close to sea level. The storm arrives on the heels of a significant loading event for higher-elevations and all elevations in the northern part of the range. The combined effect will be very dangerous storm and wind slab avalanche conditions on Tuesday. 

Storm slab is the primary concern, with denser snow falling at moderate to high precipitation rates across the west slopes from midnight onwards through the daylight hours on Tuesday. See the mountain weather forecast tab for more details. The Sunday-Monday storm has buried previously exposed terrain anchors in many locations, particularly from Stevens Pass northward. A number of interfaces now exist in the snowpack including two storm layers, sandwiched at lower elevations by a rain crust, and sitting on the variable surfaces formed during the high pressure system. This package will be overlain by initially denser new snowfall from tonight into tomorrow. The combined bacon-lettuce-and-tomato sandwich will mean that small to large avalanches in the new storm snow will be sensitive and likely and may step down to overload and trigger older, but recent interfaces (which were generally unreactive by this afternoon) to create very large avalanches. See the snowpack observations above for more information about the potential layers recently observed.

Given the strong winds, small to large, sensitive wind slabs are likely in near and above treeline, mainly on NW through SE aspects, but are possible on other aspects. It is ill advised to travel near ridgelines tomorrow.

Travel and remain near your partner and keep them in sight and direct communication at all times due to tree wells and and the threat of snow immersion. If your partner falls into tree well, you have to be very nearby to help them in time to save them.

Remember that beneath any new snow received during this storm, a myriad of early season terrain hazards exist, especially at lower elevations and in the central and south portions of the Cascades. 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.

The first frontal system since the 12-day high pressure system crossed the Cascades on Friday. New snow accumulations Saturday morning ranged from 2-5 inches along the west slopes of the Cascades with a local maximum at the top of Alpental with 8 inches of new snow. This snow buried a variety of surfaces, including melt-freeze crusts, settled old storm snow, and a mix of recently formed surface hoar or near surface facets (sugary snow) at lower elevations and some shaded terrain. We're tracking the potential for a buried facets to remain reactive in isolated patches near Stevens pass, but faceted crystals have been destroyed elsewhere along the west slopes.

The next system brought warm frontal moisture arriving on Sunday as light rain to 4000-4500 feet in the north Cascades, a battle between rain and wet snow at Stevens and Snoqualmie Passes with light snowfall accumulating at the top of Alpental, and a mix up to about 5500 feet in the south Cascades (Paradise, Crystal) through 4 pm Sunday.  No significant avalanche activity was reported. The warmer temperatures and light rain likely helped wet or potentially destroy recently buried persistent weak layers at lower elevations, including Snoqualmie Pass, but given recent observations, potentially not in all locations in the Stevens Pass area.

The system's moisture moisture feed off the Pacific intensified Sunday night and brought copious precipitation to the Cascades west slopes with a strong N-S gradient in temperature translating to snowlines approximating 5000 feet in the south, 4000 feet central, and 3000 feet north. The rain and snow came with a cooling trend on Monday.

24-hours water amounts ending 5PM on Monday were generally 1-3" with a local maximum of 3.3 inches at Alpental. 

24-hour snow totals ending 5PM Monday were far greater at higher elevation sites: The higher elevations of Alpental and Stevens receiving 16" and 20-25" respectively, with comparable elevations from Mt. Rainier southward receiving 4-9"

The 5PM Monday 24-hour snow totals were far less impressive at lower elevations with 0-2" on the lower half of Alpental and at Crystal Base, but 10" at Stevens pass level.

The North Cascades received their snow earlier and finished earlier with a relative break in the action from late morning onwards. Mt. Baker received close to 20" of snow from the storm, 13" from top to bottom in the last 24 hours, but has had a break in the action from the late morning onwards.

The overall snowpack of 2-6.5 feet along the west slopes is generally right-side up with up to 2.5 feet of new snow above the settled old snow sitting over the strong Thanksgiving rain crust, buried more deeply now.

Observations

North

On Monday afternoon, Baker Pro Patrol reported evidence of a natural 300-400 foot wide, very large avalanche with a 2-3' crown on a N-NE aspect near 6000 feet visible on the east end of Shuksan Arm. In the morning, they reported widespread explosive and ski triggered 12-14" wind slab or storm slab above 3800" with shallower slabs entraining wet snow underneath at elevations below 3800". 

Central

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 15th 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 15th 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.

Note these other observations from the Stevens Pass area, which include some additional test results on the December 15th potential weak layer: NWAC observations page.

South

On Monday, Crystal mountain Pro Patrol reported 3-5" of new snow, mild temperatures and moderate winds, with not enough accumulation or transport for avalanche activity.