Avalanche Forecast

Travel in avalanche terrain is not recommended on Thursday! 

On Thursday, strong storm arrives in the early morning hours with warm front bringing rising snow levels and strong to very strong crest level winds with heavy rain and snow. Easterly flow will keep the Cascade Passes and locations near the Cascade Crest generally snow through the morning hours with a changeover to rain during the afternoon hours at Snoqualmie Pass.

Storm snowfall will be quite intense across the Cascade West slopes with greater than 1 foot of snow expected to accumulate during daylight hours at most locations above 4,000 ft. This snow will fall at increasing density, creating upside down storm layers on top of the multitude of crusts and recent lower density snow already existing in the snowpack. This storm is very likely to produce a widespread natural avalanche cycle of large and very large avalanches in all elevations bands and a changeover to rain is likely to trigger many lower-elevation avalanche paths in the central and south Cascades.

Wind slabs forming Tuesday through Wednesday were already showing signs of significant reactivity in many areas on the crust surfaces present. On Thursday, Strong to very strong SSW crest level winds (turning westerly in the late afternoon) will quickly form very touch large to very large wind slabs on a variety of aspects, but most notably NW though SE.

Early season hazards still exist. Many creek beds have still not filled in for the winter and crusts in many areas are quite firm.

Following a brief warm-up on Tuesday, an additional 10-12" of snow was seen in many areas near treeline from Snoqualmie Pass northward, with lesser amounts to the south (e.g. 1-3" in Rainier/Crystal/White Pass area). The snowpack is generally right-side up, but is showing reactive storm slab formation on a supportable 1/5 and unsupportable 1/9 crusts in some locations. Moderate generally westerly winds late Tuesday through Wednesday re-distributed snow below treeline in exposed areas and particularly near and above treeline where it exposed the most recent crust in some locations and formed deep wind slabs nearby.

Below this recent 5-20" of recent snow reports continue to indicate a strong upper snowpack with no notable layers of concern and a multitude of crusts formed over the last two weeks. 

Observations

North

On Wednesday afternoon, Mt. Baker pro patrol reported 2’ slabs on the Monday rain layer which propagated 50 yards on steep terrain.

On Wednesday morning, Mt. Baker pro patrol reported 6-8" of lower density new snow overnight. Control results yielded explosive-triggered 4-8" storm slabs on most aspects, with isolated 1-2' slabs on north-facing aspects and a single 10" wind slab on a N-NW aspect which hadn't seen skier traffic on Tuesday. All slabs were explosive-triggered and ran on the same bed surface.

On Tuesday, the Mt Baker pro patrol reported sensitive 5-6" soft slabs releasing above the most recent rain crust (1/5), both with ski and explosive triggers.

Central

On Wednesday afternoon, Stevens DOT reported a large or very large natural slab avalanche that crossed the railroad tressel near Windy Ridge, west of Stevens Pass. The debris looked fresh.

An avalanche professional was in the Smith Brook drainage on a SSW aspect at 4600 ft and found storm slabs reactive on a 1/9 sun-crust and more notably 15 meter collapse and CTV SP, ECTPV on the 1/5 crust.

NWAC Forecasters Dallas Glass and Dennis D'Amico were on Arrowhead Mountain, non-solar aspects 2900-5600' where they found generally right-side-up surface snow bonded well to the 1/9 and 1/5 crusts with wind transport in more exposed areas. Near and above treeline they found pockets of deep wind-slab and mixed with exposed crust surfaces. The most recent crust (1/9) extended up to 5000'.

On Wednesday morning, Stevens Pass pro patrol reported all paths near ridge crest were running in the 6-10" range. 

On Wednesday, Alpental pro patrol found results with explosive trigger with 6-8" slabs widespread, deeper in places, with moderate propagation. Ski trigger was stubborn to non-reactive.

NWAC professionals on Snoqualmie Mountain Tuesday reported a major distinction in snowpack with elevation. Below treeline, a mixture of rain or wet snow maintained shallow wet snow conditions making loose-wet avalanches possible on steep slopes involving about 4-6 in. of snow above the 1/5 crust layer. Near and above treeline, especially above 5200 ft, a lack of crust, recent dry snow and light to moderate winds were actively forming shallow new wind slabs in exposed terrain.

South

On Wednesday, Crystal Pro patrol reported 8” of low density snow above the most recent crust with 4-6" on the lower mountain. Ski cuts effectively triggered 8-10” wind slabs with low energy and little propagation. At high elevations on non-solar aspects, the old snow surface had near surface facets over a firm old bed surface on the upper mtn start zones.

On Wednesday NWAC Pro Observer Ian Nicholson was in the Crystal Backcountry. He observed sensitive wind slab 6-10" in thickness in specific terrain near ridge lines. From the ridgeline he was able to produce shooting cracks off his ski tip. He also observed recent snow forming shallow 4" storm slab during the day within storm snow about 4" above the 1/5 crust.

Crystal Mountain, Wind slabs showing propagation (Photo Credit: Ian Nicholson)