Avalanche Forecast
Regions: Cascades - North West.
After a period of wild weather Wednesday night and Thursday that closed major highways and limited observations, there is a higher than normal amount of uncertainty about the snowpack heading into Friday. Account for the uncertainty with more conservative travel plans than you might otherwise given the same danger rating. New storm instabilities may shade the needle toward high danger in the Mt. Baker area Friday afternoon.
Detailed Forecast
What a difference a day makes as we quickly transition from mild and wet to cool and showery weather on Friday. Snow levels will slowly lower Thursday night and lower further during the day on Friday. Showers should decrease overnight but then increase again by mid-day Friday as a low pressure system passes to our north. Snow showers may become especially heavy in the Mt. Baker area Friday afternoon. W-SW winds will be moderate to occasionally strong Thursday night and Friday.Â
After a period of wild weather Wednesday night and Thursday that closed major highways and limited observations, there is a higher than normal amount of uncertainty about the snowpack heading into Friday. Account for the uncertainty by choosing more conservative travel plans than you might otherwise given the same danger rating.
Moderate to occasionally strong W-SW winds will transport new and recent snow onto lee slopes near and above treeline. Wind slab may build down into the below treeline band on Friday. Easterly aspects will be identified in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects. Â Â
Despite the cooling trend, new storm slab instabilities may develop during periods of intense showers. Storm slab instabilities are likely to develop in the Mt. Baker area in the afternoon. Â
For areas west of the Cascade crest including Mt. Baker, Crystal and Mt. Rainier, wet slabs will be listed as unlikely due to the cooling trend. However, avoid travel below unsupported slopes, especially near and below treeline, with the low likelihood/high consequence of large wet slab avalanches in mind. Wet slab releases are hard to predict and can happen a day or two after peak warming and rainfall.  Â
Snowpack Discussion
Weather and Snowpack
A strong storm cycle was seen Friday through Monday with about 3-5 feet of snow recorded along the Cascade west slopes. A period of brief warming on Saturday created a weak crust layer that has now been found both at Stevens and Snoqualmie down at least 3 feet.Â
A strong plume of moisture brought heavy rain to the west slopes of the Cascades (Mt. Baker, Crystal and Paradise) Wednesday evening through Thursday with a mix of snow, freezing rain, and rain seen in the Cascade Passes. A strong cold front swept through the Cascades mid-day. Bands of light to moderate showers in SW flow followed the front. A switch to westerly flow brought a bump up in temperatures in the Cascade Passes Thursday afternoon while areas further west were already beginning a slow cooling trend. Â
Precipitation totals through 4 pm Thursday:Â
Recent Observations
North
No observations Thursday.Â
Central
NWAC pro-observer Jeremy Allyn was in the Henry Creek drainage below Jim Hill on Wednesday and reported that wind transport was underway due to the incoming storm. Quick tests of the near surface snow did not show significant results and ECT and PST tests on the 2/4 interface at 75 cm down did not indicate propagation. NWAC pro-observers Jeff Ward at Skyline at Stevens and Ian Nicholson near Alpental on Wednesday did find significant signs of instability.
Stevens Pass pro-patrol and DOT reported a natural avalanche cycle Thursday morning with major paths running D2 - D3. The pro patrol also observed some natural activity off Skyline Ridge and heard an unusual report of a path running to the Nordic road in Mill Valley.
On Thursday, Alpental pro-patrol reported a breakable freezing rain crust (i.e. less stout than the 1/17 IFrc) that made for tough ski conditions. Cornices along ridgelines weakened during the warming with some cornice breaks observed in the early afternoon. The cornice failures triggered some D1-D2 storm slabs below ridges and occurred post freezing rain as there was not an ice coating on the debris. Â
South
Crystal patrol observed several loose wet slides up to size D2, all aspects, within the ski area Thursday. No large slab releases were produced with explosives Thursday afternoon. Wet slabs estimated to be about 1 ft deep were observed from the ski area in Pickhandle Basin.Â
Avalanche Problems
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, South.
Elevations: Alpine, Treeline.
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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 2