Avalanche Forecast

Issued: Feb 10th, 2017 11:58AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs and Storm Slabs.

NWAC, Northwest Avalanche Center

Cascades - West.

Strong winds Friday and Friday night will have built fresh wind slab on lee aspects mainly near and above treeline. Watch for new wind slab in the upper portion of the below treeline band. Easterly aspects will be highlighted in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.

Summary

Detailed Forecast

Light to moderate showers Friday night should taper off after midnight with only isolated light snow showers expected on Saturday. The same trend is expected for the moderate to occasionally strong W-SW winds forecast for Friday night with winds easing considerably on Saturday.

Despite the cooling trend, new and shallow storm slab instabilities may be locally sensitive. Storm slab size will range from small to large in the Mt. Baker area where more snowfall is expected.

For areas west of the Cascade crest including Mt. Baker, Crystal and Mt. Rainier, wet slabs will not be listed as they are very unlikely due to the cooling trend. However, for one more day avoid travel below unsupported slopes, especially 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.

Snowpack Discussion

Weather and Snowpack

A strong storm cycle Friday 2/3 through Monday 2/6 produced 3-5 feet of snow along the west slopes of the Cascades.

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 Thursday with bands of light to moderate showers in SW flow following the front. A switch to westerly flow brought a bump up in temperatures in the Cascade Passes Thursday afternoon and night.

Friday was windy with a slow cooling trend during the day. Light showers increased and generally became moderate in the afternoon. The Mt. Baker area had received the most snow with 13 inches recorded at Heather Meadows through 6 pm Friday.

Recent Observations

North

Mt. Baker pro-patrol reported that the cooling trend had quickly locked up the recent wet snow within the ski area. Strong W-SW winds were transporting snow to lee slopes and not bonding well to the new crust.

Central

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. A bit east of Stevens Pass, a large avalanche off of Dirty Face Peak knocked in a garage door of a house on the north shore of L. Wenatchee around 1130 am Thursday.

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.

South

Crystal pro-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.

White Pass reported similar conditions as the other Passes, with a freezing rain crust up to at least mid-mountain and very strong W-SW winds atop Pigtail Peak through Friday.

Crystal pro-patrol reported minimal results during control work Friday except... when they used a 25 lb explosive on Powder Bowl. The result was an 8 ft crown, 150-200' wide, that broke down to facets 3" above the ground. While this slide isn't relatable to human triggering, everyone loves a photo of a big avalanche.

P. Dale, Crystal Mt Pro-patrol, 2-10-17

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

Possible

Expected Size

1 - 1

Valid until: Feb 11th, 2017 11:58AM