Avalanche Forecast
Issued: Feb 11th, 2017 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Watch for and avoid lingering wind slabs near and above treeline, possible on easterly facing slopes below ridges. Watch for melting and weakening surface snow on steep sun exposed slopes where loose wet avalanches will be possible. Caution travelling on steep slopes with a slick crust.
Summary
Detailed Forecast
Sunshine or filtered sunshine through high clouds is expected Sunday with significant warming.
Watch for recent new wind slabs at higher elevations, especially below ridges in easterly facing terrain. Watch for any wind deposited snow, however, such as cross loaded slopes or specific terrain features.
Sunshine and warm temperatures Sunday should make loose wet avalanches possible on steep southerly slopes facing the sun.
Some slopes exposed to recent winds or where less new snow was received after rain changed to snow, may have a very slick crust. Be cautious of uncontrolled falls if traveling on steep slopes with slick crust.
Snowpack Discussion
Weather and Snowpack
A strong storm cycle Friday, 2/3 through Monday 2/6 deposited 3-5 feet of snow along the west slopes of the Cascades.
The great snow didn't last long as a strong SSW storm brought heavy rain to the west slopes of the Cascades (Mt. Baker, Crystal and Paradise) Wednesday afternoon and Thursday, with a mix of snow, sleet, freezing rain, and rain in the Cascade Passes. A strong cold frontal passage swept through the Cascades mid-day Thursday with bands of light to moderate showers in SW flow.
A switch to westerly flow brought a rise 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 before tapering Friday night.
Most areas along the west slopes of the Cascades received about 6-8 inches of new snow with the Baker area about 14 inches by Saturday morning. The Mt. Baker area had received the most snow with 13 inches recorded at Heather Meadows through 6 pm Friday.
The aftermath of this storm cycle left a very strong crust with whatever loose snow remains above following the rain changing to snow. This ranges from 6-16 inches where winds did not strip the new snow and expose a slick crust.
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.
Lee Lazzara was in the Mt Baker backcountry Saturday and reported a favorable increasing resistance profile in the storm snow over the very strong rain crust. Observations were not made in terrain where wind slabs more likely formed.
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.
Crystal Mt Pro-patrol, explosive triggered slab, 2-10-17. Photo, P. Dale
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
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 12th, 2017 10:00AM
