Avalanche Forecast

Issued: Jan 24th, 2017 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A further gradual decrease is expected in the avalanche danger Wednesday. Continue to watch for firmer wind transported snow and previous wind slabs on exposed lee slopes especially near and above treeline. 

Summary

Detailed Forecast

Cloudier weather should be seen Wednesday with light winds and little change in cool temperatures. A few light showers are possible but most parts of the Olympics and Cascades should just end up with clouds.

These conditions should allow for further gradual decrease in avalanche danger as recent wind and storm slabs further settle and further stabilize. 

Recently formed wind slab should further stabilize on Wednesday.  Wind slabs are most likely on SW-NW-NE aspects in the passes and Crystal area due to recent strong E-SE transport winds. In the Mt Baker area, look for wind slabs on a wide range of aspects due to shifting winds. Watch for recent wind effects near and below ridgelines and avoid steep slopes with obvious signs of recent wind loading.  Wind slabs will likely extend into the upper portion of the below treeline band in the Crystal Mountain, Snoqualmie and Stevens Pass areas.

Storm slabs should become more unlikely and more stubborn to trigger Wednesday and will be removed as an avalanche problem.

It should be cloudier on Wednesday than the past couple days and further loose wet avalanches seem unlikely. Loose wet avalanches will not be listed as an avalanche problem. Watch for loose wet snow conditions on steep solar slopes if there is unexpected significant sunshine on Wednesday afternoon.

Snowpack Discussion

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather the 2nd week of January. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. But recent observations seem to indicate that these crystals did not survive the atmospheric river that began on 1/17.

An atmospheric river moved over the Northwest about 1/17 with highly variable weather seen throughout the Cascades during this event. Heavy rain was seen up to about 5000 feet in the north Cascades and up to about 6000-6500 feet in the south with snow at higher elevations. Three day precipitation totals through about Thursday were about 8 inches at Mt Baker, about 5 inches at Crystal and 1-2 inches at the Passes. Sleet and freezing rain Tuesday at the Passes changed to snow at higher elevations at Stevens and Snoqualmie on Wednesday. An avalanche cycle likely occurred in areas west of the Passes during this period. 

With the arctic air mass finally displaced and easterly flow abating in the Passes on Thursday, Stevens and Snoqualmie Passes warmed to near or above freezing while areas further west like Crystal and Mt Baker began a cooling trend. New snow totals through Thursday were fairly light except with about 12-15 inches at Stevens, 9 inches at the top of Alpental with 13 inches at the base. Below the new snow a freezing rain crust has been reported in the Stevens Pass area and the upper half of Alpental with varying reports of a crust at the Snoqualmie Pass level.

At Stevens and Snoqualmie Pass the freezing rain crust is about 1 inch (2-3 cm) thick and generally ski supportable. Mt Baker was the benefactor of steady showers Thursday afternoon and night, picking up about 2 feet of new snow through Friday morning.  E-SE winds were strongest in the Crystal area Friday, with light to moderate E-SE winds observed or reported elsewhere.

Showers Saturday and Sunday deposited about 1-9 inches of new snow with further cooling. Strong S-SE winds Sunday built unstable wind slabs on most lee slopes receptive to transport.

Fair weather with light winds and cool temperatures has been seen Monday and Tuesday.

Recent Observations

North

A couple reports for the Mt Baker area area available via the NWAC Observations page for Sunday. Skiers triggered 6 inch storm slab on N-NE slopes at about 5500 feet, good bonds were seen to the 1/17 interface at 60 cm. Near Lake Ann wind slab was seen on S slopes and N slopes were scoured. 

Central

On Sunday NWAC pro-observers Dallas Glass and Ian Nicholson were back in the Stevens and Snoqualmie Pass areas. Strong south to east winds was actively transporting recent storm snow and building wind slabs on all but windward slopes. Many windward slopes, NE-S facing, were scoured to the 1/17 crust. Storm slabs were gaining some strength but remained touchy in some areas below treeline. In both areas, recent storm snow was highly variable due to wind effects, but in sheltered areas averaged 12-16 inches.

From the NWAC Observations page easy shears and some signs of propagation were seen Sunday at the 1/17 interface near Lanham Lake at Stevens Pass. A skier reported triggering a large storm slab in a chute above Snow Lake on Sunday. A report of snowpack whooping is seen on the Turns All Year page for Smithbrook on Sunday.

NWAC pro-observer Jeff Ward was on north slopes on Rock Mountain near Stevens Pass on Monday and found 50-65 cm on storm snow on the 1/17 crust with good stability and no recent avalanches.

A skier via the NWAC Observations page for the Source Lake-Bryant area on Monday reported 15-60 cm of storm snow on the 1/17 crust. Previous wind transport was evident at higher elevations. A skier triggered a large storm slab on a N-NE slope at about 4000 feet which ran on the 1/17 crust. Natural and ski triggered loose wet were also seen on solar slopes.

NWAC forecaster Dennis D'Amico was out at Hyak on Tuesday on N-E slopes in the 2800-4400 foot range and found about 30 cm of right side up storm snow well bonded to an uneven, thick 1/17 crust. Ski tests on steep terrain rollovers produced no results.

South

The Crystal pro-patrol reported extremely sensitive new wind slabs on NW aspects Sunday morning. About 4-6 inches of new storm snow were transported by very strong S-SE winds overnight and early Sunday morning. This built very sensitive wind slabs on lee slopes by early Sunday. Wind slabs, mostly in the 6-8" range, released sympathetically and were running long distances. Shooting cracks were extending over 100 feet from skis.

By Monday and Tuesday the Crystal pro-patrol reported that previous reactive wind deposited snow had bonded and greatly stabilized with no new avalanches.

Problems

Wind Slabs

An icon showing 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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 25th, 2017 10:00AM