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Snoqualmie Pass

Issued: 6:00 PM PST Saturday, January 21, 2017
by Kenny Kramer

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

Watch for new wind slabs forming early Sunday on exposed lee slopes at higher elevations. Allow recent storm and wind slabs further time to stabilize Sunday. Wind slabs are most likely on N-W-SW aspects. Storm slabs should continue to slowly stabilize Sunday, but still possible in the wrong spot.

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Avalanche Problems for Sunday

Wind Slabi

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.

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Storm Slabsi

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.

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Snowpack Analysis:

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. Recent observations on Thursday and Friday helped confirm that we aren't dealing with any older layers formed during this period.  

An atmospheric river moved over the Northwest Tuesday and Wednesday 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. 3 day precipitation totals through noon Thursday were about 5 inches at Crystal and 8 inches at Mt. Baker with 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 saw a slow cooling trend. New snow totals through Thursday were fairly light except with about 12-15" at Stevens, 9" at the top of Alpental with 13" 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 Pass level.

At Stevens and Snoqualmie Pass the freezing rain crust in about 1" (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.

A very pleasant day Saturday allowed temperatures to moderate to near the freezing mark in many areas. Light snow showers began to spread from south to north late Saturday afternoon. 

Recent Observations

North

The Mt Baker pro-patrol and NWAC pro-observer Simon Trautman reported several large wet slab and glide avalanches from the Mt Baker area on Wednesday following the period of heavy rain. Simon was back out in the Mt. Herman/Bagley L. area Friday. A density change about 10" (25 cm) down in the new storm snow was reactive on small test slopes and the depth of widespread natural storm slab avalanches that had occurred Thursday night. At least one unintentional skier triggered storm slab was reported on this layer in the Mt. Baker backcountry Friday. Recent wind loading was apparent on many aspects near and above 5000'. Weak wet snow below the recent storm snow could still be found below 4000 ft.     

Central

The Stevens pro-patrol on Wednesday morning reported only small wind slabs along the ridge tops and small triggered loose dry avalanches. The new snow was not bonding well to the freezing rain crust. NWAC observer Dallas Glass was out in the Stevens Pass area Wednesday and reported sensitive 25-35 cm of wind slab on westerly aspects above the 1/17 freezing rain crust in the near and below treeline bands. On Friday, Dallas reported storm slabs failing on a preserved storm interface just above the freezing rain crust in snowpack tests and on short test slopes. Wind slabs were sensitive at the same interface and most likely to be found on westerly aspects near and below treeline in the Stevens area.

NWAC observer Ian Nicholson was out in the Alpental Valley on Friday and found storm slabs 20-25 cm thick and poorly bonded to the 1/17 freezing rain crust. Wind slab distribution near treeline proved variable and touchy in pockets. Both avalanche problems required conservative terrain choices.    

South

Crystal Mt pro-patrol reported a widespread loose-wet cycle on all aspects Wednesday with slides ranging from small to large. Only shallow 2-4" winds were reported below ridgelines on northerly aspects during control work Thursday morning. NWAC observer Jeremy Allyn was in Bullion and Cement Basin in the Crystal area on Friday. Jeremy found moderate to strong E-SE winds stripping windward slopes, but only building very shallow wind slabs near and likely above treeline on lee aspects. Shallow storm snow was not cohesive or well bonded to the underlying rain crust. 

Detailed Forecast for Sunday:

A period of light snow showers is expected Sunday with a period of moderate to strong S-SE crest level winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on N-W-SW aspects due to recent and expected E-SE transport winds, but will be listed on all aspects due to variable loading patterns over the past several days. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution. Wind slab will likely extend into the upper portion of the below treeline band at Snoqualmie and Stevens Pass. Considerable avalanche danger is forecast above treeline to highlight the potential for increased sensitivity and size of wind slabs. 

Storm slabs should be more stubborn to trigger, but still possible in the wrong spot Sunday and can be avoided by sticking to lower angled slopes.

Storm slabs will not be listed in the south Cascades zone where there has been less recent snow above the rain crust.  

Observations