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East Slopes North - Canadian Border to Lake Chelan

Issued: 7:31 PM PST Thursday, February 23, 2017
by Garth Ferber

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.

Isolated lingering wind slab in the near and above treeline will continue to heal and become less sensitive to human triggering on Friday. You will need to watch for a wider variety of avalanche problems as we transition to spring. Continue to evaluate snow and terrain carefully on Friday.

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Avalanche Problems for Friday

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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Snowpack Analysis:

Weather and Snowpack

The latest of several warm, wet SW streams of moisture this season arrived Valentine's Day 2/14 through Thursday 2/16 along the Cascade east slopes. An avalanche cycle occurred Wednesday night to Thursday east of the Cascade crest.

In the northeast zone the NWAC station at Washington Pass received about 10-14 inches of moist snow over a thin freezing-rain crust formed early in the storm cycle.  In the central-east and southeast zones, light rain likely reached up about 5-6000 feet in the central-east and up to about 7000 feet in the southeast zone.

A short period of fair weather on Friday, 2/17 caused another surface crust, especially on solar aspects.

Light amounts of new snow accumulated over the weekend.

A low pressure system, tracking across the southern Washington Cascades during the day on Monday deposited a few inches of new snow to the central-east and southeast zones. A slight warming trend was observed during the storm Monday.

Light snowfall Tuesday changed to showers in the afternoon.

Sun and a few light snow showers Wednesday and Thursday will have give little if any additional snow with cool temperatures and light winds.

Recent Observations

North

The NCMG were out near Washington Pass on Sunday 2/19 and reported that ski tests on steep features gave no results and that wind slab was generally unreactive. A size 2.5 slab, 1 m x 200 m crown was seen on a north aspect at 7200 feet that probably released after cornice fall on 2/15.

On Monday 2/20, the NCMG observed a fresh and large storm slab that was triggered by cornice fall mid-day above treeline in the Hairpin Valley on a NNE aspect near 7000 feet. No other avalanche activity was observed. The slab averaged around 40 cm to the most recent crust.

The NC Heli Guides report on Tuesday 2/21 recent snow is settling nicely with no test results and good ski conditions.

Another report via the NC Mountain Guides on Tuesday for N-E slopes on Mt Burgett north of Mazama indicates older wind slab on some alpine features giving some results at about 40 and 80 cm due to flattened surface hoar layers. They avoided steep unsupported terrain features and the wind slab was not seen below ridges.

A report via the NWAC Observation page for the Washington Pass area for Tuesday indicates 12" powder well bonded to the likely 2/14 crust.

Central

NWAC observer Tom Curtis was at Jove Peak on Monday 2/20 and found the recent snow generally bonding well to the Valentine's Day crust. Some previous wind slab was apparent below ridgelines near treeline. The possibility for skier triggered, loose wet avalanches increased around midday during a slight warming trend.

Tom Curtis was in the Blewett Pass area Wednesday 2/22 and found the recent wind slabs were confined to very isolated terrain features on mostly easterly facing terrain at higher elevations. These slabs remained sensitive to ski trigger, but were very isolated in extent, ranging from 2-8 inches.

Very similar conditions were reported from Mission Ridge Wednesday 2/23, with isolated wind slabs of 6-8 inches confined to specific steep terrain features below ridges.  

South

No recent observations. 

Detailed Forecast for Friday:

A weak low pressure system should move north to south over the Northwest coastal waters on Friday. This should trigger light snow showers over the Cascade east slopes by Friday afternoon with light winds and low snow levels.

Lingering wind slabs should continue to heal, become isolated to specific terrain features, and become less sensitive to human triggering Friday. The most significant recent winds along the Cascade crest were generally S-W so NW-SE aspects in the near and especially above treeline will be indicated. Watch for signs of firmer wind transported snow.

Storm slabs are expected to have stabilized by this time.

Cloudier weather should limit the potential for loose wet avalanches on Friday and this will not be indicated as an avalanche problem. Watch for wet surface snow deeper than a few inches if you find yourself on sun exposed slopes in extended sun breaks.

Loose dry avalanches will also not be indicated as an avalanche problem. You can do tests for loose dry avalanches by pushing snow onto small safe test slopes.

Avoid areas along ridges where there may be a cornice and slopes below cornices, since cornices can fail at any time.