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East Slopes Central - Lake Chelan to South of I-90

Issued: 6:58 PM PST Wednesday, January 17, 2018
by Robert Hahn

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. 

A storm overnight will bring strong winds and new snow. The storm will load existing weak snow sitting on a crust layer in most areas. This will cause an increasing avalanche danger. Expect dangerous avalanche conditions in higher terrain with avalanches potentially running long distances. 

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Avalanche Problems for Thursday

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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Avalanche Summary:

Light amounts of new snow fell Tuesday, generally 2-5 inches. The shallow recent snow overlies a variety of older surfaces, including melt-freeze crusts formed during temperature inversion conditions last weekend (Saturday through Monday) and subsequent refreezing wet snow from rainfall that fell in some low-to-mid elevation locations on Monday night.

Buried surface hoar has been observed in the Cascade East - Central zone as of Wednesday 1/17. This layer is likely to exist in the north zone as well, but lack of direct observations leads to lower confidence of its extent and distribution. 

Last week’s storms brought 8-14 inches of snow to the east slopes of the Cascades which has been consolidated by warm temperatures.

A supportive crust (1/5) formed from a widespread freezing rain event in the central-east zone, extending to the Stevens and Snoqualmie Pass areas. This layer is not present in the northeast from Washington Pass to Holden.

Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.

Observations

North

No recent observations from this zone following the warm weather last weekend.

Central

On Wednesday, 1/17, NWAC observers Jeremy Allyn and Jeff Ward traveled in the Icicle Creek drainage to near 7000 feet. Of particular note was buried surface hoar found over a melt-freeze crust, buried about 3-4 inches on 1/16. The buried surface hoar was not limited to valley bottoms, but found to all elevations traveled, near 7000 feet. In areas void of the buried surface hoar layer, 3-4 inches of low density snow overlies a recent melt-freeze crust which also extended to about 7000 feet.   

A snow professional reported from the Mission Ridge area on Sunday where a 4-5' deep snowpack on a north aspect at 6400 feet contained two potential weak layers of concern. (1) A layer of 2 mm facets 45 cm above the ground lies beneath the 12/15 crust. This layer was reactive in column tests and propagation tests (see this report) . (2) Basal facets just above the ground were present but rounding and were not the weak layer of interest in numerous tests. These layers will need to be monitored as they could develop into potentially dangerous weak layers in the future under loading.

South

No recent observations

Forecast for Thursday:

An arriving storm overnight Wednesday will bring strong crest level winds and snowfall. The storm should come with cooling through Thursday with snowfall changing to showers Thursday with diminishing winds.

The incoming storm will load existing shallow weak snow or buried surface hoar that sits over a smooth crust in many areas. The existing weak surface snow conditions and potential sliding layer, should lead to sensitive avalanche conditions as wind and storm slabs build.

Variations in new storm snow amounts throughout the zone will lead to subsequent variation in local danger. Regardless, the current snowpack structure should not support significant snow loads well.

Building wind and storm slabs should cause dangerous avalanche conditions near and above treeline by Thursday.

Where buried surface hoar exists over a smooth crust layer, expect very sensitive avalanches that may run long distances, especially in areas receiving the greatest storm snow amounts.  

Monitor new snow and wind transport carefully and choose conservative terrain. 

No Corresponding Mountain Weather Forecast Available

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.