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

Issued: 6:08 PM PST Monday, December 11, 2017
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.

The snowpack is strong and well consolidated after a week of mild dry weather. Many areas have a very shallow snowpack below treeline making travel difficult and hazardous. Watch for wet surface snow conditions on some steep sun exposed slopes, especially below rocks or cliffs and above terrain traps. This weather provides the perfect opportunity to check out surface snow conditions where sufficient snow coverage exists for enjoyable travel.

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Avalanche Problems for Tuesday

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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.

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Snowpack Analysis:

High pressure has now been over the region for over a full week. Sunshine and very mild temperatures have been the story at mid and higher elevations with cool to cold temperatures at lower elevations. This weather has allowed for overall strengthening and consolidation of an already strong snowpack.

No triggered avalanches have been reported over the past five days.

Some small loose wet slides have occurred on mainly steep sun exposed slopes through the week, but have become much less frequent over the past several days.

Strong crest level southerly wind Friday night to midday Saturday may have built some wind slab layers in isolated terrain. However, moist surface snow on windward slopes likely limited available snow for transport.

The surface snow consists mainly of surface melt freeze crusts forming overnight on solar aspects. On shaded aspects, settled storm snow is still providing some nice skiing and riding conditions.  

The total snowpack height in these zones varies mostly from 3-5 feet, with the most in the north zone nearer the crest.

The snowpack is well consolidated and strong with settled old storm snow of about 1-2 feet bonded well to the strong Thanksgiving rain crust. 

During this high pressure pattern, surface hoar and or surface facets have been formed, and grown quite large in some areas, especially the typical sun and wind sheltered terrain at lower elevations in the cooler temperatures below the inversion. We will need to watch for these potential weak surface snow conditions as they may have an impact on future avalanche conditions when snowfall returns. 

On Monday at Washington passs, temperatures reached the mid-40's in the above treeline Monday with pass-level temperatures not making it out of the mid-twenties. Similarly cold surface temperatures were expected below 4800' throughout the Cascade east slopes, with much warm temperatures above.



A party of NCMG guides, training in the Washington Pass area on Thursday and noted the temperature inversion at the Pass, good bonding to the Thanksgiving crust, some small loose wet activity on solar slopes, with little avalanche concerns below treeline.


No recent observations.


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Detailed Forecast for Tuesday:

More of the same conditions are expected Tuesday, with slightly cooler conditions and a slightly weakening inversion. Mostly sunny and warm with light winds at higher elevations and cold and cloudy conditions in the lower passes and some low elevations. 

Warm temperatures in the near treeline and above treeline terrain have created conditions for melt-freeze conditions on solar aspects. Surface melt-freeze crusts are forming overnight and softening through the day. Loose wet avalanches are unlikely, but might be encountered in isolated steep solar exposed slopes below rocks or trees absorbing more radiation. Preserved and settled colder snow is expected on north-facing terrain at all elevations with variable conditions on all other elevations and aspects.

Wind slab has been removed as an avalanche problem for the northeast zone due to settlement time with warm temperatures near ridge-top, but without recent observations, backcountry travelers should still watch for areas of firm, wind-transported snow.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

No Corresponding Mountain Weather Forecast Available