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West Slopes Central - Skagit River to South of I-90

Issued: 8:42 PM PST Friday, December 29, 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.

Message from Garth Ferber: today Friday December 29th I worked my last shift as an avalanche forecaster at the Northwest Avalanche Center. I started my career here in 1993 and am fortunate to have been able to work here with Mark Moore, Kenny Kramer, Dennis D'Amico and now Robert Hahn. I also have been fortunate to have been able to work with many great people in the NWAC partner groups over the years. You may not have heard the last of me, I may help part time with the NWAC weather stations in the future we will see. Cheers, Garth

The recent snow and conditions observed Friday warrant patience and caution on Saturday. Don't step out into aggressive terrain before new or recent snow layers have had a chance to stabilize.

Wind slabs will be likely to trigger on a variety of aspects in near and above treeline terrain. Increasingly stubborn storm slab will likely be most prone to trigger on convex or steep slopes, but these may be larger than normal due to the depth of recent accumulated snow.

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Avalanche Problems for Saturday

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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Forecast for Saturday:

Saturday will feature decreasing light post-frontal snow showers ending around mid-day in most locations along with sharp cooling and moderate westerly winds decreasing in the afternoon.

The recent snow and conditions observed Friday warrant patience and caution on Saturday. Don't step out into aggressive terrain before new or recent snow layers have had a chance to stabilize.

The wet snow deposited Thursday is likely to freeze more solidly as temperatures drop rapidly Friday night. The anticipated cooling should help to mitigate storm snow concerns slightly at most elevations, but more notably below treeline in most areas where wet snow was reportedly widespread. The cooling should also alleviate all wet loose concerns for Saturday.

Wind transport of newer, colder snow falling Friday night along with stronger west-southwesterly winds are likely to build wind slabs. These add to recently formed wind slabs which formed at higher elevations above the rain/snow line during the storms Thursday through Friday. Wind slabs are likely to load lower density, colder snow which predated the recent storms. The combination will produce touchy wind slabs and due to the complexity of wind patterns with these storms, don't assume that only the traditional aspects will have received wind-transported snow.

Storm slab will be in the upper part of the below treeline, the near treeline and above treeline bands on Sunday. Recent reports from around Stevens Pass and Mt. Baker suggest that recently developed storm slabs may be stubborn to trigger, but don't assume that this will be true for your area or elevation band as reports are limited, slabs are becoming deeper with all the recent new snow, and more snow will be loading potential weaknesses and there are also rime and freezing rain crusts to provide bed surfaces within our upside-down snowpack in some locations. Increasingly stubborn storm will likely be most dangerous on convex or steep slopes.

Loose dry avalanches are something to keep an eye on from Stevens pass northward. Steeper terrain should be entered with caution and exposure to terrain hazards mitigated.

The snowpack is quite deep in many locations. Small trees can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.

Also despite all this new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.

Avalanche Summary:

The overall result of a very active and stormy pattern on Friday is that the upper snowpack now has an upside-down density profile.

Friday's storm brought a complex package of weather to the Cascades and the resulting weather means a variety of upper snowpack conditions could be found by late Friday on the Cascade West slopes.

NWAC stations in the central and north part of the west slopes experienced light to moderate winds on Friday, but winds were moderate at Paradise and strong at Crystal and White Pass. Wind directions were generally southwesterly.

Most areas received plenty of water with this storm and from 5AM to 5PM on Friday, precipitation totals along the major stations west of the crest ranged from 0.85" at Baker to 2.2" at Paradise. Areas in the south also saw heavy precipitation totals Thursday night. 

Temperatures increased at most locations west of the Cascade crest during the storm. However, as the storm approached it sucked in colder air from east of the Cascade crest and through the Frasier River gap on Friday afternoon, resulting in depressed snow levels, particularly at Stevens pass and to some extent Mt. Baker. Both locations received primarily snowfall.

In spite of the variation in rain/snow, temperatures were borderline for snow in many cases and new storm slab density changes within the snowpack have been bonding well as reported near Stevens Pass and at Mt. Baker, making avalanches much more stubborn to trigger on Friday. In areas that did change over to all rain below treeline, most of the dry snow problems should been erased.

Due to the complex snowlines during the storm, it is worth viewing NWAC station data graphically at a location proximal to your area of interest. As an example, the plots below illustrate the Thursday-Friday marginal snow at Stevens pass and how a switch to easterly flow toward the end of Friday's storm (when peak warming was anticipated) likely kept the event all snow at Stevens Pass.

Friday's storm follows on the heels of Thursday's storm  which brought 5-15" of snow to many locations with rain rising up the below treeline band and mixed precipitation types at Snoqualmie Pass. Areas from Stevens Pass southward experienced continuous precipitation as the frontal boundary lingered between the two storms. 

The weather late this week was in contrast to the quiet weather of the previous 5 days when weak weather systems deposited about 3-8 in of low density snow along the west slopes of the Washington Cascades. There was mainly gradual snowpack consolidation and stabilizing during this period. 

The upper snowpack along the west slopes of the Washington Cascades has been widely reported as well bonded to the December 15th interface with tests unreactive. Where found, the December 15th layer varies from about 2-3 ft in depth throughout the west slopes of the Cascades. 

Observations

North

On Friday, a professional guide in the Mt. Baker backcountry reported snow most of the day above 4,000 ft. The upper snowpack layers were stubborn. Winds were generally light with little wind transport, further limited by damp snow. He reported no wet slab or storm slab below treeline.

The Mt. Baker pro patrol on Friday morning reported one explosive-triggered 18-24” wind slab at 4500’ on a NW aspect.

The Mt Baker pro patrol reported some initial wind loading and building of new wind slab as the winds and snow began to ramp up on Thursday.

On Tuesday 12/26, NWAC Observer Lee Lazzara traveled between 2000-6000 feet in the Swamp Creek drainage. Snowpack tests failed to produce results within the upper snowpack and the 12/15 layer was not identifiable in this area.

Central 

On Friday, Alpental Pro Patrol reported evidence of a natural cycle from 1' of storm slab in the early morning hours on Friday. The snow switched to rain and rain was reported to the top of Alpental by late morning.

Storm slab seen at Alpental on 12/29. Photo: John Stimberis.

On Friday, the Crystal Mountain Pro Patrol reported deep storm snow giving large to very large mostly loose wet avalanches that were sensitive to ski cut and entrained a lot of snow, but did not propagate widely. The slides were widespread at various aspects and elevations.

On Friday, the WSDOT crew at Snoqualmie Pass reported numerous small loose wet avalanches on steep NW-facing terrain and at that elevation there was not much snow to entrain and one went to ground.

On Friday, a professional guide was around Stevens Pass and traveled on NE through S aspects near and below treeline. A resistance increase was noticeable within the 15" of 24 hour storm snow and that storm snow was bonding well to the old snow interface. The layers were not producing significant test results. Loose dry snow was a minor concern on steep slopes and tree wells were becoming more of a hazard once again.

On Thursday, the NWAC observations page received a report from Windy Mountain near Stevens. Tilt tests gave easy results in storm layers 10-30 cm down. CTHQ2RP at 12/15 crust but ECTN. Dry loose potential seen on small slopes.

On Thursday, The WSDOT crew at Stevens Pass reported heavy snow and some loose wet D1-1.5 avalanches along the highway below the pass level.

The Alpental pro-patrol on Thursday reported  that new shallow 6 in storm slab was forming mid-mountain under cliffs as the winds and snow began to ramp up on Thursday.

NWAC pro observer Ian Nicholson was in the Commonwealth Basin area on Wednesday and found an overall right side up snow pack and EC and CT tests did not give results. Wind transport was seen on Red Mountain but any wind slab was seemed limited to specific terrain features. The 12/15 layer was seen at 50-55 cm.

The Stevens pro patrol on Saturday found the 12/15 layer down 3 ft (85-90 cm) and unreactive in large column tests performed on north aspects. No wind slab was observed.

South

The Crystal Mountain pro-patrol on Friday reported  deep storm snow and ski cuts most effective giving increasingly sensitive D2-3 loose wet avalanches which were widespread  and seen on various aspects.

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.