Avalanche Forecast

Issued: Jan 13th, 2017 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

  Recent wind slabs should be primarily on N-W-S aspects but be alert on all terrain aspects firmer wind transported snow.

Summary

Detailed Forecast

The upper ridge should linger over the northeast Pacific Ocean through the weekend with some moisture moving through the ridge. In the Olympics and Cascades this should cause periods of mid and high clouds, light winds and further moderating temperatures especially west of the crest and at higher elevations.

Recent wind slabs should be primarily on N-W-S aspects. The benign weather should bring a little more stabilizing to these layers on Saturday. Remember that firmer wind transported snow is always your best sign of wind slab layers.

The wind slab compass indicator below shows loading for the recent dominant wind direction but some areas may have experienced other loading patterns, so be alert on all terrain aspects for firmer wind transported snow.

Small loose dry avalanches might be possible in steep wind sheltered terrain but will not be listed as an avalanche problem. 

Snowpack Discussion

Weather and Snowpack

A pair of warm fronts moved across the Northwest last weekend causing see-sawing temperatures to near or above freezing for areas away from the Passes with milder Pacific air finally dislodging colder air in the Passes late Sunday night. 5-10 inches of snow fell through Monday morning with snow showers depositing another 5 inches in the Mt. Baker area through Monday afternoon.

A weak low pressure system, tracking across Oregon Tuesday morning through Wednesday afternoon, deposited another 3-6 inches of snow with light winds from Stevens Pass and southward, with Paradise adding another 12 inches. A period of strong E to NE winds increased Tuesday afternoon through early Wednesday before diminishing Wednesday afternoon.

An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday and Friday over the Olympics and Cascades with mostly light winds and moderating temperatures especially west of the crest and at higher elevations.

Recent Observations

NWAC pro-observer Dallas Glass was in the Kendall Peak area below treeline Tuesday. Dallas found the new snow generally right side-up with good snow conditions. A freezing rain or rain crust at 25 cm was ski supportable. The crust disappeared above 4800 feet. The only direct signs of instability noted, were small loose dry sluffs on steeper slopes. Coming home via the highway, strong east wind were beginning to transport snow on Granite Mountain by mid-afternoon. 

NWAC pro-observers Dallas and Ian Nicholson were in the Crystal backcountry Wednesday to assess the distribution and sensitivity of fresh wind slabs from recent strong E-NE winds.  They found that wind slabs were becoming larger as they moved into the near tree line band and exposed ridges. NE-E-SE facing slopes were stripped of recent snow with new reactive wind slabs on N-W-S facing terrain and cross loaded features. Their travel in this area was therefore conservative. In wind sheltered terrain below treeline, the storm snow was right side up and lacked slab structure with excellent skiing conditions.

Some reports are available via the NWAC Observations page for Wednesday and Thursday. in summary skiers triggered wind slab layers on Mt Herman near Mt Baker and in the Slot Couloir on Mt Snoqualmie on Wednesday. Potential wind slab layers were noted at Stevens and at Snoqualmie on Thursday but no avalanches were triggered.

NWAC pro-observer Jeremy Allyn was out in the Alpental Valley on Friday and mainly found that recently formed wind slab in the near and above treeline was settling and strengthening.

Also here is advance notice that faceted surface snow and surface hoar seen in some areas lately due to the cold weather will need to be watched when snow and potential rain begins to arrive next week.

Problems

Wind Slabs

An icon showing Wind Slabs

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

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.

Aspects: North, South, South West, West, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 14th, 2017 10:00AM