Avalanche Forecast
Issued: Dec 17th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Watch for firmer wind transported snow on a variety of aspects and avoid steep terrain with evidence of wind loading on Sunday.
Summary
Detailed Forecast
Light snow showers may resume at higher elevations along the east slopes on Sunday with light to moderate west winds. A slight warming trend should be seen by Sunday afternoon.
A broad brush will be used and all aspects will be indicated for wind slab on Sunday. The greatest danger along the east slopes should be in the Northeast zone where the most transport and greatest snowpack sensitivity was seen on Friday. But watch for firmer wind transported snow on all aspects in all the east slope zones due to recent varied winds especially above treeline.
Light snow showers on Sunday may further bury intact a variety of weak surface snow types formed this week under fair cold weather. This is likely to become a weak layer starting Monday.
The December 8th layer continues to round and gain strength and should be less sensitive to trigger. Head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence of this layer such as from snowpits.
Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.
Note that the first in a series of storms is expected to begin to move across the Northwest Sunday night. This will bring a significantly increasing avalanche danger by Monday.
Snowpack Discussion
Weather and Snowpack
The last storm cycle to affect most of the Washington Cascades was from Thursday, December 8th to Monday morning, December 12th. About 1-3 feet of snow were recorded at NWAC and other stations along the east slopes ending Monday morning.
Fair and cold weather this week has been interspersed with periods of moderate or strong NE-E winds both Wednesday and Friday. These winds were strong enough to build fresh wind slabs on more atypical SW-NW facing terrain, mainly near and especially above tree line.
There has been about 4-8 inches of snowpack settlement since the storm cycle ended, allowing underlying or storm snow related weak layers to stabilize.
Light to moderate west winds are seen at NWAC station along the east slopes on Saturday such as Washington Pass and Mission Ridge and along the crest such as Crystal Mountain and White Pass. These winds may give some transport to mainly NE to SE aspects.
Several clear nights with light winds this week have also allowed for weak surface snow development. On wind and sun sheltered terrain, extensive near surface faceted snow or surface hoar has formed this week and this may be an important weak layer if buried intact by the expected return of light snowfall Saturday and Sunday.
A thin sun crust is likely to have formed on steeper southerly facing slopes, which also may be a future interface for avalanches when loaded with sufficient wind transported or future storm snow.
Recent Observations
A regional avalanche cycle was seen December 8th-11th with some avalanches releasing on the December 8th persistent weak layer in the Washington Cascades.
The pro-patrol at Mission Ridge on Monday gave a report of a generally shallow, but stable snowpack, also noting spatial variation and a sensitive snowpack on local steep wind loaded N-E slopes.
More critically, on Tuesday there was a skier triggered slab avalanche release on the December 8th Persist weak layer in the Highland Bowl back country next to the Steven Pass ski area. The Stevens pro-patrol reports it was on a steep SSW slope at 5400 feet with a 40 cm x 100 foot crown releasing in buried hoar frost and facets on a crust. If this layer is still possibly active at Stevens Pass it is still possible active along the east slopes.
A member of the Stevens Pass pro-patrol reported wind transport on Rock Mountain and especially in the Chiwaukums on Wednesday.
A report for Washington Pass for Thursday is well stated on our recent observations page!
NWAC pro-observer Tom Curtis found weak faceted snow above the ground in a still fairly shallow overall snowpack in the Blewett Pass area Thursday. No observed avalanches were noted, but snowpack tests indicate the potential. See a recent NWAC instagram post about the results of a PST test.
On Friday the NCMG at Harts Pass had strong N-NW winds causing wind transport, shooting cracks and wind slab layer collapsing.
Also on Friday the NCMG had strong E-NE winds causing major snow transport seen in the Washington Pass area and off all major peaks south to Bonanza Peak. But near Rainy Pass a generally right side up snowpack was found with a good bond was seen at the December 8th interface.
Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 18th, 2016 10:00AM
