Avalanche Forecast
Issued: Dec 19th, 2016 10:31AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
A natural avalanche cycle should occur late Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Generally shallow slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday!
Summary
Detailed Forecast
Strong westerly flow will carry another strong frontal system across the Northwest Monday night and Tuesday morning. This will cause strong winds with moderate to heavy snow near the Cascade crest with light to moderate snowfall further east for locations like Mission Ridge and Blewett Pass. A warming trend late Monday night through Tuesday morning should contribute to the formation of both new wind slab and new storm slab layers. A sharp cooling trend will take place beginning mid-day Tuesday with light to occasionally moderate snow showers continuing through the day in continued strong westerly flow. See the NWAC mountain weather forecast for more details on precipitation intensity, precipitation types and temperature trends.
New wind slab is very likely along the east slopes on Tuesday. Continued wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects due to strong gusts. Also, wind slab may develop below treeline due to the persistence of strong westerly winds.
Generally shallow new storm slab is also very likely along the east slopes on Tuesday on any slopes that rapidly accumulate new snow. Loose wet avalanches will not be listed along the east slopes but are possible in the southeast Cascades especially below treeline due to a change from snow to rain early Tuesday morning.
The December 8th layer is still a question mark along the east slopes. The upcoming storm cycle should be a good test to see if it is still reactive. Head for lower angle terrain if you see collapsing or whoomping or evidence of a buried surface hoar or buried faceted snow and crust layer.
A natural avalanche cycle should occur late Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday, especially near and above treeline!
Snowpack Discussion
Weather and Snowpack
The last storm cycle to affect the Olympics and most of the Washington Cascades was about Thursday, December 8th to Monday morning, December 12th. About 1-3 feet of snowfall was seen at NWAC stations along the east slopes.
There has been a lot of snowpack settlement since the last storm cycle ended, allowing this underlying snow to mostly stabilize.
Clear or fair and cold weather has been seen about Wednesday, December 14th to Sunday, December 18th . This caused widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts have also formed on steeper solar slopes. These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall.
Recent Observations
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.
On Monday the Mission Ridge pro-patrol reported widespread and sensitive storm and wind slab building well below ridges on lee slopes below treeline.
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: North, North East, East, South East, South.
Elevations: All elevations.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
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.
Aspects: All aspects.
Elevations: All elevations.
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 20th, 2016 10:31AM
