Avalanche Forecast
Regions: Cascades - North East.
Little change in the avalanche danger is expected Sunday with recent wind slabs primarily found on N-W-S aspects. However be alert for wind slab on all terrain aspects due to local loading patterns.
Detailed Forecast
The upper ridge will linger over the northeast Pacific Ocean through the weekend. High clouds should pass through the region Saturday night but mostly sunny skies are expected on Sunday except for low clouds along the lower east slopes. Temperatures should continue to moderate overall Sunday, but expect strong temperature inversions to continue.Â
Recent wind slabs should be found primarily on N-W-S aspects. The benign weather will continue to very slowly allow these layers to stabilize. 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.
Continue to identify the 12/17 buried PWL in snowpits and avoid areas where the overlying snowpack is shallower such that affecting this layer would be more likely.
Small loose dry avalanches are possible in steep wind sheltered terrain but will not be listed as an avalanche problem.Â
Snowpack Discussion
Weather and Snowpack
A pair of warm fronts moved across the Northwest last weekend which allowed some relatively milder Pacific air to finally work its way east of the crest Sunday evening and night. 3-8Â inches of snow fell through Monday morning, 1/9 along the east slopes.Â
A weak low pressure system, tracking across Oregon Tuesday morning through Wednesday afternoon produced 1-5 inches of snow in the central east to southeast Cascades with the most seen at the Lost Horse and Potato Hill Snotels in the southeast Cascade zone. E to NE winds increased Tuesday afternoon.
Snow showers deposited another 2-4 inches early Wednesday in the southeast Cascades zone, with little or no snow in the north and central.
An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday through Saturday over the Cascades with mostly light winds and moderating temperatures seen mainly at higher elevations along the east slopes due to strong temperature inversions. Low clouds over Eastern Washington stayed banked up against the east slopes Saturday while the mountains mostly enjoyed the sunshine.Â
Surface hoar and near surface faceting has been noted widely throughout the range in sun and wind sheltered locations below treeline. Sun crusts have formed on steeper solar aspects over the last few days. The best riding and skiing conditions have generally been reported in less wind and sun affected areas below treeline during this stretch of fair weather.
Recent Observations
NWAC pro-observer Tom Curtis was on Mt Cashmere Wednesday and pulled the plug on continuing a tour due to the conditions observed. The sound of a natural avalanche rang out loudly in the valley Wednesday morning. It was estimated the slide released from a more westerly aspect near tree line. Other concerning observations included finding a reactive buried thin crust with faceted crystals below and a 25-35 cm 4F slab above. Cracks were shooting from skis and snowpit tests also indicated human triggered slab releases would be likely in this area.
The NCMG were at Washington Pass on Wednesday and reported that wind slab from last weekend was primarily seen on E-SE terrain features and was becoming less reactive.
A couple reports via the NWAC Observations page are available for Wednesday. A report from Mt Cashmere indicated a shallow, variable snowpack with evidence of wind loading from last week. A collapse and snowpack cracking was seen on a southeast slope at 5300 feet due to a facet/crust layer at 35 cm down but these layers were not found to be widespread. A skier also reported 14-16 inches of snow and snowpack collapsing on Manastash Ridge on Wednesday.
Tom Curtis was on Diamond Head at Blewett Pass on Thursday reported many wind scoured areas with previous wind transport primarily to W slopes. The 12/17 PWL was found at 30 cm down but was not reactive. Facets at the base of the snowpack were not giving test results.
Tom Curtis was out again on Friday on Mt Cashmere and on a W aspect near treeline and he found the 12/17 PWL at 55 cm giving a PST 37/100 End. He noted some previous wind transport and cross loading on N-W-S aspects. Tom also found great skiing conditions in the trees.Â
Avalanche 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, South, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Unlikely
Expected Size: 1 - 1