Avalanche Forecast
Regions: Snoqualmie Pass.
Snow conditions and overall stability still vary a lot across the region so proceed with caution. The persistent slab concern is for Stevens and Snoqualmie and east of the crest.
Detailed Forecast
Fair weather should be seen Thursday but with continued very cold temperatures. Increasing mid or high clouds should be seen by Thursday afternoon. East winds should decrease over ridges and through the Cascade passes. The cold temperatures will maintain or increase any weaknesses and layers in the snowpack
The main concern will be new wind slab from the east winds on Wednesday. This should be most likely on west aspects near ridges and in the Cascade passes.
For those near Stevens and Snoqualmie continue to watch in the near and below tree-line zones for shooting cracks and whumpfing. These indicators of local instability even on low-angled terrain will clue you into instability that may extend into avalanche terrain. Note: The persistent slab concern for this area only applies to Stevens and Snoqualmie passes.
Small dry loose snow avalanches may also be seen or triggered on steep shaded slopes but should not be too dangerous and will not be listed as a concern.
Moderate avalanche danger specifically means that natural avalanches have become unlikely but human triggered avalanches are possible in specific areas and terrain features.
Snowpack Discussion
A storm finally arrived at the end of January with rising snow levels and rain for west slopes of the Cascades before transitioning to snow and colder temperatures. Snoqualmie and Stevens Pass saw some light freezing rain before transitioning to snow with the onset of heavier precipitation.  A switch to warmer westerly winds at the passes allowed for higher density snow or rain before the cooling trend set in. Storm totals were highest at NWAC stations near the passes and Paradise with 18-36 inches while Mt. Baker and Crystal saw closer to 6-12 inches.
The Mt Baker ski patrol/NWAC observers reported numerous skier triggered storm slab avalanches of 10-12 inches on the north slopes of Shuksan Arm on Friday and Saturday with the largest about 300 feet wide. Here the culprit was a surface hoar layer sitting on the January crust above the previous rain-line at 5500 feet.  This weak layer was not found on other nearby slopes of similar aspect and elevation showing a high degree of variability. Due to the relatively shallow and soft storm slab, there were no injuries or burials.
Slick bed surface is visible after skier triggered soft slab avalanche on Shuskan Arm 1 February, Photo by Jeff Hambelton
A hoar frost layer was also sandwiched and buried between crust layers at Stevens and Snoqualmie during the end of January storm. An extensive natural avalanche cycle was seen around Alpental on Thursday. NWAC observers at Stevens and Snoqualmie Passes reported shooting cracks and whumpfing on Thursday and Friday mainly in the below tree-line zones.
Surface hoar layer with thin rimed crust before it was buried near Stevens Pass 28 January, By NWAC Observers Jeff Ward and Dallas Glass.
Video made 30 January near Stevens Pass of tests of the buried surface hoar layer:Â https://www.youtube.com/watch?v=IsYF79R6Rio
Very little to several inches of additional very low density snow has accumulated in the Washington Cascades so far in February.
Several test profiles near Yodelin just east of Stevens Pass Tuesday found about 50 cm of settled powder over strengthening hoar frost and crust layers. No buried surface hoar was found in starting zone areas but was found at lower elevation areas. ECT results gave hard results with varied propagation which were slightly above the crust and hoar frost layers.
NWAC observer Dallas Glass at Snoqualmie today reported new small areas of wind slab to below tree line due to east winds. The hoar frost and crust are also still present on open north slopes but here are also not as reactive and less of a concern. He also reported the cold is faceting the surface snow and causing small dry loose avalanches on steep slopes.
Note: The persistent slab concern for this area only applies to Stevens and Snoqualmie passes.
The areas further south near Crystal and Mt. Rainier received more rain before a transition to snow that made the likelihood of weak layer survival slim. New snow has been reported as bonding well to the old surface by the Crystal patrol.
Sunny but cold weather is being seen on Wednesday. This will preserve or enhance existing weak layers in the upper snow pack. East winds may build new small areas of wind slab on lee west slopes near ridges and in the passes.
The mid and lower snowpack west of the crest consists mainly of crusts and melt form crystal layers which have generally not been reactive to to snow pit tests. Low snow amounts are limiting the avalanche danger on many south slopes and at low elevations due to terrain and vegetation anchors.
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: South West, West, North West.
Elevations: All elevations.
Likelihood: Likely
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1