Avalanche Forecast
Issued: Mar 9th, 2017 12:35PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Due to the potential for very large avalanches in specific areas following Thursday's complex storm, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and stay out of consequential terrain.
Summary
Detailed Forecast
Dense snow received at higher elevations in the southwest Cascades and in the Cascade Passes should be followed by a period of rain Thursday evening and then by light showers with a slow cooling trend during the day on Friday. Dense snowfall followed by moderate showers with the same slow cooling trend is expected in the northwest Cascades through mid-day Friday. W-SW winds will increase Thursday night and stay strong through Friday morning.
With a complex weather pattern afflicting the Cascades Thursday and Thursday night and stressing our deep storm snow received over the last week, travel in avalanche terrain is not recommended on Friday. Allow the recently stressed snowpack time to settle and avoid consequential terrain. Realize that very large avalanches are possible if initially small avalanches step down to deep and lingering storm instabilities or if a large natural trigger like a cornice collapse occurs.
Deep wind slab should be suspected on all aspects near and above treeline but most likely found on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.
Sensitive storm slab is mostly likely in the northwest zone and in the Cascades Passes for elevations and areas that do not see rain Thursday night. Very large storm slabs are not likely to be skier triggered but are listed in the problem set due to the step down potential.
Loose wet avalanches are likeliest in the northwest Cascade zone due to periods of moderate rain forecast to affect the below treeline band on Friday. However, large wet slab avalanches will be possible in isolated areas throughout the west slopes of the Cascades that received significant rainfall Thursday night.
Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time. Cornices have been reported as large and in charge in many areas. They will have likely been weakened during the recent storm cycle becoming more likely to fail.
Snowpack Discussion
Weather and Snowpack
The most recent warm, wet storm arrived on Valentines Day forming a strong rain crust, now buried several feet or more in most areas along the west slope Cascade zones. This crust may be a bed surface for very large avalanches during the current storm cycle.
Strong southwest flow carried a strong front across the Northwest on Friday evening March 3rd. Along the Cascade west slopes this caused strong southwest alpine winds, heavy, moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft. A region wide avalanche cycle was seen late Friday 3/3 and in certain areas on Saturday 3/4.
The first week or so of March has been remarkably snowy; NWAC stations along the Cascade west slopes have had about 3 to 8 ft of snow with the most at Mt Baker! A weak low pressure system brought another 6-10 inches of snow to the west slopes Wednesday afternoon and night.
A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. As of 6 pm Thursday, rain had pushed up to at least 6000 feet in the southwest Cascades. Easterly flow was still allowing wet and heavy snowfall at Stevens and Snoqualmie Passes, where 4-6 inches had accumulated since late morning. Mt. Baker's snow level had risen to about 4000 feet at the end of the day Thursday with another 6 inches of snow accumulating along with occasionally gusty winds.
Recent Observations
North
On Wednesday morning, Mt. Baker pro-patrol saw mostly loose dry avalanches running within the new 8" they had picked up Tuesday night. These results represent the ski area's mitigated terrain only.
Lee Lazzara traveled in non-consequential terrain in the Mt. Baker area on Thursday. He found dense wind slab quickly building and becoming sensitive to skier triggering on small test slopes. He also found several lingering storm slab instabilities within the past week's storm snow. While no one particular deeper layer was likely to be skier triggered, they represent the ability for initially shallow slab avalanches to step down and become deeper avalanches.
Central
WSDOT, Alpental ski area and NWAC Pro-observers in the Snoqualmie Pass area reported signs of a widespread natural cycle that had occurred Tuesday afternoon and evening, with D1-1.5 storm slabs featured on all aspects. NWAC pro-observers Ian and Matt found storm layers beginning to settle and not indicating likely propagation in column tests. The March 3rd melt-freeze crust was thin and down 70-85 cm in the below treeline band. Matt observed shallow wind slab on NW-N aspects near treeline. Small loose dry avalanches ran fast on steep rollovers below treeline but didn't entrain much recent snow.
Early Wednesday morning, WSDOT Stevens Pass easily released wind and storm slabs during control work involving Tuesday/Tuesday night's storm snow.
WSDOT Snoqualmie DOT reported a widespread natural cycle that occurred Wednesday night as denser snow layered on top of lighter snow received earlier in the storm. Additional heavy wet snow accumulating during the day on Thursday increased the avalanche hazard for both Snoqualmie Pass and Stevens Pass and resulted in Pass closures.
South
Crystal pro-patrol experienced touchy avalanche conditions during Tuesday afternoon's increased snowfall and warming but only limited results during control work Wednesday morning. These results represent the ski area's mitigated terrain only.
Crystal pro-patrol reported little avalanche activity mid-day but that winds and dense snowfall were beginning to increase.
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
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
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Mar 10th, 2017 12:35PM
