Avalanche Forecast
Issued: Jan 31st, 2018 2:43PM
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
Wind slabs have been slow to heal due to cold temperatures, and may be obscured by more recent lower density snow. Look for wind slabs near and above treeline, but in some areas recent winds have distributed snow well below treeline. Use caution while approaching steep wind loaded open slopes and watch for signs of active wind transport. A rising snowline through the day will allow for generally small loose wet avalanches in steep terrain below treeline.
Summary
Detailed Forecast
Scattered light snow showers in the morning should become light rain and snow by Thursday afternoon. Snow levels will slightly rise through the daylight hours and further overnight as precipitation and winds increase markedly.
Wind slabs have been slow to heal due to cold temperatures, and may be obscured by more recent lower density snow. Look for wind slabs near and above treeline, but in some areas recent winds have distributed snow well below treeline. Use caution while approaching steep wind loaded open slopes at any elevation and watch for signs of active wind transport.
Storm slabs are trending toward unlikely but may still be possible Thursday with continuous light loading in showers and a general warming trend throughout the day.
Cornices are large and should be given a wide berth as they often break back further than you expect.
A rising snowline through the day will allow for generally small loose wet avalanches in steep terrain below treeline, although loose wet avalanches could become large in isolated areas where they entrain more recent snowfall. Be aware of your exposure to terrain traps on steeper slopes especially below 4500 feet in the afternoon.
Snowpack Discussion
Light snow showers, continued cool temperatures and decreasing ridgetop winds were seen Wednesday. Heavier snow showers and stronger winds occurred Tuesday. Stevens and Snoqualmie Passes saw 6-10 inches during this post-frontal cool down while 1-2 feet accumulated at Mt. Baker, Paradise, Crystal and White Pass. The new snow is generally well bonded to the 1/29 crust but recent winds have easily been transporting the lower density snowfall. A significant skier triggered wind slab avalanche occurred in the Stevens Pass area Wednesday, see details below.
Above treeline, deep snow still sits on a strong crust layer from 1/16. The 1/29 crust is not expected to be present at these elevations. Recent poor visibility and frequent storms have limited recent observations above treeline over the last 2 weeks.
On Sunday and Monday, warm and wet weather followed a 10 day stretch of cool and snowy weather that generally featured deep and good riding conditions. Mt. Baker saw the most snow during this stretch along the west slopes of the Cascades, followed by the most rain, and as a result ended up with some of the larger natural avalanches reported during the transition.
Observations
North
On Tuesday, Mt. Baker pro-patrol reported 17" of storm snow well bonded to the recent wet and slowly refreezing snow below. Avalanche control results were minimal.
On Monday, Mt. Baker pro-patrol observed debris from several large natural avalanches that had run off of Shuskan Arm. Avalanche control work with large explosives resulted in several large avalanches with 2-3' crowns in area.
Central
On Wednesday, an avalanche professional (not ski patrol) was caught in a large (est D2.5) skier triggered wind slab avalanche in East Highland Bowl at Stevens Pass around 4800 ft in steep east facing terrain. The crown was 1-2 ft deep, broke wide and caught and carried two skiers already downslope. Both skiers were carried an estimated 100 feet into trees, partially buried but were able to self-extricate. Thankfully they sustained only minor injuries.
NWAC Pro Observer Ian Nicholson was in the Alpental Valley Wednesday. Ian found recent wind distribution of the most recent 8" of snow to be highly variable above the 1/29 crust. In some areas near treeline, the snow surface was scoured to the crust alternating with 2-3 ft wind deposits nearby. Recent wind transport patterns were difficult to pinpoint where recent low density snow had fallen. In non wind affected terrain the most recent storm snow was generally well bonded to the 1/29 crust.
South
NWAC forecaster Dallas Glass was in the Crystal backcountry Wednesday and found 15" of recent storm snow generally well bonded to the 1/29 crust. Recent wind transported snow was noted near and just below ridgelines but winds were light on Wednesday. Cornices have grown large on all exposed ridgelines. Small skier triggered and natural slab avalanches were noted in steep terrain.
NWAC Pro Observer Jeremy Allyn was in the Crystal backcountry Tuesday. He found 12-16" of wind-redistributed storm snow in the near and below treeline terrain with evidence of winds cross-loading paths. Wind-loaded pockets were very deep (>3') above 6000 ft. He found multiple storm snow interfaces, with notably reactive layer down 16". Storm slabs were possible to trigger in steep terrain.
On Tuesday NPS Rangers reported 6-20+ inches of wind-stiffened snow with moderate westerly winds depositing snow on east-facing aspects. The firm 1/29 crust was 4" thick and extended from near the Paradise area up to 7000 ft.
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: Feb 1st, 2018 2:43PM
