Avalanche Forecast
Issued: Jan 25th, 2018 10:30PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will persist on Friday due to additional light snowfall, an uptick in wind transported snow, and cold temperatures preserving recent storm instabilities. Avoid steep open slopes connected to large avalanche paths until we exit this storm cycle. Give safe margins near and below growing cornices. Loose dry avalanches will run fast in steep terrain with plenty of loose snow available, avoid steep lines with terrain traps. Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times.
Summary
Detailed Forecast
Dangerous avalanche conditions will persist on Friday due to additional light snowfall, an uptick in wind transported snow near and below ridges, and cold temperatures preserving recent storm instabilities. Due to lower density surface snow available for transport and moderate W-SW winds forecast, wind slabs may develop below treeline or further downslope than you might expect Friday. Avoid travel on slopes if you encounter wind stiffened surface snow, especially on steeper exposed terrain and steep roll-overs.
The potential for large and destructive avalanches remains in bigger terrain. Avoid travel on or below slopes connected to large avalanche paths until we exit this storm cycle. Give safe margins near and below growing cornices. There's plenty of good snow out there, so maintain a conservative approach to terrain selection.
Loose dry avalanches will run fast in steep terrain with plenty of loose snow available, avoid steep lines with terrain traps.
Significantly transformed snow conditions have taken place during this extended storm cycle. Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times.
Snowpack Discussion
An extended storm cycle continues. Over the past 8 days, 2.5 to 5 ft of storm snow has fallen over a strong rain crust buried 1/16. Mt Baker has been the exception with roughly 9 ft of storm snow received in the past week! Cool and generally light showers were seen on Thursday with light to moderate winds transporting snow near and below ridgelines from Snoqualmie Pass and southward. Recent storm snow instabilities were healing, but a large skier triggered slab avalanche was reported in the closed Alpental ski area Thursday.
Warming early Wednesday along with heavy snowfall and strong winds led to a natural avalanche cycle in some areas. Slab avalanches of 1-2 ft were common.
Poor visibility and frequent storm conditions have limited observations above treeline during this period.
Observations
Central
On Thursday morning, NWAC staff in the Alpental valley observed debris from a large to very large natural avalanche that released from Chair Peak and ran down to Source Lake. The avalanche most likely occurred within the previous 24 hours. Professionals at the closed Alpental ski area reported a large skier triggered slab avalanche 20" deep releasing in the Powder Bowl area. Fresh large cornices have built along ridgelines in this area.
On Wednesday, NWAC professional observer Jeff Ward traveled in wind sheltered terrain below treeline, just east of Stevens Pass, finding 2-2.5 ft of storm snow. In this terrain with little to no wind effect, the storm snow had a favorable density profile of gradually increasing density with depth. No reactivity was found in tests within the gradually settling storm snow.
On Wednesday, NWAC Pro observer Ian Nicholson traveled in the Kendall Trees near Snoqualmie Pass. Warming early Wednesday formed sensitive storm slabs with several natural storm slabs releasing about 12-16" on a storm density change (see instagram post).
South
NWAC forecasters in the Crystal backcountry on Thursday reported 1 to 2 feet of recent storm snow well bonded to the 1/16 crust. The new snow generally had a right-side up profile with recent storm instabilities healing, with the exception of areas of poor bonding noted on windward slopes with a shallower snowpack. Occasional gusty winds transported recent snow in the Crystal area near and below ridges but wind slab was found to be generally unreactive in the areas observed.
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 Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 26th, 2018 10:30PM
