Avalanche Forecast
Issued: Jan 24th, 2018 1:17PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will persist Thursday, especially in terrain receiving wind transported snow. Avoid steep open slopes. Give safe margins near and below growing cornices. Storm snow is very deep! Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times. Storm layers will take extra time to stabilize.
Summary
Detailed Forecast
Moderate rain and snow late Wednesday should taper to light to moderate showers with a cooling trend overnight Wednesday and Thursday.
Increasing crest level winds are expected Wednesday night and Thursday.
The avalanche danger will lower slightly Thursday, but it will take time for these deep storm slab layers to stabilize, mainly at lower elevations with little wind exposure. Near and above treeline, strong winds should continue to build wind slabs along lee slopes below ridges and exposed open terrain.
Dangerous avalanche conditions will persist due to the likelihood for triggering a large and potentially deadly avalanche. Recent storm and wind slabs will require time to heal, so be patient and match your terrain selection appropriately, namely very conservative.
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 7 days, 3 to 4+ ft of storm snow has fallen over a strong rain crust buried 1/16. Mt Baker has been the exception with over 8 ft of storm snow received in the past week!
Warming early Wednesday, along with heavy snowfall and strong winds led to a natural avalanche cycle. Slab avalanches of 1-2 ft were common.
Dense storm slabs built quickly early Wednesday, forming over weaker storm snow.
Below the recent 2 ft of storm snow, 3-4 ft or more of gradually settling snow sits above the most recent melt-freeze crust - it is very deep out there!
Poor visibility and frequent storm conditions have limited observations above treeline during this period.
Observations
Central
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.
Natural storm slab release on sheltered slope below treeline, Wednesday, 1/24. NWAC observer, Ian Nicholson
On Tuesday, NWAC avalanche forecasters in the Skyline ridge area (near treeline and below) reported 3' of snow received during this storm cycle sitting above and well bonded to the 1/16 crust. Higher intensity snowfall began creating shallow storm slabs in the afternoon, with cracking observed generally in the new shallow snow, but occasionally breaking down to an older storm instability about 1' deep. East winds were transporting snow. A professional in the Smithbrook area also reported increasing wind transported snow near treeline and one skier triggered soft slab avalanche on a south aspect near 5000'.
On Tuesday, an avalanche professional in the Alpental Valley/Source Lake area observed slabs becoming reactive 10" down by early afternoon and still building. Moderate to strong winds were transporting new snow throughout the day.
South
Professionals in the Crystal area Monday reported 1 to nearly 2 ft of snow above the 1/16 crust in wind sheltered terrain generally well bonded. In areas stripped of snow during Sunday's storm, less than 8" sits above the most recent crust and is poorly bonded. Loose dry avalanches were the biggest avalanche problem encountered Monday, found in steep terrain at higher elevations and easily ski triggered with the potential to run long distances.
Problems
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
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
Valid until: Jan 25th, 2018 1:17PM
