Avalanche Forecast
Issued: Feb 7th, 2017 9:49AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
An approaching storm Wednesday will begin to load and stress the deep recent storm snow by Wednesday afternoon. Watch for changing conditions and be prepared to alter tour plans and leave plenty of safety margin. Travel with deep snow safety precautions in place.
Summary
Detailed Forecast
An approaching storm will bring increasing clouds and strengthening winds through the morning Wednesday. Snowfall will arrive midday Wednesday and increase through the afternoon with warming.
The warming and beginning of the new snow loading Wednesday afternoon, will begin to stress the significant recent storm snow. The avalanche danger will gradually increase through the afternoon Wednesday, however the significant loading and heavier precipitation rates are likely to hold off until Wednesday night and into Thursday.
If the approaching storm arrives sooner than forecast or with, initially heavy precipitation, the avalanche danger could increase more rapidly than expected. Watch for changing conditions and be prepared to alter plans accordingly.
Note that deep snow conditions now exist in most terrain. Travel with a partner and use good communication and keep your partner in view at all times.
Snowpack Discussion
Weather and Snowpack
A strong storm cycle began Friday with 3-4 feet of snow recorded along the west slopes on average, through Monday afternoon. A slow warming trend affected all areas Saturday with rain reaching 4000-4500 feet in the south Cascades. Cool easterly flow kept temperatures locally cool at Pass level at Stevens and Snoqualmie Pass through the day Saturday, with a warm-up Saturday midday following a switch to westerly flow. This created a weak crust layer that has now been found both at Stevens and Snoqualmie passes, down about 3 feet.
Sensitive but generally shallow storm slabs were reported throughout the west slopes on Saturday from ski area control work and several backcountry observations. Intense precipitation rates Sunday morning became light to moderate during the day. Continued moderate to occasionally heavy snow showers fell through the day Monday at cool temperatures. The cooling trend Monday and settlement rates during the previous warming should all help the storm snow to become less sensitive to trigger.
A nice day Tuesday with light wind and sun or filtered sun and high clouds allowed for further snowpack settlement and stabilization of the deep recent storm snow. Most NWAC snowdepth gages settled about 6 inches or more during the day Tuesday.
Recent Observations
North
On Tuesday, Mt. Baker pro-patrol reported ski triggered soft slab avalanches of 8-10 inches on average, though they were not propagating widely as the older deep storm snow was showing signs of settlement and stabilization. There was evidence of a widespread natural cycle, likely Monday night on Table Mountain, with many crowns estimated in the 2-4 ft range.
NWAC's Lee Lazzara was up the Glacier Creek area off Hwy 542 Tuesday, reporting numerous triggered storm snow soft slabs, on average 40 cm (16") on steep test slopes on both southerly and northerly facing slopes. Another triggered slide was evident from a nearby party that may have been a bit deeper, about 2 feet.
Central
Reports from both Snoqualmie and Stevens passes Tuesday indicated continued storm snow settlement and stabilization, in the near and below treeline terrain. A thin melt freeze crust formed midday Saturday, 2/4 produced positive snowpit results but no avalanches were produced. The storm snow is gaining strength, showing increasing resistance with depth and providing some excellent conditions.
South
NWAC pro-observer Dallas Glass was in the Crystal backcountry in Bullion Basin Sunday and found a reactive storm layer 35-40 cm down in snowpit tests on all aspects below and near treeline. Shallower storm slabs could be triggered on test slopes. Storm snow from this cycle had settled to about 60-65 cm. No natural avalanches were observed and recent wind transport was localized to near the ridgeline.
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: All elevations.
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
Valid until: Feb 8th, 2017 9:49AM
