Avalanche Forecast
Issued: Feb 5th, 2017 11:03AM
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
New snow instabilities will need more time to settle out so make conservative terrain choices by choosing lower angled terrain and avoiding wind loaded slopes near and above treeline. While deeper storm slab instabilities should be healing, it is still possible to trigger a large storm slab avalanche in specific areas.
Summary
Detailed Forecast
We are going to keep the snow tap turned on for one more day with additional light to moderate snowfall Sunday night followed by a sharp cooling trend and light to moderate showers on Monday.
Moderate snow is expected for the central Cascades Sunday night and combined with a slight warming trend, this may cause a natural avalanche cycle in this zone overnight. Lighter snowfall is expected in the north Cascades with cooler temperatures. Storm slab sensitivity may peak Sunday night particularly in the central Cascades, but will continue to be touchy Monday with additional loading during intense showers. While deeper storm slab instabilities should be healing, it is still possible to trigger a large storm slab avalanche in specific areas.
There is some uncertainty to how much wind transport will occur due to a period of easterly winds in the central Cascades and northeast winds in the north Cascades Sunday night. Moderate west winds on Monday will begin to redistribute snow to more easterly aspects during the day Monday. All aspects will be listed.
Be aware that small loose dry avalanches triggered on steep slopes can entrain deeper snow layers.
New snow instabilities will need more time to settle out so make conservative terrain choices by choosing lower angled terrain and avoiding wind loaded slopes near and above treeline.
Snowpack Discussion
Weather and Snowpack
Strong NE-E winds were seen this past Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.
A storm cycle began Friday with generally 2-3 feet of snow pummeling the west slopes through Sunday afternoon. A slow warming trend affected all areas Saturday with rain reaching 4000-4500 feet in the south Cascades. Easterly flow kept temperatures locally cooler at Pass level at Stevens and Snoqualmie Pass through the day Saturday but a slight warm-up occurred Saturday night with a switch to westerly flow. Sensitive but generally shallow storm slabs were reported throughout the west slopes on Saturday from ski area control work and some backcountry observations. Intense precipitation rates Sunday morning became light to moderate during the day.
Recent Observations
North
Mt. Baker pro-patrol produced large storm slabs of 24-36" (60-90 cm) during morning control work and at least one natural storm slab avalanche was observed outside the ski area.
Central
Stevens Pass ski area and DOT professionals reported very touchy storm slabs during control work Sunday morning. Some slabs broke into deeper storm layers from earlier in the weekend with up to size D2 or D2.5 slides observed. Loose dry avalanches in steeper terrain started small but quickly gouged down and entrained additional storm snow. Pro-observer Ian Nicholson was at Yodelin just east of Stevens on Sunday and found soft storm slabs propagating around 50 cm down in ECTs and easily ski triggered on short test slopes. Collapses within the storm snow produced whumpfing while breaking trail.
Alpental pro-patrol found easily triggered storm slabs in and outside of their ski area with skier triggered storm slabs up to 50 cm. The storm slab in the photo below broke about 15-20 cm above the 1/29 rain crust in the Snoqualmie area.
Skier triggered storm slab photo from John Stimberis 2-5-17, Denny Mt
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
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: Feb 6th, 2017 11:03AM
