Avalanche Forecast
Issued: Mar 2nd, 2016 10:00AM
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
Conservative terrain selection will be essential near and above treeline as human triggered avalanches are still likely. Choose moderately angled terrain and avoid wind loaded slopes, saving those steeper lines for another day.
Summary
Detailed Forecast
A period of moderate rain and snow at rising freezing levels is expected Wednesday night along with strong S-SW ridgetop winds. Showers should taper quickly Thursday morning. A modest cooling trend is expected by early Thursday, however, the new storm related danger will continue through Thursday due to strong southwest winds loading lee slopes during showers Wednesday night.
Storm slab instabilities should be stabilizing, but may still be sensitive Thursday. Wind slabs will continue to build Wednesday night on lee slopes, facing generally NW-NE. Earlier formed wind slabs may still be reactive Thursday and may add to the increasingly complex snow structure, by Thursday.
Conservative terrain selection will be essential near and above treeline to avoid wind loaded terrain and features. Choose moderately angled terrain and avoid wind loaded slopes, saving those steeper lines for another day.
Non-avalanche hazard: Despite our seasonally healthy snowpack, many creeks are open and difficult to cross due to the periodic warm temperatures and rain events.
Snowpack Discussion
Weather and Snowpack
Very mild weather a week ago allowed for the formation of a strong melt-freeze crust, especially thick on solar slopes in most areas by Friday.
A Pacific frontal system blew through the Cascades mid-day Sunday. The most significant transport winds were from the W-SW and seen early Sunday afternoon with gusts in the 40s or 50s below or near treeline with much stronger winds above treeline. By Monday morning, 4-12 inches of new snow had accumulated along the west slopes accompanied by a cooling trend.
A warm front quickly dropped up to 10 inches of new snow early Tuesday morning at NWAC stations before snow either eased off mid-morning or changed to rain outside the Cascade Passes. Precipitation rates increased again in the afternoon as the cold front approached. In the Mt. Baker area, precipitation rates increased later in the morning with the snow level rising above 5000 ft during the day. This caused a natural avalanche cycle late Tuesday in many areas, evidenced by many field observers Wednesday morning.
Additional precipitation late Tuesday through Wednesday morning with cooling deposited an additional 6-15 inches of snow. Surface snow layers are a mix of varying density dry or moist snow all well bonded to a forming rain crust or old moist to wet snow.
The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
NWAC observer Tom Curtis was out on Jove Peak Monday. Jove Peak is NE of Stevens Pass and often overlaps snowpack characteristics of both the east and west slopes of the Cascades. Tom noted few instabilities in the upper snowpack until he observed a large natural wind slab avalanche (R2-D2) on the north side of Jove Peak that probably released Sunday night. The wind slab was 40 cm in depth and stepped down to a 2 mm buried surface hoar layer at 50 cm. The buried surface hoar was rounding, but clearly still sensitive to new loading.
A stormy day on Tuesday limited observations, but one observation submitted via the NWAC observation page from behind Windy Ridge near Stevens Pass reported increasing instability and sensitivity to triggering with a large soft slab avalanche on a WNW aspect running well and entraining moist snow lower in the path.
By Wednesday sunshine and daytime warming caused numerous natural point release, Loose-Wet avalanches on steep solar aspects seen in the Alpental Valley and the Mt Baker backcountry.
Control work at Alpental produced widespread wind slab releases in the upper elevations where wind transport was more significant.
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: North, North East, East, South East, North West.
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
Valid until: Mar 3rd, 2016 10:00AM
