Avalanche Forecast
Issued: Mar 2nd, 2017 11:11AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Very dangerous avalanche conditions are expected to develop especially near and above treeline Friday due to an incoming storm system. Travel in avalanche terrain is not recommended due to the increasing hazard.
Summary
Detailed Forecast
Several waves of warm frontal moisture will impact the west slopes of the Cascades Thursday night through mid-day Friday. A strong cold front will sweep through the area in the afternoon. The warming trend should peak early afternoon for the northwest Cascades and later afternoon for the central and southwest Cascades as the front drops southeast. Rain and snow will the the heaviest for the Mt. Baker area during the day with more light to moderate amounts seen further south until the front comes through.
Rain will creep into the below treeline band early Friday morning for the southwest Cascades and for the central Cascade Passes by late morning and mid-day as weak easterly flow weakens. Moderate to strong west-southwest winds will increase Thursday night and stay elevated through Friday evening.
Very dangerous avalanche conditions are expected to develop especially near and above treeline Friday due to an incoming storm system. Travel in avalanche terrain is not recommended due to the increasing hazard.
Storm slab should build and become more reactive during the day due to the warming trend. Storm slabs are expected to be the largest in the Mt. Baker area due to higher snowfall rates and total accumulation. While most storm slabs avalanches should fail between new storm layers, be aware of deeper storm instabilities from earlier in the week especially in the Snoqualmie Pass area where entrainment of deeper layers is possible.
Wind slabs will build on lee easterly aspects near and above treeline, but expect cross-loading and wind slab formation into the upper portion of the below treeline band due to the strength and duration of the winds.
Generally shallow loose wet avalanches are likely below treeline in the central and southwest Cascades with a switch to rain at lower elevations on Friday.
Snowpack Discussion
Weather and Snowpack
The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost, very strong rain crust in the snowpack.
A series of disturbances in cool, NW flow aloft from Saturday through Tuesday caused periods of moderate to strong SW-W alpine winds and deposited 1.5 - 3.5 feet of snow along the west slopes of the Cascades. Very strong west winds decreased on Wednesday following the last front.
Light warm frontal moisture produced a few inches of new snow from Snoqualmie Pass and north to Mt. Baker during the daylight hours on Thursday. There were several inches of settlement over the last 24 hours with near freezing temperatures seen at most sites. Precipitation was beginning to spread south late Thursday afternoon.
Recent Observations
North
NWAC observers Lee Lazarra and Simon Trautman along with a couple NWAC forecasters toured the Mt. Baker backcountry Tuesday ahead of an incoming frontal system. Surface snow conditions remained excellent with no real avalanche problems encountered beside small loose dry avalanches in steep terrain. Low cohesion snow was well bonded to the Valentine's crust and continued to provide excellent tour conditions.
The Mt Baker patrol on Wednesday reported areas of 6-8" storm slab triggered by explosives and with skis.
Central
Heavy snowfall intensities Monday afternoon and evening in convergence at Snoqualmie Pass caused an extensive natural cycle of 10-12 inch deep storm slab avalanches.
Dallas Glass toured on Mt Snoqualmie Tuesday and noted the remnants of the storm slabs that formed late Monday during heavy snowfall. These storm slabs were easily triggered with skis and released within the most recent storm snow. However, a weak layer was found at a thin melt-freeze crust about 8 inches above the rain crust and in deep tests showed propagation was likely. In the Snoqualmie Pass area 3-4 feet of snow sits above the Valentine's Day rain crust.
A couple reports from the Stevens Pass area via the NWAC Observations page for Tuesday indicate minor wind slab development near ridge tops and small ski triggered wind slab, storm slab and loose dry avalanches. The Valentine's Day crust was seen down 60 cm.
The Alpental patrol on Wednesday morning reported some 8-10" storm slab by ski tests. By afternoon wind affected snow on the upper mountain and sun affected snow on the lower mountain were producing poor ski conditions.
On Thursday, pro-observer Jeremy Allyn was in the Alpental Valley. Jeremy found storm snow instabilities still present and slowly stabilizing. Thin wind slab was forming in the upper portion of the below treeline band and showed signs of increasing sensitivity.
South
A NPS ranger at Mt Rainier reported increasing winds Tuesday evening. Surface snow was becoming wind stiffened and beginning to fail as slabs even on shallower angled terrain.
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
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Mar 3rd, 2017 11:11AM
