Avalanche Forecast
Issued: Mar 25th, 2016 11:16AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
While recent storm related avalanche problems will have greatly diminished by Saturday, isolated areas of wind slab may remain on lee N through SE slopes near and above treeline. Also, loose wet avalanches will be likely on steeper solar slopes, especially during prolonged periods of sunshine.
Summary
Detailed Forecast
Variable cloud cover on Saturday will be accompanied by generally light winds and rising freezing levels. An incoming frontal system arriving Saturday night should spread high clouds over the area in the afternoon.
Recent storm related avalanche problems will have greatly diminished by Saturday. Isolated areas of wind slab may remain on lee N through SE slopes near and above treeline, but should generally be stubborn to human triggering. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Loose wet avalanches will be likely on steeper solar slopes, especially during prolonged periods of sunshine. Be aware of terrain traps where even a small loose avalanche could have unintended consequences in the wrong terrain.
Recently formed cornices have grown large. If traveling along ridgelines, be aware that cornices break much further back than expected and you don't want to go for a ride with a chunk of cornice. Despite the recent cool weather, cornice releases can be unpredictable during the spring.
Although not listed as an avalanche problem, glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.
Snowpack Discussion
Weather and Snowpack
A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the west slopes accumulated a few inches above 4000 feet during this time. Light rain was seen on Wednesday in the below treeline band before a cold front swept through Wednesday night with moderate westerly transport winds continuing through Thursday afternoon for the south and central Washington Cascades. Orographic snow showers continued through Thursday night before tapering off Friday morning.
Post-frontal snow accumulations Wednesday night through Friday morning varied based on elevation, with only a few inches below 4000 feet and 6-21 inches above. The most snow was received at Paradise and Mt. Baker NWAC stations through Friday morning. New snow since Monday is generally well-bonded to a moist crust buried March 22nd.
Frequent March storms have built large cornices along ridges in many areas.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
Snoqualmie Pass avalanche professionals reported loose wet avalanches triggered by light rain running far on Wednesday in paths below treeline. Glide avalanches releasing to the ground off of rock surfaces were also reported near the same Snoqualmie Pass paths Thursday morning. NWAC professional observer Dallas Glass was in Kendall Peak area of Snoqualmie Pass Thursday and found small but sensitive wind slabs near treeline. Dallas also found loose snow concerns transitioning to shallow storm slab problem by mid-day with settlement and daytime warming.
The Mt. Baker pro-patrol reported only scattered and small soft storm slabs in area Thursday morning. Stevens Pass pro-patrol reported strong west winds had redistributed the new snow, but that the new wind slab was stubborn and generally nonreactive to ski cuts.
On Friday, Baker pro-patrol reported natural slab activity above treeline on Mt. Shuskan but only small loose wet releases in area and a generally right side up upper snowpack. A professional guide observation from the Snoqualmie Pass area Friday morning indicated generally well layered and bonded storm snow with increasing potential for loose wet avalanches by late morning.
Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below. These avalanches are not predictable in the sense that they are not tied to short term warming or rain events. We want to highlight the low likelihood-high consequence of this sporadic but ongoing avalanche hazard during this wet winter.
Problems
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.
Aspects: East, South East, South, South West, West.
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: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 26th, 2016 11:16AM
