Avalanche Forecast
Issued: Mar 20th, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
A mixture of typical spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Monday.
Summary
Detailed Forecast
Cooler temperatures, cloudy and showery weather is expected Monday. This weather should maintain areas of wet and weak snow below about 4-5000 feet in the Below Treeline elevation band. Moderate to strong winds at the higher elevations may build new shallow wind slabs on some leeward exposed ridges, mainly NW-E facing. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Recently formed cornices have grown large and can become sensitive during mild weather, especially following nights with above freezing temperatures. Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring. Cooler temperatures Monday may help temporarily strengthen cornices Monday.
Although not listed as an avalanche problem, unusual glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces. See the recent observations above for a good example in the Mt Baker backcountry.
Snowpack Discussion
Weather and Snowpack
A combination of sun and mild temperatures for several days gave way to mild temperatures and light rain Sunday, March 20th. The recent weather over the past five days has allowed for the 2 to nearly 4 feet of storm snow that accumulated from about March 9-15 to settle and stabilize. Wind, sun, warmth, freezing and now rain have drastically changed snow conditions over the past five days. Surface snow conditions have become highly variable, ranging from some well settled old snow on steep shaded slopes and a mix of surface crusts, wind buffed surfaces or shallow wet snow on other aspects.
Earlier avalanche problems of wind and storm slab have very likely settled and stabilized over the past several days of mild weather. Most sites have seen the snowpack settle 10-20 inches over the past five days, since the strong storms ended March 15th. The stabilizing snowpack has been verified by numerous pro-observers and snow safety personnel over the past few days.
Recent storms did build large cornices along ridges in many areas with some recent natural releases noted.
While the recent break in storms allowed for earlier storm layers to settle and stabilize, a period of strong east winds near the Cascade crest Thursday and Friday transported available loose surface snow onto many exposed westerly facing slopes, building new wind slab layers as of Friday. These slabs have been gaining strength Saturday and Sunday and are now much more resistant to trigger.
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
Pro-observer Dallas Glass was above Paradise, Mt Rainier Saturday 3/19 and found recent wind slabs on many exposed westerly facing slopes formed by strong east winds Thursday and Friday. These wind slabs in this area were generally 8-10 inches thick. Previous wind slabs formed on more typical N-E facing terrain during the storms a week ago were no longer evident, having stabilized. Several small loose wet avalanches were seen on solar slopes. No recent cornice releases were seen in this area.
NWAC pro-observer, Lee Lazzara was back in the Mt Baker backcountry on Saturday 3/19 and reported good stability in the below and near treeline with most surfaces ranging from Pencil to 1 Finger hardness and about 15 cm (6 inches) of moist to wet surface snow on all aspects, leaving ski conditions less than desirable. Plumes of snow from the high summits of Shuksan and Baker over the past few days indicated there has been significant wind transport in the above tree line and high elevations at times.
A skier on Turns All Year reported east winds and a small triggered wind slab below treeline in the Tatoosh on Thursday 3/17.
The Mt. Baker pro-patrol reported yet another glide avalanche to ground (5-6 feet deep!) on a steep rock face around 3500 feet in the below treeline band off of Shuskan Arm on Tuesday 3/15. 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. Specifically, in the Mt. Baker backcountry, we want to highlight the low likelihood-high consequence of this sporadic, but ongoing avalanche hazard during this wet winter.
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 West, West, North West.
Elevations: Alpine, Treeline.
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
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 21st, 2016 11:00AM
