Avalanche Forecast
Issued: Mar 19th, 2016 9:28AM
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 Sunday.
Summary
Detailed Forecast
Cloudy Sunday with periods of mostly light rain or snow showers and mild conditions are expected. Rain and snow amounts should be relatively light and not significantly affect the current danger.
Clouds should allow the temperatures to remain above freezing Saturday night and with the addition of light rain near and below treeline, small loose wet avalanches may be possible on Sunday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep terrain.
Moderate winds near ridgetop may combine with light precipitation to build new areas of shallow wind slab on lee aspects in the upper elevations above treeline. Any new wind slabs will mostly likely be found in NW-NE facing slopes. East winds over the past few days will also have built wind slab on westerly aspects in places along the Cascade crest and in the Cascade passes. Hence the wind slab avalanche problem will be indicated on a variety of aspects near and above treeline.
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.
Although it won't be listed as an avalanche problem avoid areas below steep slopes or rock faces where unusual glide avalanches can release unexpectedly. See the recent observations above for a good example in the Mt Baker backcountry.
Snowpack Discussion
Weather and Snowpack
An active weather pattern in early March has seen several strong storms since March 10th. Cool and showery weather Monday and Tuesday with prolonged westerly winds deposited additional snow at lower snow levels. Storm totals from Thursday 3/10 through Tuesday morning 3/15 were 2 - 3.5 feet!
A combination of sun and mild temperatures over the past three days have allowed for the 2-3 plus feet of storm snow received since March 10th to settle and stabilize while forming a strengthening surface crust on most slopes. The stabilizing snowpack has been verified by pro-observers and snow safety personnel over the past few days.
Recent storms also built large cornices along ridges in many areas with some recent natural releases.
While fair weather since Thursday of this week has allowed for earlier storm layers to settle and stabilize, strong east winds near the Cascade crest transported available loose surface snow onto many exposed westerly facing slopes, building new wind slab layers as of Friday. .
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 Thursday 3/17 and reported good stability in the below and near treeline with little softening of surface crusts on solar slopes and good skiing on non-solar slopes. Plumes of snow from the high summits of Shuksan and Baker indicated lots of wind transport in the above tree line and high elevations.
A skier on Turns All Year reported east winds and a small triggered wind slab below treeline in the Tatoosh on Thursday 3/17.
Dallas Glass was in the Kendall Peak area of Snoqualmie Pass Wednesday 3/16. He found density differences persisting in the layering of recent storm snow, however, propagation was unlikely and that overall instabilities had settled out within the 60 cm of storm snow over a 3/12 rain crust. As the sun poked out midday Wednesday, generally small loose wet avalanches were observed in the steep terrain of Kendall Peak on S-SW aspects.
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: Treeline, 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 20th, 2016 9:28AM
