Avalanche Forecast
Issued: Mar 17th, 2016 11:36AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Cornices.
NWAC,
Northwest Avalanche Center
A mixed bag of spring avalanche problems Friday will keep heightened avalanche conditions in specific terrain. You can mitigate the loose wet and cornice risk by being aware of the terrain you are connected with and by recognizing the rapidly increasing hazard in locations that warm quickly. Firm wind transported snow and snowpack cracking that might be found on any aspect can be good signs of wind slab layers.
Summary
Detailed Forecast
Sun on Friday morning should give way to some high clouds by Friday afternoon with warmer temperatures. Strong east winds are likely in places along the Cascade crest especially over lower parts of the crest and in the Cascade passes.
Stronger mid-March sun should make loose wet avalanches possible on steeper solar aspects on Friday after quickly breaking down sun or melt-freeze crusts. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in specific terrain. Watch for rapidly increasing hazard in locations that warm quickly.
Northwest winds on Wednesday likely continued to build wind slab on lee aspects in the upper elevations of the above treeline band. This wind slab will mostly likely be found on N-SE facing slopes.
But east winds Thursday and Friday will also possibly build new wind slab on west aspects in places along the crest especially over lower parts of the Cascade crest and in the Cascade passes.
So recent and new wind slab will be indicated on most aspects in the above and near treeline on Friday. Wind slab may also be locally possible in the below treeline on west slopes in areas that have east winds on Friday. 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 when it is warm and sunny. 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
Storms moved across the Northwest at a nearly daily frequency the past few weeks with fluctuating but generally moderate snow levels. Two deep surface low-pressure systems moved across the Olympic Peninsula last week with the first passing last Thursday, March 10th and the next on Sunday, March 13th. These systems caused very stormy weather.
Cool and showery weather Monday and Tuesday with prolonged westerly winds deposited additional snow at lower snow levels. Storm totals from Thursday, March 10th through Tuesday morning were between 2 - 3.5 feet!
Daytime warming and settlement have allowed recent storm-related weak layers to gradually settle and mostly stabilize. The stabilizing has been verified by pro-observers and snow safety personnel over the past few days.
Storms have also built large cornices along ridges in many areas with some recent releases.
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
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. 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.
NWAC pro-observer, Lee Lazzara was in the Mt Baker backcountry Tuesday near Table Mt and Mt Herman. The main takeaway was that wind slabs were still sensitive to ski release on steep open slopes, with a skier triggered size D1 wind slab on a steep NE facing slope. The mid-March solar radiation was quickly settling storm slab instabilities and causing 5-10 cm on wet surface snow on solar aspects, producing several small loose-wet slides.
Pro-observer Dallas Glass was in the Kendall Peak area of Snoqualmie Pass Wednesday. He found that while density differences persisted in the layering of recent storm snow, propagation was unlikely and that overall instabilities had settled out within the 60 cm of storm snow over the 3/12 rain crust. As the sun poked out mid-day Wednesday, generally small loose wet avalanches were observed in steep terrain of Kendall Peak on S-SW aspects.
Lee 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 wind transport at the highest elevations.
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, South West, West, North West.
Elevations: Alpine, 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 18th, 2016 11:36AM
