Avalanche Forecast
Issued: Mar 15th, 2016 12:33PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Cornices and Loose Wet.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions should persist above treeline, where recent wind slabs gradually stabilize. Conservative decision making will be essential Wednesday, especially in higher exposed terrain, as recent wind slabs gradually settle and stabilize. Avoid steep open slopes showing signs of recent wind transport. Best to confine travel to lower angled terrain away from wind effects.
Summary
Detailed Forecast
A few scattered showers are possible, but little accumulation is expected Wednesday. Temperatures should remain relatively cool, but expect increasing sun breaks to allow for daytime warming to have its effect on the snowpack.
Light to moderate westerly winds at ridge level should persist Wednesday, possibly redistributing available loose surface snow.
Recent wind slabs will be the main avalanche problem.
New wind slab is mostly likely to be found on NW-SE facing slopes. Firm wind transported snow and snowpack cracking can be good signs of wind slab instabilities.
Increasing sun breaks may development wet surface snow as the sun is gaining power so watch for the development of loose wet snow on solar slopes during extended sun breaks.
The avalanche danger should continue to gradually decrease Wednesday with less wind and slow settlement and stabilization of new snow and fairly cool temperatures.
Snowpack Discussion
Weather and Snowpack
Storms have 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 with the first passing last Thursday and the next on Sunday. These systems caused very stormy weather.
Cool showery weather Monday and Tuesday with continuing moderate westerly winds have deposited additional snow in showers at lower temperatures.
New storm amounts along the west slope sites have mostly been in the 10-18 inch range each of the past two days with 2-3.5 feet of storm snow since Thursday!
Periods of strong to very strong winds during recent storms created widespread wind and storm slab problems, creating dangerous avalanche conditions. These recent storms have also built large cornices along ridges in many areas.
Daytime warming and settlement have allowed storm-related weak layers to quickly settle and stabilize, therefore, storm slabs have been removed from the avalanche problem list. The quickly stabilizing storm slabs have been verified by many field observers and practitioners over the past two days.
The solid mid and lower snowpack along the west slopes remains a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
A park ranger early Monday morning 3/14, assessing avalanche conditions for plow drivers to Paradise, found sensitive storm slabs with cracks extending from ski tips and one slab releasing remotely. 24 hr storm snow was 15 inches and recent storm snow 30 inches down to the crust formed after rain Tuesday night, 3/9.
Storm slab sensitivity Monday morning 3/14, near Paradise, Mt Rainier, near tree line. Photos: NPS Peter Ellis
By Tuesday morning in the same local that park ranger found those storm-related weak layers unreactive, despite an additional 18 inches of new snow! The loading itself increased the settlement rate of those delicate storm weaknesses and by Tuesday morning, the deep new snow was not producing avalanches in the same terrain as Monday.
NWAC observer Dallas Glass in the Crystal Mountain backcountry Monday 3/14, found surprisingly good stability within the deep storm snow. However, these observations were limited to below tree line and were confined to slopes less affected by recent strong winds. In general, 24 hr new snow depths were 10-12", with previous storm totals of about 2 feet down to the 3/9 rain crust. Storm snow exhibited a favorable density profile. A cornice dropped onto a north facing slope produced no avalanche below while a subsequent cornice drop on a west aspect pulled out a moderate storm slab.
NWAC observer, Lee Lazzara was in the Mt Baker backcountry today 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 triggering a 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.
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
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
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: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 16th, 2016 12:33PM
