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, Storm Slabs 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 and storm slabs will be the main avalanche problems Wednesday.
New wind slab is mostly likely to be found on N-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.
The persistent slab avalanche problem will not be listed in the northeast zone due to an abundance of other avalanche problems. This problem is most likely to be found on non-solar aspects in the above and near treeline band and stretching into the upper portion of the below treeline band. This interface is likely getting harder for a human to trigger but if triggered is capable of producing large avalanches. Be aware that at depths approaching the 1 m mark the extended column test becomes a less reliable indicator of propagation across a column. Deeper tests like the propagation saw test or deep tap test may help but layer identification and terrain selection are your best friends for managing a persistent slab danger.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain. A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.
Storms have moved across the Northwest almost every day or two the past couple 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.
Recent storm snow amounts along the east slope sites have varied but have been mostly in the 1.5-3.5 foot range since last Wednesday!
Periods of strong to very strong winds during recent storms created widespread wind and storm slab problems, creating dangerous avalanche conditions.
We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations.
Recent Observations
Reports from the Washington Pass area Tuesday 3/15 reported an active avalanche day with several storm slab avalanches triggered on NE aspects from 5600-6600 feet. One large size 3 avalanche was seen on an NNE aspect about 7200 feet, likely releasing by a cornice failure triggering a storm slab on the slope below.
The main avalanche problems related to recent wind slab and storm slab. The storm slab problem seems to be variable in this locale as a great deal of steep terrain was negotiated without producing avalanches. The storm slabs are balancing between becoming more sensitive with solar effects and those same solar effects allowing for storm weakness to settle and gain strength. Best to stay conservative at this point as there still remains great quality snow in lower consequence terrain.
NWAC observer Tom Curtis tested conditions in the Blewett Pass area Monday, 3/14 following the strong storm Sunday. Sensitive storm and wind slabs of up to 20 inches were found in open terrain, especially wind affected slopes. Easy failure in test pits was noted on a crust-facet layer buried beneath the current storm snow, about 16-20 inches. Failures were found with PST and ECT tests on this interface.
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
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
