Avalanche Forecast
Issued: Feb 16th, 2016 10:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
A mixed bag of conditions seems possible on Wednesday with winter like snow conditions possible in the above treeline and wet snow conditions in the near and below treeline. Several types of avalanche problems will need to be watched for on Wednesday.
Summary
Detailed Forecast
An occluded front should rotate out of a low pressure system over the Northwest coastal waters late Wednesday. This should cause light rain or snow to spread to the Olympics and Cascades Wednesday afternoon. Further increasing rain or snow should be seen Wednesday evening especially on the volcanoes. Temperatures will remain mild on Wednesday.
A mixed bag of conditions should continue on Wednesday with winter like snow conditions possible in the above treeline and wet snow persisting near and below treeline.
This forecast will be conservative and list new wind slab as likely in the above tree line on Wednesday especially in the central east and northeast zones. Watch for new firmer wind transported snow mainly on lee N to SE slopes near ridges. You will need to be able to assess this problem for yourself if you travel above treeline.
You should continue to watch for loose wet avalanche conditions in the near and below treeline on Wednesday. Watch for wet surface snow deeper than a few inches that usually precede loose wet avalanches or other triggered or natural loose wet avalanches. Steep slopes near and below treeline are probably still best avoided.
Wet slab avalanches won't be listed as a problem but quickly change your plans if you see evidence of wet slab avalanches. You can check for significant liquid water in layers in snow pits.
Glide avalanches also won't be listed as a problem but avoid areas below steep rocks still holding snow since these slopes can release at unpredictable times.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat since several warm and wet systems have tested this layer and since it has been unreactive where it can still be identified in recent snowpit tests.
A warm and wet system late last month caused a rain crust to form in most areas and elevations along the east slopes on Jan 28th.
Dry weather with the warmest temperatures of the winter occurred February 7-10th with temperatures climbing into the 50's in most areas east of the crest. This caused loose wet avalanches and overall snowpack consolidation with a crust as temperatures cooled late last week.
A pair of warm fronts brought a wide range of new snow amounts east of the crest about Thursday to Saturday but areas at higher elevations had 6-12 inches with the most near Washington Pass.
Rain and and mild temperatures should have predominated along most of the east slopes Sunday and today. NWAC sites had rain up to a couple inches ending this morning. But about 8-14 inches of new snow fell at higher elevations in the central east and northeast zones at indicated by NRCS sites at Harts Pass and Lyman Lakes.
Recent Observations
A professional observation from the northeast zone Saturday indicated that storm snow of about a foot remained cold and provided good ski conditions near and above treeline and was well bonded to the recent melt-freeze crust. Areas of wind slabs formed on some lee slopes below ridges due to moderate west wind transporting loose surface snow Friday night.
The North Cascades Mountain Guides reported significant firm wind slab formation in the above tree line on Sunday. Ski cuts gave roller balls and hand tests in the recent storm snow gave easy shears in upside down storm snow. A crust buried Thursday February 11th was identified in a snow pit at 6000 feet at 25 cm but did not indicate propagation.
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.
Elevations: Alpine.
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
Valid until: Feb 17th, 2016 10:10AM
