Avalanche Forecast
Issued: Feb 17th, 2016 10:11AM
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
The avalanche danger should increase Thursday afternoon and evening due to building wind and storm slab. A cooling trend Wednesday night and Thursday should limit the loose wet problem to below treeline but wet snow hazards will continue. Change your travel plans accordingly if conditions deteriorate more quickly than forecast.
Summary
Detailed Forecast
An occluded front pushing through Wednesday night should be followed by light showers on Thursday morning and a cooling trend. A surface low tracking inland near the Oregon/Washington border mid-day Thursday should provide a burst of moderate snowfall in the afternoon and evening hours for the southern Washington Cascades, followed by strong W-SW winds. Lighter snowfall is expected further areas further north during the daylight hours.
New wind slab should build on lee slopes near and above treeline Thursday afternoon and evening. Watch for new firmer wind transported snow mainly on lee N to SE slopes near ridges.
Storm snow with a cooling trend Wednesday night and Thursday should generally bond well to surface snow comprising of wet grains or various crust layers. Heavier precipitation rates Thursday afternoon and evening should increase the storm slab likelihood especially for the southern Washington Cascades.
Despite the cooling trend, loose wet avalanches will remain possible below treeline Thursday especially on steeper slopes. Watch for wet surface snow deeper than a few inches that usually precede loose wet avalanches or other triggered or natural loose wet avalanches.
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. Look for glide cracks in steep terrain to offer clues of the slab undergoing downhill creep.
Snowpack Discussion
Weather and Snowpack
Dry weather with the warmest temperatures of the winter occurred February 7-10th with mild temperatures and high freezing levels.
A pair of warm fronts passing through Thursday, Feb 11th through Saturday, Feb 13th brought a wide range of snowfall with the biggest storm totals of a foot to 18 inches at higher elevations stations and Mt. Baker and Paradise.
Rain and mild temperatures dominated along the west slopes Sunday and Monday with snow mixed in at the beginning and end of the event. NWAC sites in the near and below treeline recorded 2-6+ inches of mostly rain over the 2 days ending early Tuesday morning. The Paradise station recorded over 5 inches of precipitation (mostly rain) in the 24 hours ending Monday 4am!
Mild temperatures with spotty light rain were seen on Tuesday and Wednesday.
The upper snowpack should consist of a mix of crusts and wet grains with no deeper instabilities of note.
Recent Observations
Reports from the Mt Baker pro-patrol Saturday and Sunday indicated the 19 inches of storm snow was reluctant to move during avalanche control. A few isolated wind slab pockets were noted along ridges otherwise the warming Sunday afternoon was causing a few loose wet slides on steep slopes. Reports from Stevens Pass Sunday indicated the 8-10 inches of storm snow became sensitive to ski triggers by Sunday late morning. These slides behaved as loose-wet avalanches as surface snow become increasingly wet.
NWAC pro-observer Dallas Glass was out near Mt Baker Monday and reported several recent large loose wet avalanches on steep north slopes near and below treeline. He also reported a large glide avalanche on a similar slope. Dallas was out again in the same area Tuesday and reported a widespread cycle of loose wet and wet slab avalanches occurred Monday night during heavy rain and snow on most aspects in the 4300-5000 foot range. A 1-2 inch surface crust by this morning had improved surface snow stability.
NWAC pro-observer Tom Curtis was near Stevens Pass Tuesday and reported many recent loose wet avalanches mainly on north to east slopes at about 5000 feet. A pit had water pooling above a crust at 35 cm with weak wet snow below the crust and an PST test indicating propagation below the crust. He also noted glide cracks on nearby slopes.
Mt. Baker and Alpental pro-patrols continue to report glide cracks and glide avalanches on steep rock faces in their areas due the continued mild temperatures and frequent rainfall.
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, 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.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Feb 18th, 2016 10:11AM
