Avalanche Forecast
Issued: Feb 16th, 2018 11:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Travel in avalanche terrain is not recommended. Heavy snowfall and wind will create very dangerous avalanche conditions. Stay far away from slopes steeper than 30 degrees and out from under any terrain where avalanches could come from above.
Summary
Detailed Forecast
If you travel in the backcountry today, you must be certain of your ability to avoid all avalanche terrain. Put a wide buffer of terrain between where you travel and any slopes over 30 degrees. Use extra caution in areas where avalanches could come down from above. Avalanches may be surprisingly large and run farther than you expect.
Watch out for areas where the wind has recently drifted snow. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a Wind Slab avalanche. You can trigger Storm Slab avalanches on steep slopes where you find at least 1 foot of recent snow resting on a slick crust. Storm Slabs could break near the surface or involve up to 4 feet of recent snow resting on a slick crust.
Snowpack Discussion
Natural and triggered avalanches were reported Wednesday through Friday from around the West Slopes. On Friday a party was involved in an avalanche in the Snoqualmie Pass backcountry. Skiers and riders triggered numerous slab avalanches in the Bagley Lakes area of the Mt Baker Backcountry. These were all about 1 foot deep and ran in the recent storm snow. Dangerous conditions limited access and observations.
As of late Friday snowfall since the 13th has added up to about 2 feet of snow on the ground. Rain fell up to 3000 feet late Friday. In many areas observers have reported weak sugar-like facets adjacent to a hard, slick crust formed in early February. This combination has been a slick surface and not allowed the recent snow to bond well. You can find these layers 2-4 feet below the snow surface.
On Thursday observers reported a cycle of natural shallow slab avalanches at Snoqualmie Pass. Meanwhile, Loose Dry avalanches were easily triggered in steep terrain on Stevens Pass. These involved snow that fell since February 13th. Backcountry travelers triggered or were caught in avalanches at both Snoqualmie and Stevens Passes.
Observations
On Friday, an NWAC observer in the Mt Baker backcountry reported numerous small triggered slab avalanches in steep terrain near and below treeline. One of these was close to 300 feet wide.
NWAC staff in the field Wednesday and Thursday in the Stevens and Crystal Mt backcountry areas found loose surface snow conditions resulting in numerous loose dry skier triggered avalanches. Wind transported snow was seen to be isolated to exposed features near and above treeline.
At Snoqualmie Pass, NWAC staff and Alpental ski patrol reported natural and triggered slab avalanches in recent snow Wednesday through Friday. On Friday, observer Jeremy Allen reported snowpack tests indicating propagation (Propagation Saw Test) on a crust that was buried February 8th. Dennis D'amico was able to trigger a small storm slab on a steep E-NE test slope at 3900 ft, which ran on the old snow interface 8" down. Matt Schonwald reported many small slab avalanches and about 4 inches of heavy snow on the surface resting on softer recent snow.
On Friday, Forecaster Dallas Glass toured near Paradise on Mt Rainier. He reported 2.5 feet of recent snow on a crust and fresh wind drifts up to 3 feet deep. While no avalanches were reported cracks were observed in recently drifted snow.
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: All aspects.
Elevations: All elevations.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 17th, 2018 11:10AM
