Avalanche Forecast
Issued: Feb 5th, 2016 10:15AM
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
New wind and storm slab layers should be seen on Saturday. These layers could be especially sensitive and remotely triggered where they further build over surface hoar buried on February 3rd. Conservative decision making will be essential until you know the extent of the new potential buried surface hoar layer in your area.
Summary
Detailed Forecast
Strong southwest winds aloft will accompany a cold front will cross the Northwest on Friday evening. West winds will follow with orographic snow showers and a good cooling trend on Saturday morning.
The cooling trend may help bond new snow to old snow surfaces where the snow starts at above freezing temperatures. With a little luck many areas along the east slopes will have several inches or better of new snow by the time snow showers taper off on Saturday.
The main avalanche problem should be new wind slab on lee slopes in the near and above treeline. Watch for signs of snowpack cracking and firmer wind transported snow on lee slopes.
A secondary avalanche problem should be new storm slab in areas with less wind if there is rapid loading. The cooling trend may help limit this avalanche problem.
These layers could be especially sensitive and remotely triggered where they further build over surface hoar buried on February 3rd. Conservative decision making will be essential until you know the extent of the new potential buried surface hoar layer in your area.
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 throughout the Cascades.
A warm front about January 21st caused up to about a foot of snow in the northeast zone. Freezing rain or rain created a crust in the central east and southeast zones.
Another wet and warm front was seen Jan 27-28th. This brought up to another 1.5 ft of snow in the northeast zone and likely formed another crust in the central and southeast zones.
A strong occluded front with strong winds crossed the Northwest January 29-30th. NWAC stations along the east slopes had about 4-14 inches of new snow.
Cool, benign weather followed Sunday through Tuesday. A sun crust formed on many solar slopes and surface hoar was seen on many non-solar slopes.
A cold front and then a warm front crossed the Northwest on Wednesday and Thursday. NWAC stations along the east slopes for the 2 days ending Friday morning had about 2-10 inches of snowfall.
Recent Observations
The January 3rd and 11th layers were reactive with widespread triggering occurring during the late January storm cycles. But the latest observations indicate that these layers have gradually become non-reactive and the persistent slab problem has been removed from the east slope zones. You can still check for these layers in snow pits in your specific area of travel.
NWAC pro-observer Tom Curtis observed healthy surface hoar growth in the Blewett Pass area on all aspects Monday and in the Icicle Creek drainage near and above 4500 feet Tuesday. Increased cloud cover in the Washington Pass zone may have limited surface hoar development in that area.
Tom Curtis was out again at Jove Peak on Thursday and found snow transported to E-SE slopes and some sensitivity in a storm layer at about 25 cm but a surface slab was lacking.
The North Cascade Mountain Guides generally report good snow conditions and and little avalanche activity the past few days.
The likely most significant observation comes from an avalanche class touring in the 4-7000 foot range near Mission Ridge today. The group was remotely triggering small to large 20-45 cm wind slab on lee N-SE slopes due to surface hoar layer buried on February 3rd. The layer was also found at about 10 cm in non-wind affected areas.
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
Valid until: Feb 6th, 2016 10:15AM
