Avalanche Forecast
Issued: Feb 3rd, 2016 10:00AM
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
Wind slabs formed Wednesday, as well as expected continued wind transport Thursday, should maintain dangerous avalanche conditions, especially near and above treeline on a variety of aspects. Snow pits may still be a worthwhile effort to determine if persistent weak layers from January 3rd and 11th are still present or reactive in your local area.
Summary
Detailed Forecast
Light to occasional moderate snow showers are expected Wednesday night and early Thursday with moderate WSW ridgetop winds. Generally light expected snowfall amounts and gradual warming should allow for settlement of storm related weaknesses. However, winds may still be strong enough to transport loose surface snow, continuing to deposit wind slabs on lee aspects, more NE-E-SE facing.
New snow may have initially bonded poorly to a variety of snow surfaces Wednesday, including possible surface hoar or weak lower density snow on sheltered and shaded slopes, or a possible thin sun crust on southerly facing slopes. Travel cautiously and make route choice decisions conservatively Thursday.
Watch for signs of cracking or firmer wind transported snow.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods earlier this month 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 on Friday. NWAC stations along the east slopes had about 4-14 inches of new snow by Saturday morning. Cool weather and some sun have been seen along the east slopes Sunday through Tuesday.
Another Pacific front is arriving late Wednesday, beginning to deposit new shallow snow, potentially burying recently observed surface hoar.
Recent Observations
The January 3rd and 11th layers were reactive, with widespread triggering occurring during the late January storm cycles. However, 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. However, it's still important to locally track these layers in your specific area of travel.
NWAC pro-observer Tom Curtis was at Blewett Pass on Saturday and found remnants of the January 3rd and 11th layers but they were not reactive.
The January 3rd and 11th layers were seen at 60 cm and 50 cm in a test pit reported via the NWAC Observations page for Saturday for the Clara Lake area. The January 11th layer gave no result and the January 3rd layer gave a hard test for propagation.
A report on Turn All Year for the Clara Lake area indicated that the January 3rd and 11th layers are no longer reactive there. Extensive new surface hoar was noted.
The North Cascades Mountain Guides were on Driveway Butte near Mazama Sunday and also found the January 3rd layer unreactive.
NWAC pro-observer Jeff Ward was in the Chumstick Mountain area Monday. The January 3rd and 11th layers were found at about 75-85 cm and 70-80 cm and gave hard or no results in tests. Jeff noted isolated, stubborn to release small wind slab near ridges and new widespread surface hoar. Jeff was in the Stevens Pass area Tuesday. He generally found a stable upper snowpack, but observed new surface hoar growth most prominent in Highland Bowl near ridgecrest.
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 this area.
Surface hoar will have to be tracked following new snowfall being received Wednesday and Thursday.
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, North West.
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 4th, 2016 10:00AM
