Avalanche Forecast
Issued: Feb 6th, 2016 10:23AM
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
Lingering wind slab on lee slopes near and above treeline may be tougher to trigger, but still possible on Sunday. Wet loose avalanche potential will vary considerably through elevation bands and location, largely impacted by how the weather forecast plays out. Slabs overlying surface hoar buried on February 3rd in the Mission Ridge area may still be sensitive, so conservative decision making will be essential until you know the extent of new persistent weak layers in your area.
Summary
Detailed Forecast
A warm front lifting north across the area should bring light rain and clouds to the northeast Cascades and filtered sunshine to the southeast Cascades on Sunday. A warming trend will begin Sunday, but will likely be tempered by moderate winds above treeline, light but cool easterly flow near the Passes and extensive cloud cover for the northern 2/3rds of the Cascades. However, there is a decent amount of uncertainty for Sunday's forecast.
Lingering wind slab on lee slopes in the near and above treeline elevation bands may be tougher to trigger, but still possible Sunday. Watch for signs of snowpack cracking and firmer wind transported snow on lee slopes.
Wet loose avalanche potential will vary considerably through elevation bands and location, largely impacted by how the weather forecast plays out Sunday. Light precipitation in the north and sunbreaks in the south will encourage natural wet loose activity on steeper slopes predominately near and below treeline. Be aware of loose wet avalanche potential above terrain traps (like above cliffs or near gullies), where even small wet avalanches can become powerful and have unintended consequences.
Slabs overlying surface hoar buried on February 3rd may still be sensitive particularly in the Mission Ridge area. We don't have enough information to list a persistent slab avalanche problem for the centraleast Cascades, but 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.
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.
A quick but powerful front crossed the Cascades Friday night. New snow amounts were light, 2-6 inches with the most in the NE Cascades, but westerly transport winds were strong.
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 Friday. 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
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 7th, 2016 10:23AM
