Avalanche Forecast
Issued: Dec 25th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
You can trigger recent or freshly forming shallow wind slabs above treeline near ridgelines. Watch for signs of recent or active snow transport, or snow surfaces cracking as you move. While we think that storm slabs associated with a massive loading event late last week have healed, wisdom dictates safe travel practices, particularly on large avalanche slopes.
Summary
Discussion
We expect one more day of generally benign weather with winds starting to pick up ahead of a storm approaching from the NW, later in the day.
Reports indicated 8-15” ski penetration of right-side-up powder on all aspects on Tuesday. This remained preserved on sheltered northerly aspects Wednesday following wind transport which occurred Tuesday night. Sunshine early Tuesday and partial sunshine on Wednesday created thin crusts on southerly aspects. Wind Slabs near ridgelines or steep terrain where a small Loose Dry avalanches could push you into unintended places are the identifiable avalanche concerns at this time.
The West North zone received 3-4ft+ of snow (7.5in water) from December 18-21. The event produced a widespread avalanche cycle with numerous natural and triggered avalanches to size D3 occurring Friday and Saturday. The storm snow has now settled 11” with a slowing rate of settlement indicative of a stabilizing snowpack. Some Loose Wet avalanche activity occurred earlier in the week as reported to NWAC here. Tuesday and Wednesday, folks have been recreating in Bagley Lakes and on Shuksan arm, testing a variety of steeper terrain features without signs of recent avalanche activity. This has given us some confidence to take storm slabs off the problem list.
On 12/24, Mt. Baker Pro Patrol evaluated several nicely rounding weaker layers near the base of the snowpack on a NE aspect at 5080 ft and found these layers were not reactive in tests. It’s worth keeping these early season weaknesses in the back of your mind given the recent loading event, but we don’t have evidence that they are of concern without a major loading event.
Happy Holidays! We extend our gratitude in advance for submitting observations to NWAC.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Problems
Wind Slabs
We expect shallow recent and fresh wind slabs mostly above treeline on Thursday. E winds transported and drifted snow near treeline on Tuesday night and SSW winds should increase Wednesday afternoon and may re-distribute more snow. There is plenty of dry snow to move around, so expect wind slab formation if average winds are more than 10 mph in your area. Watch for signs of wind-scoured ridgelines, recent cornice formation, or filled in tracks to indicate winds are moving snow in your area. Such snow transport tells you to avoid wind-loaded slopes and seek sheltered or lower-angle terrain.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 26th, 2019 10:00AM
