Avalanche Forecast
Regions: Mt Hood.
Recent or new wind slab and new storm slab is expected to be the main avalanche problem on Friday. These layers could be more reactive where they build over a hard smooth crust.
Detailed Forecast
A weak front will move across the Northwest Cascades late Thursday. This should cause southwest winds and a cooling trend. Snowfall should be pretty light except with 5-10 inches looking likely in the Mt Baker area. By Friday a large digging trough offshore should cause much lighter winds and but with renewed snow mainly over the south Cascades with low snow levels.
Wind slab should be the primary problem Friday. Southwest winds in the last storm cycle and for the late Thursday system make this most likely on north to southeast slopes. Watch for firmer wind transported snow mainly north to southeast slopes. New or recent wind slab could also be more reactive where it builds over a hard smooth crust.
Several inches or more of new snow make new storm slab possible at Mt Hood on Friday. Watch for new storm slab in areas that experience rapidly accumulating snow deeper than a few inches. New storm slab could also be more reactive where it builds over a hard smooth crust.
The surface crust formed in some areas following the storms early this week is reported to be strong and hard enough to present an out of control fall danger. Avoid steep hard slopes where there will be fall consequences if you are not confident you can manage this problem by walking or using ski or boot crampons.
Snowpack Discussion
Weather and Snowpack
Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts formed on steeper solar slopes during sunny periods.
However PWLs likely have not survived at Mt. Hood due to the copious water amounts received over this latest storm cycle. Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night with generally 3 inches of water accumulating at Mt. Hood NWAC stations through early Tuesday morning.
Unfortunately, much of the heavy precipitation fell in liquid form with rain likely reaching up to about 7000 feet late Monday night and Tuesday morning.Â
 A sharp cooling trend followed mid-day Tuesday with about 2 inches of snow in post-frontal showers.  A strengthening rain crust was noted near and below treeline by late in the day Tuesday with the arrival of colder air. Â
Cold and clear weather overnight Tuesday and early Wednesday helped strengthen surface or near surface crusts up to at least 7000 feet.
Recent Observations
The Mt Hood Meadows pro-patrol on Tuesday morning reported that the snowpack BTL appeared to be handling the heavy rain remarkably well with little natural or human avalanche activity observed. However, by late Tuesday afternoon and after some partial clearing, debris from a very large wind slab avalanche was observed above treeline on a ENE aspect at 7500 feet. This R4.5 - D3 avalanche likely occurred Monday night and cleared out two start zones running roughly 1500 feet vertical. From a distance, the crown was estimated to be several feet thick!Â
Reports from the Mt Hood Meadows pro-patrol Wednesday, reported a significantly different snowpack following the rain, avalanches and cooling. A stout surface crust was found on all elevations up to at least 7200 feet. On exposed terrain, the crust was very supportable while in treed terrain the crust ranged from breakable to supportable.
A report via the NWAC Observations - Recent Observations tab for the Cooper Spur area on Wednesday indicated a thick surface crust that likely presented an out of control fall danger in the ATL. Local reactive wind slab to 1 foot deep was reported in the NTL.
Avalanche 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: Possible
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1