Avalanche Forecast
Issued: Jan 29th, 2018 11:50AM
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
Heavy snowfall and strong winds will create touchy wind and storm slabs near and above treeline. The new loading may also reactivate very large and dangerous avalanches above treeline, which may be triggered by cornice-fall. Conservative route-finding will be essential to avoid wind-loaded slopes, steeper terrain, and terrain connected to higher-elevation avalanche start zones.
Summary
Detailed Forecast
A relatively brief (6 hour) period of heavy rain and snow in the above treeline band will be changing to snow showers later Monday night and will continue on Tuesday. Strong SW winds should decrease slightly on Tuesday. The colder temperatures will allow wet snow to begin to refreeze.
Fresh winds with the new snow will form small to large wind slabs near and above treeline, which will be touchy on Tuesday. Avoid wind-loaded terrain as it won't take much new snow to redistribute the snow to form a large slab. Give potentially wind-loaded terrain near ridges a wide margin for error on Tuesday.
Older wind slabs which which were triggered by large explosives over the past weekend are likely very stubborn to trigger, but may still be reactive to cornice or other large triggers on Tuesday. A facet/crust combo exists and this layer has produced very large avalanches, so avoid travel on or connected to a large avalanche path Tuesday. While these avalanches may be unlikely, they will be unsurvivable.
Large storm slabs forming Monday night in the above treeline band will likely be sensitive to human triggering on Tuesday. Avoid steeper terrain (>30 degrees) and convex or unsupported slopes at higher elevations. At lower elevations where a rain crust will be refreezing, expect storm slabs to be possible if heavier snowfall rates are seen within the shower activity. Make more conservative terrain choices where new snow exceeds 8" on top of the recent crust.
Snowpack Discussion
A relative break in the action was seen from late morning Sunday through Monday after a long and stormy period which has impacted the Mt Hood area since January 17th leaving 30 inches of settled storm snow over the most recent crust layer. Saturday and Sunday featured more intense precipitation, significant warming temperatures, and strong winds created very dangerous avalanche conditions and natural avalanches in many areas.
The break in the weather allowed good visibility of multiple very large artillery-triggered wind slabs which entrained lots of wet snow and even in one case knocked down timber.
Rain on Saturday morning was followed by cooling, creating a thin, breakable crust in some areas.
Winds throughout this active weather pattern redistributed snow forming deep and firm wind slabs on lee slopes near and above treeline.
Observations
Mt Hood Meadows Pro Patrol Sunday reported multiple very large artillery-released avalanches with very deep crowns up to 12 ft range, running on steep NE and E-NE terrain above treeline and travelling very long distances, in one case snapping 20+ year old trees. Snow pits in the 5000 and 7000 ft range identified 1 mm rounding, wet facets on top of a 1/18 freezing rain crust as the likely weak-layer/bed surface for these very large slides. Given the large amount of recent wind transport, the depth of this layer varied widely.
A Mt. Hood Meadows Patroller provides scale for the sidewall of one of these very large explosive-triggered avalanches on Mt. Hood.
Mt Hood Meadows Patrol Saturday reported sensitive storm slabs which were entraining snow as they ran. A thin breakable rain crust formed in the morning to 5500 feet. By mid-day additional snow was falling at most elevations.
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: All aspects.
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: Jan 30th, 2018 11:50AM
