Avalanche Forecast
Issued: Jan 18th, 2018 12:31PM
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
Human-triggered avalanches are likely on Friday. New snow and moderate SW winds will combine to create sensitive wind slabs near and above treeline. Both storm and wind slab will increase in size throughout the day. Avoid steep wind exposed terrain receiving wind transported snow.
Summary
Detailed Forecast
Cool temperatures, moderate snow showers and periods of moderate S-SW winds will continue to deposit new snow and further build existing wind and storm slab layers.
Note that a refreezing 1/17 rain crust will continue to harden and may provide a bed surface for wind and storm slab avalanches on Friday.
Building wind and storm slabs should maintain dangerous avalanche conditions near and above treeline. Storm slabs may form quickly and become sensitive to human triggering, especially in those areas receiving the greatest amounts of new snow (more than about 8" of new snow accumulated) and higher snowfall rates (extended periods of more than about 2" per hour).
Conservative terrain choices will be essential in higher and wind exposed terrain. Best to travel in wind sheltered, lower elevation terrain well away from steep open slopes until this new storm snow stabilizes.
Snowpack Discussion
New storm snow amounts varied according to elevations as rain gradually changed to snow Wednesday night. As of Thursday evening, storm snow since Wednesday below 5000' is about 4-6 ", near 6000' about 8-10' and above 6000' about 12-16".
Above treeline, greater new snow amounts and periods of very strong winds have caused dangerous avalanche conditions. New wind and storm slabs in higher exposed terrain will require careful terrain choices and conservative decision making.
Near and below treeline the precipitation began as rain and transitioned to snow. This has caused less dangerous conditions with new snow forming good bonding to the old rain soaked snowpack. However, heavy snowfall and moderate to strong winds have built increasing wind and storm slabs at progressively lower elevations. There are still dangerous avalanche conditions with new and increasingly deep wind and storm slabs of 1-2 ft or more as of late Thursday.
The recent high pressure created melt-freeze surface conditions followed by rain on Monday, which formed into a 2 inch thick crust at Mt. Hood Meadows, providing a good bed surface for recent snowfall. The shallow recent snow reported did not bond well to this bed surface.
Observations
On Tuesday, Mt. Hood Meadows Pro Patrol reported two natural 6-12" wind slabs on NE aspects between 7000 and 7300 ft.
Observations Friday from Mt Hood Meadows reported a supportable rain and freezing rain crust to 7000 feet. The ice crust prevented winds from transporting snow near and below treeline.
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: Jan 19th, 2018 12:31PM
