Avalanche Forecast
Issued: Jan 19th, 2018 10:35PM
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
Deep areas of wind and storm slabs should make triggering an avalanche in steep terrain possible or even likely. Avoid travel on steep slopes receiving wind deposited snow, where fresh wind slab deposits will be sensitive to human triggering. Expect dangerous avalanche conditions at higher elevations. Choose conservative terrain, by travelling in wind sheltered terrain well away from avalanche paths.
Summary
Detailed Forecast
A band of enhanced snow showers should arrive Friday night with diminishing showers Saturday. This should freshen storm snow with winds continuing to redistribute new cold snow to lee slopes by Saturday.
This weather should cause widely variable snow conditions with some wind exposed slopes scoured to recent crusts and lee slopes building deeper wind drifts and slabs. Watch for changes in snow conditions, particularly stiffer more dense snow, likely a wind or storm slab. Avoid travel on slopes if you encounter wind stiffened surface snow, especially on steeper exposed terrain and steep roll-overs.
The temperatures should remain cool, depositing new low density snow that will be easy for winds to re-distribute on exposed terrain.
Wind slabs near and above treeline remain the primary avalanche problem in this area, so remain conservative in your terrain choices especially as you venture into exposed terrain.
The best and safest snow will remain in sheltered terrain away from ridges.
Snowpack Discussion
New storm snow amounts since late Wednesday vary by elevation and wind exposure, ranging from being striped to old firm layers or up to 2 ft in higher sheltered terrain.
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 Friday.
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 Friday, NWAC observer Laura Green traveled to observe terrain near treeline. Recent storm snow had been highly re-distributed by strong winds. Sensitive wind slabs were found on exposed slopes, mainly N-E facing. Mostly shallow storm snow was found well bonded and not reactive to tests below treeline.
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 20th, 2018 10:35PM
