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Avalanche Forecast

Archived

Jan 17th, 2018–Jan 18th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

A storm overnight will bring strong winds and new snow. The storm will cause a rapid increase in the avalanche danger overnight and through Thursday. Expect dangerous avalanche conditions.  Choose conservative terrain and avoid slopes exposed to wind affected snow.

Detailed Forecast

An arriving storm overnight Wednesday will bring strong winds and snowfall. The storm should begin with rain then transition to snow overnight and through Thursday with snowfall changing to showers Thursday with diminishing winds.

The incoming storm will build increasing wind and storm slabs over a thickening crust layer. The smooth crust layer may form a potential sliding layer, allowing any avalanche releases to run long distances as wind and storm slabs build.

Building wind and storm slabs should cause dangerous avalanche conditions near and above treeline by Thursday.

Monitor new snow and wind transport carefully and choose conservative terrain. 

Snowpack Discussion

Very mild temperatures Wednesday allowed for softening surface snow conditions. 

Small amounts of snow fell above 6400 feet on Monday night. Breezy W-SW winds have redistributed snow and created shallow wind slabs (see observation below).

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.

Friday, a rain and freezing rain event created icy conditions reported up to at least 7000 feet. 

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

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.