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Avalanche Forecast

Archived

Jan 18th, 2018–Jan 19th, 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.

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.

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

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.