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Avalanche Forecast

Archived

Dec 18th, 2016–Dec 19th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.

Regions

Mt Hood.

Travel in avalanche terrain is not recommended at Mt Hood on Monday. 

Detailed Forecast

Strong west flow will carry a strong cold front across Mt Hood on Monday. This should cause strong alpine winds with moderate to heavy snow and a warming trend. The warming trend should contribute to the formation of both new wind slab and new storm slab layers.

New wind slab is very likely at Mt Hood on Monday. Wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects.

New storm slab is also very likely at Mt Hood on Monday on any slopes that rapidly accumulate new snow of more than a several inches.

Travel in avalanche terrain is not recommended at Mt Hood on Monday. 

Another strong front should be seen Monday night and Tuesday. This should maintain or increase the avalanche danger in the Olympics and Cascades.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect all of the Olympics and Cascades was from Thursday, December 8th through Monday morning, December 12th. About 4 feet of snow were recorded at the NWAC stations in the Mt Hood area.

Another system moved across the Mt Hood area late Wednesday through Thursday morning depositing an additional foot of new snow with moderate southeast winds shifting to the west and decreasing. 

There has been a lot of snowpack settlement since the last storm cycle ended, allowing this underlying snow to mostly stabilize.

Clear or fair and cold weather has been seen about Thursday at Mt Hood. This should have caused new widespread surface hoar and near surface faceted snow to develop to develop at Mt Hood as in the Cascades. Thin sun crusts have also formed on steeper solar slopes. These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall.

Recent Observations

NWAC pro-observer Laura Green was out in the Newton and Clark drainages on Friday a found a mix of surface conditions of recent snow and 3-4 mm surface hoar with a thin sun crust on solar slopes. The upper snow pack on N-NE slopes was right side up with no direct signs of instability.

A report via the NWAC Observations tab also indicated a small slab avalanche near the Ski Bowl ski area on Friday.

The Mt Hood Meadows pro-patrol on Sunday reports fairly widespread surface hoar and near surface faceted snow up to about 7300 feet. Calm winds have allowed it to develop or grow the past couple days.

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.