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Avalanche Forecast

Archived

Dec 19th, 2016–Dec 20th, 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 avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

A natural avalanche cycle should occur Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to very strong westerly winds on a variety of aspects. Slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday!

Detailed Forecast

Strong westerly flow will carry another strong frontal system across the Northwest Monday night and Tuesday morning. This will cause strong winds with moderate to heavy snow along the west slopes and a warming trend overnight. The warming trend should contribute to the formation of both new wind slab and new storm slab layers. A sharp cooling trend will take place beginning late Tuesday morning with light to moderate snow showers continuing through the day in continued strong westerly flow. See the NWAC mountain weather forecast for more details on precipitation intensity, precipitation types and temperature trends.

New wind slab is very likely for Mt. Hood on Tuesday. Continued wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects due to strong gusts. Also, wind slab may develop below treeline due to the persistence of very strong westerly winds. 

New storm slab is also very likely Tuesday on any slopes that rapidly accumulate new snow. A change from snow to rain near and below treeline Monday night will also make storm slabs more likely to initiate or cause natural loose wet avalanches in areas lacking a slab structure. Loose wet should only be likely in the morning due to a rapid cool down beginning later Tuesday morning.

A natural avalanche cycle should occur Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday! 

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.

Strong westerly winds affected most locations in the Olympics and Cascades including Mt. Hood Monday morning before easing off Monday afternoon. 5-8" of new snow fell at NWAC stations on Mt. Hood with a warming trend through Monday afternoon.  

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.

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.

No observations were received on Monday. 

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.