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Avalanche Forecast

Archived

Feb 9th, 2017–Feb 10th, 2017

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

Regions

Mt Hood.

Moderate to occasionally strong W-SW winds will transport new snow onto lee slopes near and above treeline Thursday night and Friday. Wind slab may build down into the below treeline band by Friday afternoon. NW-SE aspects will be identified in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.

Detailed Forecast

What a difference a day makes as we quickly transition from mild and wet to cool and showery weather on Friday. Snow levels will slowly lower Thursday night and lower further during the day on Friday. Showers should decrease overnight but then increase again Friday as a low pressure system passes to our north. W-SW winds will be moderate to occasionally strong Thursday night and Friday. 

Moderate to occasionally strong W-SW winds will transport new and recent snow onto lee slopes near and above treeline. Wind slab may build down into the below treeline band by Friday afternoon. NW-SE aspects will be identified in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.   

Despite the cooling trend, new storm slab instabilities may develop during periods of intense showers. 

Wet slabs will be listed as unlikely due to the cooling trend. However, avoid travel below unsupported slopes, especially near and below treeline, with the low likelihood/high consequence of large wet slab avalanches in mind. Wet slab releases are hard to predict and can happen a day or two after peak warming and rainfall.   

Snowpack Discussion

Weather and Snowpack

A strong storm cycle was seen Friday through Monday with about 3-4 feet of snow recorded at the NWAC Mt Hood stations. Temperatures were mild through the weekend with strong southwest winds followed by cooling and lighter winds on Monday. Light snowfall was seen at Mt Hood on Tuesday, depositing about 2-3 inches of new snow with fairly light winds.  

A strong plume of moisture brought light to moderate rain to Mt. Hood Wednesday afternoon through Thursday. About 2 inches of water fell mainly as rain at Mt. Hood from Wednesday afternoon through Thursday afternoon with significant snowpack settlement. A strong cold front swept through around 2 pm Thursday followed by bands of light showers in SW flow along with the start of a slow cooling trend.  

Recent Observations

The Meadows pro-patrol closed their gates by mid-day Wednesday due to deteriorating weather. Ski tests were beginning to produce small loose wet avalanches. Cornices were reported to be large after the last storm cycle.

On Thursday, Meadows pro-patrol reported saturated snow down 25 cm but no avalanche activity in the ski area. The upper mountain was not observed due to strong winds and limited visibility. 

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.

Wet Slabs

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.