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Avalanche Forecast

Archived

Feb 12th, 2016–Feb 13th, 2016

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

Regions

Mt Hood.

Moderate to strong W-SW winds will transport new snow onto lee slopes near and above treeline Saturday. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow will likely build unstable wind slab on lee aspects above treeline. The avalanche danger will increase rapidly Saturday night as snowfall increases along with a slow warming trend. 

Detailed Forecast

Light snowfall accumulations are expected Friday night through Saturday above 4000 feet on Mt. Hood.

Moderate to strong W-SW winds will transport new snow onto lee slopes near and above treeline. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow will likely build unstable wind slab on lee aspects above treeline.

Wet loose avalanches may still be locally a concern on steeper slopes at lower elevations so continue to watch for loose wet avalanche potential above terrain traps such as above cliffs, near gullies or where avalanche debris would deeply accumulate. 

Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.

The avalanche danger will increase rapidly Saturday night as snowfall increases along with a slow warming trend. This unstable combination of moderate to heavy precipitation and warming should continue on Sunday leading to high avalanche danger in many areas.

Snowpack Discussion

Weather and Snowpack

A large upper ridge and warm air mass aloft over the West Coast from this past Sunday through Wednesday led to the warmest weather of the winter. During this stretch temperatures were generally well above freezing.  A warm front brought periods of rain to Mt. Hood Thursday night through mid-day Friday, with snow levels falling to 5500 ft after the frontal passage Friday afternoon. 

The warm temperatures and solar effects earlier in the week caused loose wet avalanches and overall snowpack consolidation while recent rainfall has kept the upper snowpack moist in many areas. 

Recent Observations

Limited loose wet snow conditions were seen by Tuesday at Mt Hood by the pro-patrol. Moderate winds limited surface snow melt near and above treeline. More significant wet snow conditions were seen below treeline Tuesday through Friday.

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.