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Avalanche Forecast

Archived

Feb 17th, 2016–Feb 18th, 2016

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.

The avalanche danger should increase rapidly Thursday afternoon and evening as new wind slab builds on lee slopes near and above treeline. A cooling trend Wednesday night and Thursday should limit the loose wet problem to below treeline. 

Detailed Forecast

An occluded front pushing through Wednesday night should be followed by light showers on Thursday morning and a cooling trend. A surface low tracking inland near the Oregon/Washington border mid-day Thursday should provide a burst of moderate snowfall in the afternoon and evening hours, followed by strong westerly winds. 

New wind slab should build on lee slopes near and above treeline Thursday afternoon and evening. Watch for new firmer wind transported snow mainly on lee N to SE slopes near ridges. 

Shallow storm slabs will be a secondary concern in less wind affected areas. 

Despite the cooling trend, loose wet avalanches will remain possible below treeline Thursday especially on steeper slopes. Watch for wet surface snow deeper than a few inches that usually precede loose wet avalanches or other triggered or natural loose wet avalanches. 

Snowpack Discussion

Weather and Snowpack

Dry weather with the warmest temperatures of the winter occurred February 7-10th with mild temperatures and high freezing levels. 

A pair of warm fronts brought about 9-15 inches of new snow to Mt Hood from Thursday Feb 11th to Saturday Feb 13th.

Rain and and mild temperatures resulted roughly 2.5 inches of rain at the Mt. Hood NWAC stations over  2 days ending early Monday morning.  Mild and benign weather occurred Tuesday and Wednesday. 

Recent Observations

The Mt Hood Meadows pro-patrol reported a widespread natural cycle of loose wet avalanches on Sunday morning. On Monday the Meadows pro-patrol reported no avalanches with consolidated snow and surface runnels up to 7000 feet.

On Tuesday the Meadows pro-patrol reported numerous large 1 foot wet slab avalanches in White River Canyon up to 100 yards wide at 5-6000 feet below a ridge with cornices. A small loose wet avalanche was also reported in Heather Canyon.

NWAC observer Laura Green reported no significant layers in a test pit near treeline, with a homogeneous upper snowpack consisting mainly of wet grains. 

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.