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Avalanche Forecast

Archived

Feb 18th, 2016–Feb 19th, 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 Friday afternoon and evening due to building wind and storm slab. Cooler temperatures should limit the loose wet problem to below treeline but wet snow hazards will continue. Change your travel plans accordingly if local conditions differ from forecasted weather and/or snowpack conditions.

Detailed Forecast

Continued periods of light to moderate snow at cool temperatures, both Thursday night and renewing during the day Friday are expected, along with strong winds.

This weather should build wind slabs on lee slopes near and above treeline, especially by Friday afternoon. Watch for firmer wind transported snow, mainly on the lee slopes facing N to SE, below ridges. 

Heavier precipitation rates Thursday afternoon and evening, and again Friday should increase the storm slab likelihood.  

Despite the cooling trend, loose wet avalanches will remain possible below treeline Friday especially on steeper southerly facing 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, leaving behind well settled older snow and a strong melt-freeze crust. 

About 9-15 inches of storm snow fell on Mt Hood from Thursday - Saturday, Feb 11-13.

Rain 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. 

Cooler weather and new storm snow began Thursday 2/18 and by Thursday afternoon has deposited about 4-6 inches of new snow. Strong winds have transported new snow and built wind slabs up to 1 foot by midday Thursday near and above treeline.

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 new unstable wind slabs of 1 foot had built on lee N-E facing terrain by mid-day Thursday, 2/18. The new storm snow was bonding well to the old moist snow surface and had not become deep enough yet to cause a significant increase in danger.

The mid and lower snowpack remain relatively homogenous with no significant layers of concern. 

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.