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Avalanche Forecast

Archived

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

Regions

Mt Hood.

The avalanche danger should be greatest early Saturday following the storm Friday. Watch for wind loaded terrain and avoid steep slopes suspected of wind deposits. Conditions should improve as the snow pack slowly stabilizes through Saturday afternoon.

Detailed Forecast

Continued periods of moderate snow at cool temperatures, Friday night, along with strong winds.

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

Heavier precipitation rates Friday night should increase the storm slab possibility by early Saturday. 

Showers should end quickly early Saturday with partial clearing and diminishing winds. Cooler temperatures should continue Saturday. 

Watch for greater depths of storm snow and avoid wind loaded terrain and steep rollover features. 

Storm snow problems should improve through the day Saturday as upper snow layers settle and stabilize.

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. 

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. 

Snow showers at cooler temperatures arrived Thursday 2/18 followed by a strong front Friday. These storms have deposited storm snow amounts of about 1 foot by Friday afternoon in wind protected areas.

The new snow has been wind affected at higher exposed elevations, but in general is well bonded to the old wet but draining snowpack. 

The upper snowpack, below the recent storm snow, consists of a mix of crusts and wet grains with no deeper instabilities of note.

Recent Observations

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.

By Friday afternoon, 2/19 Laura found increasing wind slab deposits on lee slopes near and above treeline with about 1 foot of new storm snow. On wind protected slopes, the storm snow was unreactive to ski tests. 

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.