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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

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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 light to 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

Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat since several warm and wet systems have tested this layer and since it has been unreactive where it can still be identified in recent snowpit tests.

Dry weather with the warmest temperatures of the winter occurred February 7-10th with temperatures climbing into the 50's in most areas east of the crest. This caused loose wet avalanches and overall snowpack consolidation with a crust as temperatures cooled late last week.

Storm snow of 6-12 inches accumulated at higher elevations a week ago loading the melt freeze crust by February 12th. The greatest recent snow was received near Washington Pass where the 2/12 crust is now buried 1-2 feet or more.

Rain and mild temperatures have dominated along most of the lower east slopes Sunday and Monday. NWAC sites had a couple inches of mainly rainfall ending Monday morning. About 8-14 inches of new snow fell at higher elevations in the central east and northeast zones indicated by NRCS sites at Harts Pass and Lyman Lakes. Mild temperatures with spotty light rain were seen on Tuesday and Wednesday. 

Cooler weather with increasing storms arrived Thursday and a stronger front Friday 2/19. Increasing new storm snow deposits with strong winds continue as of Friday afternoon.

The upper snowpack below the recent storm snow consists of a mix of crusts and wet grains.

Recent Observations

The North Cascades Mountain Guides reported significant firm wind slab formation in the above tree line on Sunday. Ski cuts gave roller balls and hand tests in the recent storm snow gave easy shears in upside down storm snow. A crust buried Thursday, February 11th was identified in a snow pit at 6000 feet at 25 cm but did not indicate propagation.

Tom Curtis was in the Icicle Creek drainage Wednesday and found soft wet snow in the upper snowpack along with challenging travel conditions. A thin melt-freeze crust limited loose wet activity Wednesday. Tom also noted open creeks due to the ongoing warmth and rainfall at lower elevations. 

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.