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Avalanche Forecast

Archived

Jan 19th, 2016–Jan 20th, 2016

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

Regions

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New wind and storm slab is most likely in the central and south Cascades on Wednesday. A potential lingering persistent weak layer also makes conservative decision-making and cautious routefinding is essential.

Detailed Forecast

Light snow showers mainly near and west of the crest should end on Wednesday morning. A short relative break should be seen Wednesday afternoon.

New wind slab is most likely in the central and south Cascades. Watch for signs of firmer or hollow wind transported snow mainly on lee northwest to southeast near and above treeline.

New storm slab due to rapid loading is also most likely in the central and south Cascades in areas that receive more than a few inches of snowfall especially in areas with any warming like the Cascade passes.

Persistent weak layers had a good test from loading and warming over the weekend. On slopes that have not released on these layers, the likelihood of triggering is slowly decreasing, but the consequences are increasing as the slab depth increases. Wind or storm slabs that step down to persistent weak layer depths could become large enough to bury, injure or kill.

There are still reports of trees loaded with snow. Watch for the hazard above!

Also here is a heads up that a strong wet warm front is headed our way for Thursday.

Snowpack Discussion

Weather

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on January 3rd and 11th. One or both of these layers have been seen in snow pits by NWAC pro observers the past few days ranging from Mt Baker to the Crystal Mountain area. These persistent weak layers are most likely to be found on non-solar (northerly) aspects in the below and near-treeline band. However, we cannot rule out the above treeline elevation bands so the problem is listed for all three bands.

A parade of weather systems this week added about 1-2 feet of snowfall to the west slopes from about Friday to Sunday.

A fairly vigorous surface low pressure and frontal system will cross the south Cascades Tuesday and Tuesday night. This should bring heavy snow to the south Washington Cascades, moderate snow to the central Cascades and lighter amounts of snow to the Olympics and north Cascades. A minor drop in snow levels should be seen following this system except for a bump in snow levels in the passes when winds change to west there Tuesday night.

Snow and Avalanche Observations

NWAC Pro observer Dallas Glass found the buried surface hoar layer well intact 45-50 cm down on a west aspect of Kendall Peak Saturday. Natural avalanches he observed were limited to recent storm snow, with  storm  slabs averaging around 8-12 inches (20-30 cm). 

On Sunday the Baker pro-patrol  found  16 inch storm  slabs releasing during control work on wind-loaded aspects and a natural release of similar depth from Saturday night. The Alpental pro-patrol reported a natural cycle Saturday night then less activity Sunday. Back country rangers at Paradise on Sunday reported rain up to 4500 feet with the buried surface hoar seen in snow pits at 5900 feet.

NWAC pro observer Ian Nicholson at Yodelin on Monday found both the 1/3 and 1/11 surface hoar layers buried about 40 and 75 cm respectively on north aspects, within the below treeline elevation band. Tests indicated propagation was likely, with the deeper layer slightly more reactive.

 

 

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.