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Avalanche Forecast

Archived

Jan 18th, 2017–Jan 19th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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The snowpack in the Olympics and Cascades is undergoing a lot of change and there is less than the usual certainty in which avalanche problems and danger levels to indicate. It might be a good plan to wait until Friday when conditions are better known and should be more stable.

Detailed Forecast

Winds and snow showers will decrease Wednesday night and Thursday with cooler temperatures.

The heavier storm snow should be seen in the northeast zone with lighter amounts elsewhere east of the crest.

The greatest likelihood of new wind slab should be at higher elevations where storm snowfall is heaviest. Underlying crusts may make may make wind slab sensitive on some slopes.

New storm slab will be most likely in any areas that experience rapidly accumulating snow of more than an inch an hour for several hours. Underlying crusts may make may make storm slab sensitive as well on some slopes.

The 12/17 PWL and other persistent weak layers closer to the surface have been unreactive lately in the northeast zone and PWL's have been removed from that zone. If these layers does not snow reactivity in the central east and southeast zones soon they will be removed from those areas as well.

The snowpack in the Olympics and Cascades is undergoing a lot of change and there is less than the usual certainty in which avalanche problems and danger levels to indicate. It might be a good plan to wait until Friday when conditions are better known and should be more stable.

Snowpack Discussion

Weather and Snowpack

An Arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals probably formed in most areas during that period.

A drastic change was seen when an atmospheric river moved to the Northwest Tuesday and Wednesday. This caused heavy snow in the northeast. Rain was seen up to about 5-6000 feet in the central east and southeast with snow at higher elevations. Sleet or freezing rain was also probably seen in some lower elevation areas east of the crest.

Observations the next couple days should help determine to what extent any of the surface hoar and near surface faceted crystals from the fair cold weather might have survived the atmospheric river event.

Recent Observations

NWAC pro-observer Tom Curtis was out on Friday on Mt Cashmere and on a W aspect near treeline and he found the 12/17 PWL at 55 cm giving a PST 37/100 End. He noted some previous wind transport and cross loading on N-W-S aspects. Tom also found great skiing conditions in the trees. 

An observation via the NWAC observation page from Saturday continued to identify weak persistent grains near the base of the relatively shallow snowpack in the Mission Ridge area. While these layers were reactive in column tests, no recent avalanche activity has been observed involving these layers.

The NCMG were in the Cutthroat area near Washington Pass on Monday and reported unreactive hand shear test and unreactive wind slab.

The Mission Ridge pro-patrol on Wednesday morning reported that building new wind and storm slab was still shallow and not yet reactive. The 12/17 PWL at about 75 cm down is not reactive or is giving hard lower quality tests.

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.

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.