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Avalanche Forecast

Archived

Jan 17th, 2015–Jan 18th, 2015

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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Sunday should be fairly stormy day at higher elevations east of the crest. New storm and wind slab layers are likely at higher elevations east of the crest.

Detailed Forecast

A wet cold front will cross the Northwest Saturday night. Snow levels will be highest on Saturday night. A good period of southwest-west flow, moderate to heavy orographic snow showers, and cooling with lowering snow levels should be seen on Sunday. Snow showers should spill to the east of the crest. New snow by the end of the day Sunday east of the crest should vary a lot from lower to higher elevations with 10 inches at higher elevations.

Building storm slab is likely mainly near and above treeline on Sunday. Watch for cracking and releases on steep sheltered slopes where snowfall exceeds an inch an hour for more than a few hours.

Building wind slab is also likely mainly near and above treeline on Sunday. Watch for firmer wind transported snow on lee slopes.

Buried surface hoar may act as a weak layer in some areas east of the crest. This may be mainly in the upper parts of valleys that had foggy conditions in the later part of the dry period.

The cooling should cause some good snow conditions on other slopes. Don't let powder starvation change plans to stick to safe slopes on Sunday. It is always a good plan to give new snow layers a day to stabilize before venturing to steeper slopes after a storm.

Snowpack Discussion

A front late Thursday was followed by showers Friday. East of the crest this gave up to about 2-7 inches of storm snow.

A dry period was seen about 6-15 January. East of the crest the later part of the period was marked by warm temperatures and sunshine at higher elevations and sometimes foggy conditions in the valleys. NWAC observer Tom Curtis was north of  Blewett Pass on Thursday and found widespread 5mm+ surface hoar up to 5400 feet. The non-reactive persistent facet layer found on Jove Peak on Wednesday was also found at 85 cm Thursday.

The overall snowpack remains well below normal for this time of year and some windward or southerly aspects have little if any snow cover.

Problems

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.

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.