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Avalanche Forecast

Archived

Dec 23rd, 2015–Dec 24th, 2015

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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Further wind and storm storm slab and wind slab are likely Wednesday but also watch for where a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap like creeks or trees.

Detailed Forecast

A cool upper trough will continue to sink south over the Pacific Northwest on Thursday. West winds should begin to decrease Thursday initially in the north Cascades. Snow showers Thursday should bring another few inches mainly to the central and south part of the east slopes.

Dangerous avalanche conditions should persist Thursday especially in the near and above treeline zones where new or recent storm and wind slab should be likely. Storm and wind slab avalanches should remain within recent storm layers but could move fast. Watch for evidence of wind deposited snow or pillows. Test for inverted strong over weak storm snow and give cornices a wide margin.

Loose dry avalanches in non-wind affected terrain will be difficult to manage on steep slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap like creeks or trees.

Snowpack Discussion

The Christmas snow globe keeps giving here in the PNW! The east slopes are getting in on the action as well! It's already well beyond anything we achieved last year and getting deeper by the day. By Thursday morning the east slope areas will have received 2-4 feet of snow in less than a week.

A report via the NWAC Observations page indicates 2 skiers were caught in a storm or wind slab avalanche and carried a short distance on Driveway Butte on Tuesday in the Methow River area. The crown at the trigger point was 6 inches but ranged up to 2 feet. This was on a north slope at about 5840 feet.

NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found wind slab on steep open slopes even to below tree line and often above terrain traps like creeks or trees. A storm layer was seen at 35 cm with wind and storm slabs reacting to tests to this interface. He noted wind slab releases of 20-35 cm on northeast to east slopes near tree line and storm slab releases near and below treeline on steep terrain features of varied aspect and on small steep slopes above Forest Service roads. He also found some difficult trail breaking in deep powder.

The southeast zone should have a shallower snow pack. However, we have no recent observations from the southeast zone.

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.

Loose Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.