Avalanche Forecast
Issued: Jan 17th, 2018 10:58AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
A storm overnight will bring strong winds and new snow. The storm will load existing weak snow over a crust layer in most areas. This will cause a rapid increase in the avalanche danger. Expect dangerous to very dangerous avalanche conditions with avalanches travelling long distances over a smooth crust layer. Travel in avalanche terrain above treeline is not recommended, otherwise choose conservative terrain.
Summary
Detailed Forecast
An arriving storm overnight Wednesday will bring strong crest level winds and snowfall. The storm should come with cooling through Thursday with snowfall changing to showers Thursday with diminishing winds.
The incoming storm will load existing shallow weak snow or buried surface hoar that sits over a smooth crust in many areas. The existing weak surface snow conditions and potential sliding layer, should lead to sensitive avalanche conditions as wind and storm slabs build.
Variations in new storm snow amounts throughout the zone will lead to subsequent variation in local danger. Regardless, the current snowpack structure should not support significant snow loads well.
Building wind and storm slabs should cause dangerous to very dangerous avalanche conditions by Thursday.
Where buried surface hoar exists over a smooth crust layer, expect very sensitive avalanches that may run long distances, especially in areas receiving the greatest storm snow amounts.
Travel in avalanche terrain is not recommended above treeline.
Elsewhere, monitor new snow and wind transport carefully and choose conservative terrain.
Snowpack Discussion
Light amounts of new snow fell Tuesday. The shallow recent snow overlies a variety of older surfaces, including melt-freeze crusts formed during temperature inversion conditions last weekend (Saturday through Monday) and subsequent refreezing wet snow from rainfall that fell in some low-to-mid elevation locations on Monday night.
Buried surface hoar has been observed in the Cascade East - Central zone as of Wednesday 1/17. This layer is likely to exist in the north zone as well, but lack of direct observations leads to lower confidence of its extent and distribution.
Last week’s storms brought 8-14 inches of snow to the east slopes of the Cascades which has been consolidated by warm temperatures.
A supportive crust (1/5) formed from a widespread freezing rain event in the central-east zone, extending to the Stevens and Snoqualmie Pass areas. This layer is not present in the northeast from Washington Pass to Holden.
Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.
Observations
North
No recent observations from this zone following the warm weather last weekend.
Central
On Wednesday, 1/17, NWAC observers Jeremy Allyn and Jeff Ward traveled in the Icicle Creek drainage to near 7000 feet. Of particular note was buried surface hoar found over a melt-freeze crust, buried about 3-4 inches on 1/16. The buried surface hoar was not limited to valley bottoms, but found to all elevations traveled, near 7000 feet. In areas void of the buried surface hoar layer, 3-4 inches of low density snow overlies a recent melt-freeze crust which also extended to about 7000 feet.
A snow professional reported from the Mission Ridge area on Sunday where a 4-5' deep snowpack on a north aspect at 6400 feet contained two potential weak layers of concern. (1) A layer of 2 mm facets 45 cm above the ground lies beneath the 12/15 crust. This layer was reactive in column tests and propagation tests (see this report) . (2) Basal facets just above the ground were present but rounding and were not the weak layer of interest in numerous tests. These layers will need to be monitored as they could develop into potentially dangerous weak layers in the future under loading.
South
No recent observations
Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 18th, 2018 10:58AM
