Avalanche Forecast
Issued: Jan 20th, 2018 10:59AM
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 stormy day on Sunday will cause the avalanche hazard to rise throughout the day. Watch for developing wind and storm slabs, and be prepared to select more conservative terrain as avalanche conditions deteriorate.
Summary
Detailed Forecast
Another round of precipitation and winds will arrive on the east slopes of the Cascades during the day on Sunday. The avalanche danger will increase throughout the day, peaking in the afternoon as precipitation and winds ramp up.
Moderate to strong winds will transport snow forming new and sensitive wind slabs. Watch for signs of winds actively transporting snow such as plumes, moderate wind speeds, and newly forming cornices. Identify and avoid avalanche terrain where the wind is depositing snow.
Shallow storms slabs will likely develop with precipitation during the day. As storm slabs grow, consider the consequences of the terrain where being caught in even a small avalanche may have more drastic consequences such as above cliffs, gullies, and open creeks.
Be prepared to select more conservative terrain as the storm intensifies and avalanche conditions deteriorate throughout the day.
Snowpack Discussion
An active storm cycle beginning Wednesday and extending into the weekend deposited 10-20 inches of new snow around the region. Observations indicate this new snow is bonding well to the 1/16 melt-freeze crust.
In exposed terrain winds have transported recent snow forming wind slabs on a variety of aspects.
Observations above treeline have been very limited leading to a high level of uncertainty in this terrain.
Buried surface hoar has been observed in the Cascade East - Central zone as of Wednesday 1/17. This layer has not been found recently in the north zone or in the area above Lake Wenatchee, however the layer cannot be ruled out entirely as its extent and distribution is still uncertain.
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 was 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
On Thursday snow professionals in the Washington Pass area found good bonding of new snow to the old surfaces with favorable snow profiles and no avalanche activity, even traveling on steep slopes. No buried surface hoar was found in this zone.
Central
On Saturday an avalanche professional on Dirty Face Mountain near Lake Wenatchee observed a well bonded upper snowpack near and below treeline. No buried surface hoar was found and no avalanches were observed.
On Friday, NWAC observer Jeff Ward traveled near Mt Poe near Lake Wenatchee. No buried surface hoar was found over the Jan 16 crust layer in this terrain. Wind re-distribution was evident, especially in higher exposed terrain. Storm snow totals ranged from 16-20 inches in this area with no recent avalanches observed.
On Wednesday, 1/17, NWAC observers Jeremy Allyn and Jeff Ward traveled in the Icicle Creek drainage to 6800 ft. Buried surface hoar was found over a melt-freeze crust. The buried surface hoar was not limited to valley bottoms, but found to all elevations up to 6800 feet.
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 21st, 2018 10:59AM
