Avalanche Forecast
Issued: Jan 22nd, 2018 1:20PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Step back as new higher density storm snow layers and winds create more widespread slab conditions than seen in recent days. New storm and wind slabs may entrain significant amounts of recent snow, creating the potential for larger avalanches. Plan for increasing avalanche danger as the day unfolds.
Summary
Detailed Forecast
Increasing stormy weather on Tuesday will include rising winds and increasing moderate snowfall, along with a slight warming trend for the southeast and central-east Cascades.
Conditions for storm slab development will be in place on Tuesday. These slabs will be sensitive to trigger and may run farther than you might expect. In most areas the weak layer is expected to be the recent lower density snowfall, but in some areas the new loading may awaken weak layers near the 1/16 crust. You are likely to trigger a large storm slab on steeper or convex terrain on Tuesday. Uncertainty exists in the timing and intensity of precipitation on the east slopes, so scale back your terrain selection as storm conditions evolve.
Increasing snowfall, recent snow available for transport, and wind on Tuesday will create sensitive new wind slabs near and above treeline. The past several days of snow also formed wind slabs on a variety of aspects near and above treeline. Fresh or old, the wind slabs will entrain significant new snow to create large avalanches that run farther than you expect. A backcountry traveler will be likely to trigger a wind slab on wind-loaded slopes on Tuesday. Identify and avoid areas of wind deposited snow such as below cornices, snow drifts, and on cross-loaded terrain features.
Route-finding and travel conditions will deteriorate throughout the day.
Snowpack Discussion
Along the east side of the Cascades a stormy period from 1/16 through the present produced 8-16 inches of settled snow now sitting on the 1/16 melt-freeze crust. Observations generally show the bond between the new snow and the crust continuing to gain strength.
Winds observed during the stormy period formed 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 was observed on the 1/16 crust in the Cascade East - Central zone on Wednesday, January 17th. The layer was also found on the crust in the Twisp River drainage on Monday, January 22nd, but faceted grains below the crust were even more concerning. The crust layer interfaces were the suspect culprit for a large avalanche on Sunday in the Cascade East-North zone. This layer will need to be watched during the upcoming stormy period as the extent and distribution of these weak layers remain highly uncertain and likely sporadic.
A widespread freezing rain event in the Cascade East-Central zone formed a supportive crust (1/5) which extends to the Stevens and Snoqualmie Pass areas. This layer was not present in the northeast from Washington Pass to Holden.
Snow depth 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 Monday, snowpit tests by NCMG on a SE aspect Near Treeline gave sudden collapses within faceted grains below the 1/16 crust. Test results on a NE aspect yielded consistent sudden planar results above the 1/16 crust with partially decomposed surface hoar on the interface.
NWAC received a second-hand report of a large skier-triggered avalanche on west-facing slope (likely near treeline) on Abernathy Peak in the upper Twisp River drainage. The avalanche propagated widely and was suspected to have failed on the 1/16 crust.
On Saturday guides from North Cascades Heli reported 10-14 inches of snow over the 1/16 crust. Observations indicate potentially touchy wind slabs above treeline.
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
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
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
Valid until: Jan 23rd, 2018 1:20PM
