Avalanche Forecast

Issued: Dec 14th, 2017 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

With a pattern change upon us, begin using extra caution Friday. Watch for areas where freshly wind-transported snow may build shallow wind slabs over near surface facets, surface hoar, or melt freeze crusts - any of which may act as a sliding surface for storm slabs or small wind slabs forming near ridgelines.

Summary

Detailed Forecast

Expect rapidly lowing freezing levels and an abrupt pattern change on Friday morning as a short round of rain and snow adds a shallow blanket of new snow on top of the highly variable existing snowpack. We'll be watching to see whether faceted crystals are preserved and buried as this round of precipitation moves through.

Snow accumulation and wind will combine to produce snow transport that should be marginally sufficient for wind slab formation. With some uncertainty in the strength of the winds and new snow amounts and the surfaces buried underneath, watch for signs of denser snow on top of settled powder or faceted crystals on non-solar aspects or increasing snow density on top of firmer melt-freeze layers on solar aspects. With a lot of variety in the surface snow conditions right now and additional snow arriving through the weekend, Friday is an excellent day to submit an observation to NWAC to help us better define the extent of developing avalanche problems.

Early season terrain hazards have become an increasing hazard in recent days with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.

Snowpack Discussion

Strong high pressure has now been over the region since December 3rd with Thursday's high clouds portending Friday's pattern change. Sunshine and very mild temperatures have been the story with cooler temperatures and areas of low clouds at lower elevations and in the Cascade Passes. This weather has allowed for overall strengthening and consolidation of an already strong snowpack. Total snow consolidation along the west slopes of the Cascades ranges from 5-18" since December 3rd. The upper snowpack consists of about 1.5 to 2.5 feet of settled old storm snow over the strong Thanksgiving rain crust. 

No human-triggered avalanches have been reported in over a week. Some natural small loose wet slides have occurred on mainly steep sun exposed slopes over the last week, but have become much less frequent over the past several days. Active wind loading has not been observed since late last week.

Surface conditions are highly variable. On solar aspects, surface melt-freeze crusts reform every night. On shaded and sheltered aspects, settled storm snow or near surface faceted snow are still providing some nice skiing and riding conditions. Many aspects in higher terrain now feature wind stiffened snow. Near surface faceting and surface hoar growth has been reported on colder, non-solar aspects throughout the Cascades but especially in the cooler Cascade Passes. These persistent grain types will become important when snowfall returns and watched as potential future weak layers. 

Observations

North

On Wednesday, NWAC Pro Observer Jeremy Allyn was on Green Mountain and found settled, stable snow conditions. At the summit, northeast slopes were wind affected, but there was no snow available for transport. At peak solar warming, spring-like conditions were found on steep solar aspects. There was little to no snow below 5000 feet. 

On Wednesday, the Mt. Baker professional patrol reported no glide or other avalanches. Surface snow softened in the sun. Snow on non-solar is staying cold and drier, but without near surface facets or surface hoar, likely due to being above the inversion.

On Monday, Pro patrol at Mt Baker Ski area reported two climax/glide avalanches off steep rock faces during the late morning hours. The slides were on an east and a south face, both were about 30 feet across and 3 feet deep, with debris piles 4-5 feet deep. No other avalanche activity was observed.

NWAC observer, Lee Lazzara travelled extensively Sunday, December 10th in the Swamp Creek / Winchester area from 3000-6100 feet. While the temperatures were quite warm and melting evident, surprisingly little current or previous loose wet avalanche activity was noted, mainly tree bombs. No avalanche activity was noted with variable but good ski quality in well settled storm snow, well bonded to the T-Day crust. Very little to no surface hoar or near surface facets were seen, other than in non-avalanche terrain near valley bottoms and creeks. 

Central

On Wednesday, NWAC Pro Observer Jeff Ward was on Jove Peak where he found little to no avalanche concerns. He found a mix of conditions, ranging from corn-snow on solar slopes, to powder on lower angle slopes at lower elevation, and large (1 cm+) surface hoar in the valleys. With the variable snow surfaces, he suggests it is going to be difficult to predict how the next storm snow is going to bond to these surfaces.

Also Wednesday, see a nice observation from the Stevens Pass area posted to the NWAC observations page:

Surface hoar over a breakable melt freeze crust, image Matt Hartman.

Professional Observer Dallas Glass skied non-solar slopes in the Alpental valley on Monday. He observed a very strong temperature inversion up to 4800' with cold, dry snow below the inversion. He found moderate easterly winds near treeline at 5200’. No snow was being transported. He found no avalanche problems that affected his travel. He noted the biggest travel hazard was low snow cover. Surface snow conditions were extremely variable.

South

Professional observer Ian Nicholson was in the Crystal backcountry on Wednesday. He found an overall consolidated, stable snowpack. He found limited good skiing on NE-facing slopes on a surface layer of settled powder, near-surface facets, and surface hoar. He noted an impressive volume of snow lost during this high pressure ridge. He noted that the easterly wind was picking up over the course of the day, but was not transporting snow.

A DOT crew was at Chinook Pass on Tuesday and reported noteworthy surface hoar up to 1 cm below the 4500' inversion. 

Problems

Wind Slabs

An icon showing 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: North, North East, East, South East, South, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Dec 15th, 2017 10:00AM