Avalanche Forecast

A period of light snow showers is expected Sunday with a period of moderate to strong S-SE crest level winds early Sunday. The winds early Sunday moved available surface snow and build new sensitive wind slabs on a variety of aspects, primarily N-W-SW facing terrain.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on N-W-SW aspects due to recent and expected E-SE transport winds, but will be listed on all aspects due to variable loading patterns over the past several days. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution. Wind slab will likely extend into the upper portion of the below treeline band at Snoqualmie and Stevens Pass.

The considerable avalanche danger above treeline is forecast to highlight the potential for increased sensitivity and size of wind slabs. 

Storm slabs should be more stubborn to trigger, but still possible in the wrong spot Sunday and can be avoided by sticking to lower angled slopes. 

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. Recent observations on Thursday and Friday helped confirm that we aren't dealing with any older layers formed during this period.  

An atmospheric river moved over the Northwest Tuesday and Wednesday with highly variable weather seen throughout the Cascades during this event. Heavy rain was seen up to about 5000 feet in the north Cascades and up to about 6000-6500 feet in the south with snow at higher elevations. 3 day precipitation totals through noon Thursday were about 5 inches at Crystal and 8 inches at Mt. Baker with 1-2 inches at the Passes. Sleet and freezing rain Tuesday at the Passes changed to snow at higher elevations at Stevens and Snoqualmie on Wednesday. An avalanche cycle likely occurred in areas west of the Passes during this period. 

With the arctic air mass finally displaced and easterly flow abating in the Passes on Thursday, Stevens and Snoqualmie Passes warmed to near or above freezing while areas further west like Crystal and Mt. Baker saw a slow cooling trend. New snow totals through Thursday were fairly light except with about 12-15" at Stevens, 9" at the top of Alpental with 13" at the base. Below the new snow a freezing rain crust has been reported in the Stevens Pass area and the upper half of Alpental with varying reports of a crust at Pass level.

At Stevens and Snoqualmie Pass the freezing rain crust in about 1" (2-3 cm) thick and generally ski supportable. Mt. Baker was the benefactor of steady showers Thursday afternoon and night, picking up about 2 feet of new snow through Friday morning.  E-SE winds were strongest in the Crystal area Friday, with light to moderate E-SE winds observed or reported elsewhere.

A very pleasant day Saturday allowed temperatures to moderate to near the freezing mark in many areas. Light snow showers began to spread from south to north late Saturday afternoon. 

Recent Observations

North

The Mt Baker pro-patrol and NWAC pro-observer Simon Trautman reported several large wet slab and glide avalanches from the Mt Baker area on Wednesday following the period of heavy rain. Simon was back out in the Mt. Herman/Bagley L. area Friday. A density change about 10" (25 cm) down in the new storm snow was reactive on small test slopes and the depth of widespread natural storm slab avalanches that had occurred Thursday night. At least one unintentional skier triggered storm slab was reported on this layer in the Mt. Baker backcountry Friday. Recent wind loading was apparent on many aspects near and above 5000'. Weak wet snow below the recent storm snow could still be found below 4000 ft.     

Central

The Stevens pro-patrol on Wednesday morning reported only small wind slabs along the ridge tops and small triggered loose dry avalanches. The new snow was not bonding well to the freezing rain crust. NWAC observer Dallas Glass was out in the Stevens Pass area Wednesday and reported sensitive 25-35 cm of wind slab on westerly aspects above the 1/17 freezing rain crust in the near and below treeline bands. On Friday, Dallas reported storm slabs failing on a preserved storm interface just above the freezing rain crust in snowpack tests and on short test slopes. Wind slabs were sensitive at the same interface and most likely to be found on westerly aspects near and below treeline in the Stevens area.

NWAC observer Ian Nicholson was out in the Alpental Valley on Friday and found storm slabs 20-25 cm thick and poorly bonded to the 1/17 freezing rain crust. Wind slab distribution near treeline proved variable and touchy in pockets. Both avalanche problems required conservative terrain choices.    

South... Updated Sunday 10:15 am

UDATED Observations! Crystal Mtn Pro patrol reported extremely sensitive new wind slabs on NW aspects Sunday morning. About 4-6 inches of new storm snow were transported by very strong S-SE winds overnight and early Sunday morning. This built very sensitive wind slabs on lee slopes by early Sunday. Wind slabs, mostly in the 6-8" range, were releasing sympathetically and running far distances. Shooting cracks were extending over 100 ft from skis.