Avalanche Forecast

Snow showers on Thursday should taper down overnight. A frontal band rotating up from the south Friday should bring generally light snow showers mainly for the south and central Cascades in the afternoon. Out ahead of this frontal band, there should be a period of moderate E-SE winds affecting again mainly the south and central Cascades. 

A period of moderate E-SE winds Friday may build new wind slab near and above treeline on westerly aspects, especially in places with lower density snow available for transport. All aspects are indicated to cover older wind slab on more easterly aspects. Look for recent wind effects near and below ridgelines.

Storm slabs may be the most prevalent in the Mt. Baker zone Friday morning due to moderate snowfall rates Thursday afternoon and evening. Storm slabs may also be locally sensitive in the Passes if poorly bonded to an underlying freezing rain crust. Give these layers time to settle and approach steeper slopes with caution.

We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area. 

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. Observations over the next few days should help determine to what extent any of these weak persistent grain types might have survived the atmospheric river event and remain relevant to the forecast moving forward.

An atmospheric river moved over the Northwest Tuesday and Wednesday. Heavy rain was seen up to about 5000 feet in the north Cascades and up to about 6000 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 over the last 24-48 hours were fairly light except with about 12-14" at Stevens and Mt. Baker, 9" at the top of Alpental with 13" at the base through 4 pm Thursday. Below the new snow a freezing rain crust has been reported in the Stevens Pass area and the upper half of Alpental (no freezing rain crust reported at the Alpental base). 

Recent Observations

The Mt Baker pro-patrol and NWAC pro-observer Simon Trautman reported wet slab and glide avalanches from the Mt Baker area on Wednesday. A natural glide avalanche was seen on a NW slope at 3800 ft with a 1-2 ft x 100 ft crown. Explosive control gave a very large wet slab with a 3 ft x 1000 ft crown above the road to the ski area on a NNW slope at about 4000 ft. Another wet slab was seen on a west slope at about 4000 ft. Lastly a wet slab ran starting at about 5000 ft ran on a S slope on Church Mtn to near Highway 542.

The Stevens pro-patrol on Wednesday morning reported only small wind slabs along the ridge tops and small triggered loose dry avalanches. 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. 

Crystal Mt pro-patrol reported a widespread loose-wet cycle on all aspects Wednesday with slides ranging from small to large. Only shallow 2-4" winds were reported below ridgelines on northerly aspects during control work Thursday morning.