Avalanche Forecast

Issued: Jan 29th, 2017 1:11PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The main avalanche problem in the northwest to southwest Cascades should be possible wind slab formed over the weekend mainly on northwest to southeast slopes near and above treeline.

Summary

Detailed Forecast

A weak upper trough should begin to linger over BC and the Northwest on Monday. West winds should generally decrease in the Olympics and Cascades by Monday. The trough will cause decreasing clouds in the Olympics and north Cascades but more clouds and some light rain or snow in the south Cascades mainly at Mt Hood Monday afternoon and night with further cooling.

The main avalanche problem in the northwest to southwest Cascades should be possible wind slab formed over the weekend mainly on northwest to southeast slopes near and above treeline. This problem may be greatest in the northwest Cascades. But watch for firmer wind transported snow on other aspects. Note the skier triggered wind slab avalanches that were reported on Mt Herman on Saturday.

Snowpack Discussion

Weather and Snowpack

An atmospheric river arrived over the Northwest Tuesday 1/17 with highly variable weather seen throughout the Cascades during this event. Heavy rain was seen up to about 6000 feet or higher in the North Cascades and up to about 7000 feet or higher in the south with snow at higher elevations. Sleet and freezing rain Tuesday 1/17 at Stevens and Snoqualmie changed to snow at higher elevations on Wednesday 1/18.  At Stevens and Snoqualmie Pass the freezing rain crust is about 1 inch thick.

A storm cycle from Thursday 1/19 through Sunday 1/22 deposited 1-2 feet of snow along the west slopes of the Cascades with the greatest totals seen at Mt. Baker.

A large upper ridge and warm air mass built over the Northwest Thursday to Saturday with temperatures reaching the 40s in many places along the west slopes away from the lower parts of Stevens and Snoqualmie. Strong southwest winds were seen Saturday in the Northwest Cascades, with moderate southwest winds in the central west Cascades and light winds in the southwest Cascades. East winds were seen at Stevens and Snoqualmie.

A front is crossing the Northwest on Sunday afternoon. The front is causing strong southwest winds in many areas with cooling. Rain or snow amounts should be light by Monday morning.

Recent Observations

North

An observation via email for Saturday came to the NWAC with 2 groups of skiers reporting 3 triggered wind slab avalanches of about 2 feet on N-E slopes at 5000-5800 feet on Mt Herman near Mt Baker with a burial to the waist of 1 person.

NWAC pro-observer Lee Lazzara was in the Bagley Lakes area on Saturday and reported wind slab formation. He also reported a large cornice failure on Saturday night onto a north slope of Table Mountain near Mt Baker at about 5000 feet.

Central

Reports on Turns All Year for the Snoqualmie Pass for Friday and Saturday indicated some good conditions still to be found on some north slopes.

A report from Stevens for Saturday is available via the NWAC Observations page. Trees were shedding snow but roller balls were not entraining snow. Small loose wet avalanches were seen on steep solar slopes. But N-W slopes still held good snow. The 1/17 crust was seen at 40 cm giving CTH RP results in small faceted snow on the crust.

A similar report is available for Sunday from NWAC pro-observer Ian Nicholson at Stevens where some small loose wet activity was seen in the morning. A layer of small faceted crystals that are becoming rounded at about 40 cm was not reactive on the 1/17 crust.

South

NWAC pro-observer Dallas Glass was at Paradise on Sunday up to about 6400 feet. He found a new surface crust from Saturday over about 15 inches of settled recent snow on the thick crust from 1/17. Shady slopes still held cold settled surface snow. Some wind transport was seen at higher elevations. Numerous size 1-2 wet slabs from Saturday of up to 6-10 inches were seen on solar slopes such as the east side of Alta Vista.

Wet slabs on Alta Vista at Paradise on Saturday 1/28. Photo by Michael Madden.

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, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 30th, 2017 1:11PM