Avalanche Forecast

Issued: Jan 18th, 2017 11:09PM

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

NWAC, Northwest Avalanche Center

Cascades - West.

The snowpack in the Olympics and Cascades is undergoing a lot of change and there is less than the usual certainty in which avalanche problems and danger levels to indicate. It might be a good plan to wait until Friday when conditions are better known and should be more stable.

Summary

Detailed Forecast

Winds and snow showers will decrease Wednesday night and Thursday with cooler temperatures.

Heavier storm snow should be seen above 5000 feet in the north such as at Mt Baker and above 6000 feet in the south such as at Mt Rainier by Thursday with lighter storm snow in the Cascade Passes.

The greatest likelihood of new wind slab should be at higher elevations on the volcanoes where storm snowfall is heaviest. But bed surfaces such as underlying freezing rain crusts may make wind slab sensitive in the Cascade passes.

New storm slab will be most likely in any areas that experience rapidly accumulating snow of more than an inch an hour for several hours. Bed surfaces such as underlying freezing rain crusts may make storm slab sensitive as well in the Cascade passes.

Wet slab or glide avalanches seem possible for another day in the below treeline on Thursday while water from the atmospheric river drains from the snowpack. Wet slab avalanches will be indicated as an avalanche problem in the below treeline since most reports on Wednesday indicated wet slab avalanches. Avoid areas below unsupported steep terrain without trees especially if the slope is known to have a smooth ground surface.

Snowpack Discussion

Weather and Snowpack

An Arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals probably formed in most areas during that period.

A drastic change was seen when an atmospheric river moved to the Northwest Tuesday and Wednesday. This caused heavy rain up to about 5000 feet in the north and up to about 6000 feet in the south with snow at higher elevations. Sleet or freezing rain Tuesday has been followed by a change to snow at higher elevations at Stevens and Snoqualmie on Wednesday. An avalanche cycle is expected to have occurred in some or many areas.

Observations the next couple days should help determine to what extent any of the surface hoar and near surface faceted crystals from the fair cold weather might have survived the atmospheric river event.

Recent Observations

NWAC pro-observer Dallas Glass was at Stevens Pass on Tuesday and reported that the sleet and freezing rain had already formed a 1/4 inch crust over the snowpack by midday.

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 feet with a 1-2 foot x 100 foot crown. Explosive control gave a very large wet slab with a 3 foot x 1000 foot crown above the road to the ski area on a NNW slope at about 4000 feet. Another wet slab was seen on a west slope at about 4000 feet. Lastly a wet slab ran starting at about 5000 feet 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.

Problems

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.

Likelihood

Likely

Expected Size

1 - 1

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Wet Slabs

Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.

Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty

A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.

Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 19th, 2017 11:09PM