Avalanche Forecast

Issued: Feb 13th, 2019 11:18AM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Persistent Slabs and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

If you venture in the backcountry be certain of your ability to avoid avalanche terrain. Wind and more snow will stress the snow and make it easy to trigger very large and life-threatening avalanches. Continue to avoid steep slopes and put plenty of space between where you travel and large avalanche paths.

Summary

Discussion

Snow and Avalanche Discussion

Observers have reported a widespread and prolonged cycle of avalanches involving a variety of aspects and elevations. Common characteristics of these avalanches are very widely propagating crowns about 3 feet deep. Local ski patrols, highway workers, and backcountry travelers all reported extensive avalanching on the night of the 11th, into the 12th, with triggered slides and a few naturals continuing into the 13th. While some avalanches ran in storm layers in the upper snowpack, many appear to be running an old weak faceted layer over 3 feet below the surface. Visibility and dangerous conditions have limited our ability to gather more detailed information.

Mission Ridge has received over 40 inches of snow since February 8th. Low elevation terrain, including slopes close to towns are now locations of concern for avalanches. On Tuesday, forecaster Matt Primomo reported natural avalanches and found a very weak and avalanche prone snowpack on the Rooster Comb, on the outskirts of South Wenatchee. Expect similar conditions near the towns of Peshastin and Leavenworth.  In addition to avalanches, all the deep snow is building concern for Snow Immersion Suffocation, or tree well hazards higher in the mountains. Travel with partners and keep them in sight.

A natural avalanche in the Icicle Canyon. Grindstone Mtn, SE, 6,600ft. Photo: Matt Primomo

Snowpack Discussion

Regional Synopsis coming soon.

Problems

Persistent Slabs

An icon showing Persistent Slabs

Today is not the day to travel in avalanche terrain. If you venture into the backcountry, you must be confident in your ability to put enough distance between you and slopes that could produce life-threatening avalanches. Stay far out from under big avalanche paths and make sure not to get too close to start zones. Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. These avalanches are difficult to predict and break in ways much wider and in more surprising ways than avalanches that we typically see in the Northwest. 

The snowpack has seen a rapid and major change. Give the snow some time to adjust. Continue to stick to slopes under 30 degrees and smaller terrain features. Observers have consistently reported; 1) Remotely triggered avalanches. 2) Widespread collapsing of the snow underfoot or a "whumphing" sound. 3) A layer of weak, sugary facets and surface hoar resting on a crust 4-5 feet below the snow surface. Watch for these signs indicating that you can trigger a Persistent Slab avalanche.

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 2

Wind Slabs

An icon showing Wind Slabs

Easterly wind will drift snow at upper elevations, forming fresh slabs and further stressing an already tenous snowpack. Avoid any slope over 30 degrees with drifts or where you see the wind blowing snow. Avalanches on wind loaded slopes could be up to 6 feet deep. 

In sheltered areas you can still trigger large storm slab avalanches in the snow that fell in the past week. The upper layers may slide easily. Today, you can't simply avoid or manage the storm instabilities. Any avalanche triggered in the recent snow could result in a larger and more deadly persistent slab avalanche.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Feb 14th, 2019 11:18AM