Avalanche Forecast

Issued: Feb 9th, 2019 10:03PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

UPDATE: WE EXPECT VERY DANGEROUS AVALANCHE CONDITIONS DEVELOPED OVERNIGHT, ESPECIALLY IN THE WENATCHEE MOUNTAINS AND AREAS NEAR HIGHWAY 97. AVOID TRAVELING IN LOCATIONS WHERE AVALANCHES CAN START, RUN, OR STOP. AVALANCHE MAY BE VERY LARGE, DESTRUCTIVE, AND EASY TO TRIGGER. HELP US SPREAD THE WORD BY TELLING YOUR FRIENDS AND OTHERS YOU SEE IN THE MOUNTAINS. 

Sunday has avalanche accident written all over it. The biggest snowstorm of the year just came through the valley. The pre-existing snowpack was an ugly setup for all this new snow, and very deep slabs have formed. Be very cautious of any steep slopes from the foothills to the mountaintops. 

Summary

Discussion

 Snow and Avalanche Discussion

UPDATE: ADDITIONAL SNOW FELL OVERNIGHT, ESPECIALLY IN THE WENATCHEE MOUNTAINS AND HIGHWAY 97 CORRIDOR. DURING THE SAME TIME PERIOD, THE SNOW HAS BECOME MUCH MORE SLAB-LIKE, WHICH WILL MAKE IT VERY EASY TO TRIGGER AVALANCHES. THIS IS AN UNUSUAL SET-UP WHERE THE AVALANCHE DANGER IS HIGHEST AT MID-ELEVATIONS. THE WEAK-LAYERS WE'VE BEEN TALKING ABOUT FOR WEEKS NOW HAVE A SLAB OVER THEM. THIS IS A CLEAR RECEIPT FOR AVALANCHES. MAKE SURE TO TALK TO YOUR FRIENDS AND THOSE YOU SEE WHILE YOU TRAVEL. 

This was the biggest single storm to impact the zone this season. Up to 23" of snow fell in the area and counting, with Mission Ridge getting the most as of 1700 on Saturday. The snow was so light, and the winds so strong that the weather stations did a poor job of measuring the water weight of this new snow. This creates an uncertain avalanche forecast. What we do know is that any of Sunday's avalanches could be big enough to bury or kill you. A widespread weak layer consisting of facets over a crust has been documented across the region. In addition to the new snow, deeper, old snow layers are a very real concern, especially in the Wenatchee Mountains and eastern Entiat Mountains near and east of Highway 97.

Most avalanche accidents occur with Considerable Danger. Sunday’s danger is a solid Considerable at all elevations. Avalanches may be possible in unusual places like steep sagebrush foothills. Be aware of your surroundings and ask yourself, "Am I in avalanche terrain? Could the snow slide?" Traveling one at a time is good practice, but remember, it does not eliminate the hazard of choosing to enter avalanche terrain. Slopes of less than 30 degrees, and places well away from steep open slopes would be my preferred terrain choice for Sunday.  

Image courtesy of Bruce Tremper.

Snowpack Discussion

Coming Soon.

Problems

Wind Slabs

An icon showing Wind Slabs

Full whiteout conditions occurred in many places, with sustained winds in the 60mph range and a gust of 87mph on Saturday at Mission Ridge. This creating areas devoid of new snow, and areas of very thick slabs. A low level barrier jet set up over the Columbia with strong winds from the foothills to mountaintops. Dangerous wind slabs may be found in steep gullies and ravines even in sagebrush country above Wenatchee. This is an unusual situation that calls for the utmost caution. While the winds may have tapered off, the wind slab instability is primed and ready for a person to easily trigger. You may be able to trigger avalanches from a distance as well. Stay off of slopes steeper than 30 degrees that are freshly loaded by the wind. 

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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Persistent Slabs

An icon showing Persistent Slabs

These are seriously dangerous times in the mountains. As time goes on, the new snow will settle into more of a cohesive slab. The shear weight of this slab is likely stressing weak layers deeper in the snowpack. Persistent slab avalanches will be most likely in the Wenatchee and Entiat Mountains, where they could break deep, or all the way to the ground. Watch for cracking, listen for collapses, or dig down to look for these layers of concern. If you can find layers of facets of surface hoar, all you need is a slab on top (which we have in most places with the recent storm), and you’ve got a recipe for dangerous avalanches. This structure may be most dangerous in the 5,000 to 6,500ft band, and on west to north to east aspects where weak snow is well preserved.

Was this past storm enough to reawaken old persistent weak layers in the ranges of the East Cascades? We don’t know yet.

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

Likely

Expected Size

1 - 2

Valid until: Feb 10th, 2019 10:03PM