Avalanche Forecast

Archived

Feb 9th, 2019–Feb 10th, 2019

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

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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.

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

Wind Slab avalanches are the 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.