Avalanche Forecast Cascades - East

Date Issued: Valid Until:

Dennis D'Amico,

Avalanche Forecast

Sat Jan. 19th · 7:11AM

Alpine

Danger Ratings High

Treeline

Danger Ratings High

Below Treeline

Danger Ratings Considerable

Problems

Persistent Slabs Persistent Slabs
Wind Slabs Wind Slabs

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
The Bottom Line: Very dangerous conditions have developed in the East Central Cascades. Incremental loading is beginning to stress multiple persistent weak layers to their breaking point. This is the time to step back and avoid recreating in avalanche terrain.

Snow and Avalanche Discussion

The snowpack near and east of Highway 97 in the Wenatchee Mountains may become extremely dangerous this weekend. Old persistent weak layers appear to be groaning from a mere 7” of snow that fell on the 17th, and will continue to get more heavily loaded into Saturday. A recently buried layer of surface hoar and facets may produce widely propagating avalanches throughout Eastern Cascades. 

Toward the end of the cold and dry period, shaded aspects harbored surface hoar and facets east of the crest. One observer reports skiing on a ‘glass carpet’ of surface hoar between 4,000 and 5,500ft. Another noted ‘dramatic faceting’ near and below treeline on shaded aspects. The new snow doesn't appear to be bonding very well, especially where it sits over facets and surface hoar. A few natural avalanches occurred overnight on the 18th at Mission Ridge, these ran on a crust from early January. Observers have noted loud whumphs and collapses near Blewett Pass the past few days. As the snow accumulates with warming temperatures and the winds blow into Saturday, we will create a cohesive slab and further stress these layers. More on the recently buried snow surface in this observation here.

 
Recently buried layer of large surface hoar (1/17). Where a slab sits over this layer, expect dangerous conditions to exist.


Weather Forecast

Sat 19th Jan 14:37 - Dennis D'Amico

Weather Synopsis for Saturday night through Monday

Leftover showers should wind down this evening as a weak low pressure system develops along the old frontal boundary to our south. This system will develop and lift north Sunday morning, spreading light rain and snow into Mt. Hood and perhaps the south Washington Cascades. Clouds and light precipitation should decrease near and north of Snoqualmie Pass in the afternoon as the system pulls further east.

Shortwave ridging anchored offshore will flop into the region late Sunday night and Monday making for a fair weather day. Relatively cooler air will filter into the region Monday and no significant winds are expected over the short term.  

Regional Synopsis

Wed 16th Jan 09:00

January 16, 2019

Since Friday January 11, we’ve enjoyed a spell of generally nice weather in the mountains. This allowed for some great views, enjoyable outings, and lots of snow observations. A more active weather pattern beginning Thursday January 17th will bring this nice weather to a close.

During this time period, the snow surfaces around the area have changed dramatically. This forms the foundation for a few current trends we are seeing in the mountains.

New Snow Problems

Reports from around the area indicate a wide variety of snow surface conditions prior to new snow on January 17th. We’ve heard about breakable crust, very icy surfaces, sugar facets, surface hoar, and rime. What snow surface you encounter can depend on aspect, elevations, and general location.

As a series of winter storms impact the area, how will the new snow bond to the old snow surface. This can be tricky to predict. As the snow starts to pile up make lots of shallow snow observations. Shovel tilt test, hand shears, and small slope test can all help you track how the new snow is bonding to the old snow at different aspects and elevations. Stop and take a look at where the snow is failing. Do you see large grains of snow? Do they look like feathers? Do the act like sugar? How far below the current snow surface are they?

Old Snow Problems

There are still lingering persistent weak layers in the eastern zones of the cascades. You are most likely to find weak older snow in areas further east from the Cascade crest where snowpacks are shallower, more variable, and generally weaker. In some locations weak snow near the ground can still be found. These basal facets have hung around all season. The only way to gain information about these old persistent weak layers is to get out your shovel and dig. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.

Problems

Persistent Slabs

Persistent Slabs

The snowpack has become ripe for large, human triggered avalanches. There are multiple persistent weak layers that all have the potential to fracture and avalanche with this incoming storm. These layers are tricky because they’ve sat around for weeks not producing avalanches. Just the past few days they have been incrementally loaded and appear to be ‘waking up’ with large whumphs being reported near Blewett Pass -even as low as 5,000ft. They are likely to be doing the same in other areas on the eastern edge of the range where less than 3 or 4 feet of snow is on the ground. Slides could fail a few feet down, or on the ground, taking the entire seasons snowpack with it. Slides could be very wide and encompass entire terrain features. Don't get lured onto slopes steep enough to avalanche, don't be the trigger. Avoid runout zones below steep slopes, as it may be possible to trigger slides from below as well. 

Further west, the main layer of concern will be surface hoar that has been recently buried by new snow in the past few days. Slides may wrap around terrain features and surprise you and your group.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 3

Wind Slabs

Wind Slabs

The winds will rip near ridgelines on Saturday morning. These, combined with a burst of new snow will form dense slabs, increasing the avalanche danger. Anticipate that these slabs won’t bond well with the old, weak snow surface that recently got buried. Look for shooting cracks within the new snow. Does the snow sound hollow? Are there deep dunes of drifted snow? What is underneath that slab, is there strong over weak? If so, it is time to avoid slopes steep enough to avalanche.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 2