Avalanche Forecast
Issued: Dec 27th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Use visual clues to identify areas where winds drifted snow into firmer and thicker slabs. Steer around wind-loaded slopes steeper than 35 degrees at upper elevations where it is possible to trigger a large avalanche.
Summary
Snowpack Discussion
December 26, 2019 (The regional synopsis is updated every Thursday @ 6 pm)
No matter where you are in the northwest, the snowpack looks a world different than it did a week ago. A major storm, widespread avalanches, developing weak layers, and an initially thin snowpack made for a hectic week. Let’s take a look at a few large scale trends for our holiday season.
A very large natural avalanche (D3) on the Shuksan Arm near Mt Baker that occurred during the storm cycle. Photo: Adam U.
Wet and Wild
“Some of the highest precipitation rates I’ve ever seen.” “Absolutely puking!” “Rivers running in the streets.” “Snowing snow hard I can’t see my hand in front of my face.” These were all statements we heard describing the wet and wild storm that impacted the Northwest from about Thursday (12/19) through Sunday night (12/22). Water totals were staggering in many locations (Table 1). For Hurricane Ridge, Mt Baker, Washington Pass, and high elevation terrain this deluge translated into significant snowfall. However, warm air in the central and southern parts of the region brought rain well into the near treeline bands. Whether your favorite spot saw rain or snow, four things are clear. 1. This was a huge loading event. 2. A widespread natural and triggered avalanche cycle (up to D3) occurred. 3. Avalanche danger spiked during and just after the storm before trending down through the week. 4. The snowpack changed dramatically.
Location
Precipitation 12/19-22
Hurricane Ridge
4.97”
Mt Baker Ski Area
7.35”
Washington Pass
3.53”
Stevens Pass
6.28”
Leavenworth
2.94”
Snoqualmie Pass
7.89”
Crystal Mt Ski Area
7.45”
Paradise, Mt Rainier
6.57”
Mt Hood Meadows
2.18”
Table 1: Precipitation totals for select weather station locations December 19-22.
The Emergence of Persistent Slab Avalanche Problems
Coming out of this huge loading event, there was hope that many of the early season weak layers had been destroyed. Unfortunately, this was not the case in some locations. Old weak snow layers reared their heads in areas east of the Cascade Crest and near Crystal Mountain. While the exact extent and character of these layers can differ slightly, most locations are finding a layer of weak sugary facets associated with a crust about a foot above the ground. Persistent slabs are tricky to assess. Do not solely rely on snowpits and snowpack tests to help you choose terrain. How will these layers change going forward? Only time and observations will tell.
You may find weak sugary facets near a crust about 12 inches above the ground similar to what you see here. Photo: Jesse Charles
Low Tide Snowpacks During the Holidays
The storm this past week definitely helped the meager early-season snowpacks in all locations. Areas like Hurricane Ridge, Mt Baker, and Washington Pass experienced a jump of 20 or more inches in their snow depths over the past week. However, even with these increases, an early season snowpack still describes most locations. Expect numerous obstacles as you travel including open creeks, rocks, and trees. With this thin snowpack, limited access, and difficult travel we still have limited observations in some regions.
Open, deeply incised creeks in the Alpental Valley. Photo: Dallas Glass
Lack of information leads to a higher than normal degree of uncertainty. If you travel to higher elevations or more remote trailheads, recognize you could experience different conditions than the forecast suggested. You can help us fill in the gaps by submitting your observation here.
Happy Holidays!
-Dallas Glass
Problems
Wind Slabs
Although we’ve only picked up a few inches of new snow, southerly winds are efficiently transporting snow and building fresh slabs. Pay attention as you climb up in elevation and transition into wind affected terrain. Blowing snow, fresh cornices, and hollow sounding snow are all good indications that wind slabs likely exist nearby. It may be easy to trigger an avalanche where wind slabs form on weak old snow surfaces, particularly on north and east aspects at upper elevations. These may not be your run of the mill wind slabs and could surprise you. Gather information as you travel, and carefully assess the old/new snow interface. When in doubt, default to lower-angled terrain.
In sheltered areas away from the wind, loose dry avalanches are possible. With unconsolidated surfaces, these avalanches could entrain snow as they move downslope and grow larger.
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
Expected Size
Valid until: Dec 28th, 2019 10:00AM
