Avalanche Forecast
Regions: Cascades - East.
Another round of snow with strong winds will create deep drifts at upper elevations. Large avalanches can be initiated on steep slopes where thick slabs of wind deposited snow may be found. There is quite a bit of uncertainty with the upper elevation snowpack structure. Best to avoid steep slopes that are actively filling in with drifted snow.
Discussion
On Friday I observed a couple of recent, large wind slab avalanches in Icicle Creek (see photo below). During the day on Friday, limited snow was observed being transported as the day progressed and temperatures warmed up. This was likely due to wet snow that fell up to at least 5,000ft. An observer on Thursday at Mission Ridge found multiple layers of buried surface hoar stacked between wind slabs on a northeast aspect at 6,500ft in Stemilt Basin. Uncertainty is fairly high regarding how widespread these potentially problematic layers are, though tests indicated these layers could initiate fractures, and she did get some whumphing collapses on low angle slopes.
A large human triggered avalanche occurred near Washington Pass on Thursday. A skier remotely (from a distance of ~150ft away) triggered a 4ft deep avalanche on an east aspect at 7,000ft above Hwy 20. No one was caught or carried, but check the East North zone for more.Â
Recent crowns from natural wind slab avalanches in Big Slide Creek in Icicle Creek. East at ~7,400ft, January 3, 2020. Matt Primomo photo.
Snowpack Discussion
Happy New Year!Â
The pacific northwest rung in the new year with a winter storm cycle that brought changing conditions to our region. This storm exhibited three characteristics: warm, wet, and windy. Let’s take a look at the end of 2019, the beginning of 2020, and where we can go from here.Â
Wrapping up 2019
The last few days of December were generally quiet and cold. The snowpack seemed to enjoy this break in the weather. Lingering unstable snow from the pre-Christmas storms gained strength, persistent weak layers appeared to stabilize, and avalanche hazard decreased in all forecast zones. All in all 2019 ended quiet and uneventful… until the New Year’s Eve weather party showed up … Â
Quiet weather led to lower avalanche danger during the last week of 2019. Hogsback, White Pass, WA. Photo: Andy Harrington
Blowing into 2020
A warm, wet, and windy weather system blew into the northwest for New Year’s Eve and New Year’s Day. This brought rapidly changing conditions and increasing avalanche hazard to all areas.Â
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Warm: Unfortunately this system brought with it warm air. Freezing levels measured near the coast reached 9000’ on the afternoon of the 31st. Many weather stations recorded above freezing temperatures during the onset of precipitation.Â
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Wet: While this system wasn’t as wet as the atmospheric river prior to Christmas, it still produced impressive water numbers in many areas. The bulk of the precipitation seemed to be focused on the Passes and Volcanoes, and water spilled over the crest to places like Washington Pass and Leavenworth. Sadly, when combined with the warm temperatures, this translated to rain well into the near treeline band (or higher) for most areas. The main exception appeared to be in the northeastern cascades, where locations like Washington Pass remained all snow.Â
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HurRidge
MtBaker
WaPass
Stevens
Leavenworth
SnoqPass
MtRainier
MtHood
Precipitation (in)
1.84
3.39
1.42
4.56
1.06
6.19
5.39
5.41
Snow (in)
-
15
-
12
0
3
10
8
Table 1: Precipitation and storm totals from selected weather stations during the New Year’s Eve Storm. “-†24hr storm snow not measured.Â
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Windy: While the warm and wet were impressive, it’s the winds that may set this storm apart. Most weather stations recorded very strong and extreme winds during the storm. Alpental exceeded 100mph just after midnight to ring in the new year. Any dry snow at high elevations was redistributed by the wind and snow surfaces were transformed.Â
Table 2: Wind speeds from New Year’s Eve from selected wind sites. Note the sustained period of winds between 40-60mph.
Eventually, temperatures cooled, the rain turned back to snow, and winds calmed. Many locations picked up additional snow as the storm wound down, but 2020 was already off and rolling with its first major storm.
New Year’s Resolutions
The active weather pattern that started the new year appears to continue. The snowpack and avalanche conditions will continue to change. So, what can your New Year Avalanche Resolutions be?Â
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Read the forecast. This is a great way to monitor conditions even if you aren’t heading into the mountains.Â
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Get out in the snow! Enjoy the wonderful mountains in your backyard.Â
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Submit an observation. Tell the avalanche center what you saw while out in the snow by submitting an observation and sending in a photo.Â
Thanks for all of your support in 2019 and here’s to 2020!
-Dallas
Â
Avalanche Problems
Wind Slabs
Another round of cold snow and gale force winds will create sensitive new wind slabs. These drifts will be deeper and more significant at higher elevations, and closer to the crest where more snow will accumulate. Check to see how these slabs are bonding with the layers below. Are you experiencing shooting cracks through the snow or getting whumphing collapses? Is the wind actively transporting snow? Look for recent avalanches, and use small test slopes to check how easily the snow can slide.
We have limited information for much of this area regarding existing snowpack structure. It will be necessary to be skeptical and make your own observations as you go. In some areas, deeper weak layers may be a concern. Slabs may or may not exhibit direct signs of instability before releasing, so think twice before entering any features that appear heavily duned by the wind. Consider the consequences before entering slopes steeper than 35 degrees. Be extra cautious beneath steep open slopes that are getting top loaded, below ridgelines, near the edges of cross loaded gullies, and near terrain traps and strainers, such as stands of trees.
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