Avalanche Forecast
Regions: Cascades - East.
Snowfall and nearly continuous strong winds over the past couple of days have created thick slabs in the western half of the zone. Large avalanches are possible where wind deposited snow rests on steep slopes. Opt to avoid features that appear freshly drifted by wind.
Discussion
On Saturday I experienced blustery conditions and snowfall rates of about 1" per hour. There was 8" of storm snow sitting on the recently buried crust, which thins out and tapers away above 5,500ft in the Icicle. Winds were heavily loading east facing slopes on a north-south oriented ridgeline above 6,000ft. Here, I was able to easily initiate cornice failures, which caused loose dry avalanches that entrained more snow while moving downhill. More loose dry avalanches may be possilble on Sunday in sheltered areas.Â
Observers have identified a layer of buried surface hoar in very isolated areas (6,500ft on NE in Stemilt Basin, 6,400ft on E in Icicle Crk). This layer has had the chance to survive above ~6,000ft, as the New Years rain event melted it away below that. We have a fair amount of uncertainty regarding the distribution and reactivity of slabs that continue to develop above this layer. We know more about recent activity of similar layers in the East North Zone, where recent natural and human triggered slides up to 4ft deep have occurred. More snow has fallen there in the previous week. As snow continues to accumulate a high elevations, is it only a matter of time before this layer begins to get overloaded in the East Central zone?
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
Light snowfall and winds strong enough to transport it will continue on Sunday. These drifts are becoming more significant at higher elevations, and closer to the crest where it has been snowing more. Pay attention to how the snow changes as you enter higher elevation terrain. Look for recent avalanches, and use small test slopes to check how well the slabs are bonded to layers below. If you find shooting cracks or recent avalanches, best to steer around heavily loaded slopes. Remember to always consider the consequences before entering slopes steeper than 35 degrees. Be extra cautious beneath steep open slopes with cornices looming above, below ridgelines, near the edges of gullies, and near terrain traps and strainers, such as stands of trees.
In isolated areas above 6,000ft, a layer of buried surface hoar may become a concern. Make your own observations as you ascend above this elevation. Look for a thin grey stripe that may be found 1 to 2ft beneath the snow surface. Are you experiencing shooting cracks through the snow or hear whumphs? If so, heed these signs of instability and avoid travel on slopes steep enough to avalanche.
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: Possible
Expected Size: 1 - 1