Avalanche Forecast
Regions: Cascades - East.
The avalanche danger will be on the rise Monday with very dangerous avalanche conditions developing at upper elevations in the western part of the zone. In these areas, avoid traveling on or in terrain connected to large open slopes capable of producing large avalanches. You are likely to trigger a shallow slab on steep unsupported slopes or convex rolls at lower elevations.
Discussion
We know that storm or wind slab avalanches will become increasingly likely overnight through Monday as denser snow builds quickly on top of the colder drier snow received recently. What we don't know is if spotty reports of buried weak layers above 6000' will become reactive over this period. Previously, observers had 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 elevation. We have a large amount of uncertainty regarding the reactiveness of these layers heading into an extended storm cycle.Â
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
Storm Slabs
West winds have been loading the same aspects over the last few days. These same aspects will be continually loaded by a stronger storm producing more snowfall through Monday. As the chances of natural or human triggered avalanches increases, avoid exposing yourself to any terrain capable of producing large avalanches. For areas further east like the Wenatchee Mountains, there will be plenty of wind but far less snow to build thicker slabs.
In less wind affected areas, watch for shallow storm slabs to develop on any steep unsupported slopes. Watch for clear signs of instability like shooting cracks as you travel or recent avalanches.
At lower elevations where snow turns to rain, loose wet avalanches are possible on steep slopes. Remember that even small loose wet avalanches that release at lower elevations can be more powerful than you expect and can push you into trees, rocks or other obstacles.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 1