Avalanche Forecast
Issued: Jan 4th, 2020 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
Dangerous avalanche conditions should become more widespread on Sunday with deeper slab avalanches possible on steep wind loaded slopes near and above treeline. Choose lower angled and non-wind affected terrain to avoid triggering an avalanche Sunday.
Summary
Discussion
Reports from Saturday indicated that the 5-7" of fresh snow that fell with rapidly cooling temperatures Friday night was easily transported by sustained strong westerly winds. Small wind slab avalanches were easy to find and trigger near treeline as well as on open slopes below treeline Saturday morning. Winds were strong enough that loading patterns were erratic with snow surfaces stripped down to the most recent crust adjacent to areas of wind drifted snow.
Forecaster Peter Moore found the New Year's Day crust easily identified in the upper snowpack. A stout crust with some weaker snow above creates the potential interface for new snow to build and fail on moving forward. During the heavy NYE rain event, water penetrated deep into the snowpack.
You don't need a loupe to pick out the most recent thick crust that was buried New Year's Day. Photo: Peter Moore 1-3-20
Snowpack Discussion
Happy New Year!
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.
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 …
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.
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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.
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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.
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.
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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.
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?
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Read the forecast. This is a great way to monitor conditions even if you aren’t heading into the mountains.
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Get out in the snow! Enjoy the wonderful mountains in your backyard.
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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.
Thanks for all of your support in 2019 and here’s to 2020!
-Dallas
Problems
Wind Slabs
A similar weather forecast is leading to similar avalanche danger and problems for Sunday. Dangerous avalanche conditions should continue on all steep wind loaded slopes near and above treeline with another round of new snow and strong winds forecast. Watch for cracking in the snow as a clear sign of instability and avoid open slopes steeper than 35 degrees that have firmer wind drifted snow. It's on larger open slopes with deeper drifted snow that avalanches may be large enough to injure, bury, or kill a person.
Winds are forecast to be strong enough that wind slabs may form further downslope and below ridgelines than usual and may form on open slopes below treeline.
With cool temperatures continuing, storm slabs are not expected to develop at lower elevations in more wind-protected areas. If you find more than 6" of new snow below treeline, approach smooth and steep unsupported slopes with caution. Test how the new snow bonds to the old snow surface on small inconsequential test slopes.
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
Expected Size
Valid until: Jan 5th, 2020 10:00AM
