Bulletin d’avalanche

Stevens Pass

Jan 2 – 3, 2020Northwest Avalanche Center
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Archivé · expiré 7 years ago

Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Points saillants

Expect a warm, wet, and dangerous day on Friday. Rising temperatures, snow, rain, and strong winds will create hazardous avalanche conditions at all elevations. To stay safe conservative terrain choices will be key, avoiding steep slopes until this warmup runs its course. 

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    Heavy, warmer snow over weak, colder snow will create storm slab instabilities near and above treeline. These may involve interfaces between recent storm layers, and may propagate widely at upper elevations where wind creates deeper drifts. Cracking in the snow or fresh avalanches are direct signs of instability. You can also use small, inconsequential test slopes to check how easily the snow may fracture and slide. 

    Use conservative terrain choices, meaning look to travel on slopes less than 30 degrees and avoid getting underneath steep open slopes. Be extra cautious around terrain traps such as gullies, rocks, and cliffs where even a small avalanche could have serious consequences.

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    Rain on fresh snow over a crust is a great recipe for loose wet avalanches. Wet snow may entrain more wet snow as it moves downhill on steep slopes, creating large debris piles. We now have enough coverage and new snow below treeline to be concerned with these in places where summer hiking trails exist. Rollerballs and recent, fan shaped avalanches with ball shaped debris piles are signs that you may be able to trigger these. Stay off of, and out from underneath steep open slopes to avoid these.

Discussion

Thursday was a relatively quiet day with 8-12" of new snow at Stevens Pass thanks to a Puget Sound Convergence Zone. Travel conditions were excellent, soft powder in the morning. The new snow was initially cold and uncohesive, limiting danger for storm slab avalanches. By mid-day, the next system arrived, bringing with it steady snowfall for the remainder of the day. The conditions are likely to become much more dangerous overnight as the temperatures rise and precipitation continues to fall. Expect a warm, wet, and generally dangerous day on Friday.

 

 

Snowpack Discussion

January 02, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

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. 

  • 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. 

  • 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

Leanenworth

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. 

  • 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? 

  1. Read the forecast. This is a great way to monitor conditions even if you aren’t heading into the mountains. 

  2. Get out in the snow! 

  3. 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

 

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