Issued: Sat, Dec 16, 2017 at 8AM

Expires: Sun, Dec 17, 2017

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Final 11/22 avalanche accident report here

Above 3,500ft Moderate

2,500 to 3,500ft Moderate

Below 2,500ft Low

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

BOTTOM LINE

A moderate avalanche hazard exists for wind slabs and persistent slabs at mid to upper elevations on wind loaded and leeward aspects, generally West to Northeast, on slopes 35º and steeper. Natural avalanches unlikely, human triggered possible.

Whumphing (audible collapsing) and shooting cracks are immediate signs of potential avalanche danger, and indicate buried weak layers failing.

Winds from earlier in the week transformed the snowpack, scouring all raised features, and re-depositing and cross loading the snowpack into gullies, and depressions. The height of snow varies greatly from 0.5 to 3 feet deep.

5 hours and 21 minutes of daylight today. Sunrise at 10:12am, sunset at 3:33pm.


WIND SLAB AVALANCHE PROBLEM

Wind slabs, 3 -9″ deep, are stubborn to touchy to trigger. Natural avalanches are unlikely and human triggered are possible in specific areas, at mid to upper elevations, on leeward aspects and on cross-loaded features, generally West to Northeast, on slopes 35° and steeper, and they will be small, D1, in size.  Without much snow left available for transport, further wind slab development will be stunted.

Where you find the most snow will also be the same locations where you find wind slabs. Even small wind slabs may have the ability to wash you off your feet and into terrain traps, rocks, cliffs, and other hazards. Choose specific terrain wisley.

 

PERSISTENT SLAB AVALANCHE PROBLEM

Our snowpack with poor structure contains buried persistent weak layers (PWL’s) in the mid-pack. Expect these PWL’s to continue to fluctuate through activation and inactivity throughout the season. Each time the snowpack is rapidly loaded the balance will be tipped, and avalanches will occur until the snowpack slowly adjusts to the new load.

Currently the avalanche hazard for persistent slab avalanches is moderate; natural avalanches unlikely, human triggered avalanches possible. Strong winds this week (peak gust ESE 65 mph on Dec 11) scoured and removed the avalanche problem in specific locations, on raised features, such as ridgelines, leaving the basal melt freeze crust and less than a foot of snow in place.

However, the persistent slab problem is located in other specific areas, including loaded features, such as any depression or gully, and generally on leeward, West to Northeast aspects, on slopes above 35º at mid and upper elevation- and anywhere the wind was not able to rip it apart. They will be stubborn to trigger and may break above you. PWL’s are buried 1-3 feet. The potential size of avalanches will vary greatly, as the snowpack depth is extremely variable. Expect avalanches up to D2 in size, large enough to bury, injure or kill a person.

Avoid slopes with terrain traps. Use safe travel protocol.

CORNICE WARNING

While cornices are not an avalanche problem today and have not reached full size yet, they are precariously overhanging due to strong winds. With poor visibility today, be cautious around leeward ridgelines where stiff cornices will be strong enough to allow you to step out on them, but may fail. Don’t walk the plank.

 

General representation of the snowpack

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

winds transporting 2″ of new snow on Dec 15

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dec 15 – Wind transporting snow to leeward aspects

 

Avalanche danger scale here

Recent Avalanche Activity

Remnants of recent avalanche activity from earlier in the week, caused by rapid wind loading, can easily be seen in some places at Hatcher Pass, including Rae Wallace and the SW face of Marmot. These avalanches were approximately D2 in size.

Dec 11 – Rae Wallace N, 4500′ – Old SS-N-D2-O, crown depth est. 2′ deep max

Dec 11 – SW face Marmot, above the death gully (severe terrain trap), SS-N-D1.5-2-O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dec 8 – ENE, 2800′, Fishhook Creek

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

See OBSERVATIONS for more information.

Recent Weather

This week’s weather at 3550′:

Temps averaged 31ºF, with a low of 21ºF and a high of 40ºF.

2″ of new snow was recorded at IM snotel this week.

Overnight at 3550′:

Temperature averaged  27° F.

1″ new snow overnight.

This week’s weather at 4500′:

Temps averaged 25ºF, with a low of 17ºF and a high of 32ºF.

Winds averaged SE 16 mph, max 44mph . Gusts averaged SE 26, max gusts SE 65 mph.

Overnight at 4500′:

Temps averaged  21ºF overnight.

Winds averaged SE 19 mph overnight, with a max gust of 50 mph.


NWS recreational forecast for Hatcher Pass here


NWS point forecast here


State Parks snow report here

Additional Info & Media

A small amount of new snow today should not increase the avalanche hazard. Even if winds increase today, there is not much available snow left to transport to significantly increase the avalanche hazard.

 


Final 11/22 avalanche accident report here

Posted in HPAC Forecasts.
Allie Barker

Forecaster: Allie Barker