A strong storm beginning this morning will bring up to a foot of new snow today and an additional 1-2 feet tonight. This will rapidly load slopes and if the storm verifies, large natural avalanches will be likely along with likely human triggered slides. These avalanches will be large and could run far into runout zones. Areas closer to the coast, such as Portage and Placer valleys will see the majority of the snow, but heavy snow is also expected in Girdwood Valley, Turnagain Pass, Moose Pass and Seward. The Summit Lake area is likely to see lesser amounts (16-20″ storm total compared to 3+’ of storm total for the other zones).
Storm snow avalanches will be the primary concern as the day progresses. Avalanche danger is directly related to how much new snow has fallen combined with increasing winds loading slopes. The new snow will fall on a few inches of light powder that sits over hard wind affected surfaces. This set-up creates a slick surface for the new snow and bonding is expected to be poor. It is a day to be mindful of changing conditions and leaving the mountains before sun down.
Types of storm snow avalanches that are expected to be occurring naturally tonight:
– Wind slabs: 1-3 feet of new snow combined with strong Easterly winds = wind slabs up to 4-6′ thick
– Storm slabs: Soft slabs are expected in areas out of the wind due to rapid loading and poor bonding with the old surface
– Loose snow sluffs: Sluffing in the new snow on steep slopes is expected
– Cornice falls: Fresh cornices are expected to build and break off during the storm, often this will trigger a wind slab or storm slab below
Forecaster excercise: Imagine 2-3+ feet of new snow by tomorrow morning, with wind, falling on the slopes pictured below. We are powder starved and in need of snow, but sticking well out of avalanche terrain (including runout areas) as the storm sets in will be key.
Seattle Ridge, with the up-track on right of photo
Southwest face of Sunburst
New snow and wind loading could overload old weak layers buried in the existing snowpack. In the case this occurs, or a storm related avalanche ‘steps down’ into a deeper weak layer, avalanches could be very large and send debris well into valley bottoms. Another reason to let the mountains sit during the storm. Areas on the South end of the Pass and the Summit Lake area are most suspect for having deeper weak layers release.
Weak faceted snow on a crust sits about a foot below the surface in the mid-elevations in the Summit Lake region. An example of weak layers in the pack that could become overloaded and release during the storm
Partly sunny skies with valley fog was over the region yesterday. Ridgetop winds were light from the North and temperatures were in the teens along ridgelines and in the mid 20’sF at 1,000′. Overnight, winds shifted to the East and are increasing along with temperatures as a small, but strong, low pressure develops South of Seward.
Today, Thursday, a quick and potent storm will move in with heavy snowfall and strong wind. Snow has just begun this morning and by 6pm this evening should bring 8-14″ (.5-.7″ water). From 6pm to 6am tomorrow, another 12-24″ (1-1.5″ water) is expected. Favored areas will be Placer and Portage Valleys with Summit Lake likely seeing the least in these ranges. Ridgetop winds will be Easterly in the 30-50mph with stronger gusts. Temperatures should remain close to 30F at sea level (#snowtosealevel) and in the low 20’sF along ridgetops.
Quote from the NWS’s Special Weather Statement:
Depending on the track the low center takes, the road system from Seward to Portage could see snow accumulations anywhere from one to three feet through Friday evening. Heaviest snowfall is expected to be from Moose Pass southward.
Friday, this system will slowly exit and light snow showers are expected. Ridgetop winds are forecast to remain strong from the East and South however. Periods of light snowfall should continue through the weekend as a Southerly flow keeps pumping moisture into our area.
|Temp Avg (F)||Snow (in)||Water (in)||Snow Depth (in)|
|Center Ridge (1880′)||22||trace||0.05||67|
|Summit Lake (1400′)||20||0||0||29|
|Alyeska Mid (1700′)||23||trace||0.05||59|
|Temp Avg (F)||Wind Dir||Wind Avg (mph)||Wind Gust (mph)|
|Seattle Ridge (2400′)||20||SE||5||21|
|12/10/19||Turnagain||Avalanche: Tincan and Sunburst from the air||CNFAIC Staff|
|12/10/19||Turnagain||Observation: Tincan||Nancy Pfeiffer|
|12/08/19||Turnagain||Avalanche: Tincan||Ryan Van Luit Forecaster|
|12/06/19||Turnagain||Avalanche: Sunburst||Billy Finley|
|12/04/19||Turnagain||Observation: Sunburst||A.Johnston-Bloom/ W.Wagner/ R.Van Luit Forecaster|
|12/03/19||Turnagain||Observation: Hippy Bowl||Nick Langowski|
|12/01/19||Turnagain||Observation: Tincan, All elevations||Eric Roberts|
|12/01/19||Turnagain||Observation: Tincan||Andy Moderow|
|11/30/19||Turnagain||Observation: Tincan Treeline Plateau/ Common Bowl/ Ridge||Eric Roberts|
|11/29/19||Turnagain||Observation: Sunburst Ob #2||Aleph Johnston-Bloom Forecaster|
Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: email@example.com
|Area||Status||Weather & Riding Conditions|
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