ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday November 25th, 2018
Posted by Wendy Wagner on 11/25/18 at 7:00 am.
The Bottom Line
High Avalanche Danger
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

A powerful storm moving through is expected to increase the avalanche danger to HIGH in the upper elevations today. Over 1′ and up to 2′ of snow accumulating by this afternoon should cause natural avalanches to occur.  Warming temperatures will also increase the likelihood of avalanche activity at these high elevations and bring rain to 2,200′.  The avalanche danger is CONSIDERABLE in the treeline band as avalanches from above may run into these mid-elevations. Travel above treeline is NOT recommended.

4. High
Alpine
/ Above 2,500'
Avalanche risk
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
3. Considerable
Treeline
/ 1,000'-2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk
Avalanche Problem 1
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.
  • TYPE
    Storm Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

CHANGING WEATHER AND INCREASING AVALANCHE DANGER

A warm, wet and windy storm is over the region currently. Roughly 10 to 12″ of snow has fallen in the Alpine zones with another 10 to 14″ expected today (above 2,500′). At the mid-elevations, roughly 6 to 10″ of wet snow has fallen with an additional 1″ of rain (yes rain) expected up to 2,200′ this afternoon. Easterly ridgetop winds will continue to be in the 20’s with gusts in the 50’s mph. 

Although there was bare ground before this storm up to 2,000′, above this a shallow snowpack has been forming (see photos below). Rain last weekend followed by cool temperatures formed a crust up to the ridgetops on this pre-existing snow surface. The kicker is, surface hoar formed on the crust before it was buried yesterday. So, what we have is a perfect recipe for avalanche activity in the upper elevationsAny new snow accumulation is expected to slide off the old snow surface easily. How much avalanche activity we will have and how big those avalanches will be is directly related to how much snow falls. As of this morning, there is roughly a foot of snow from yesterday and another foot expected today – again, this is all above 2,500′. Therefore, storm slabs up to 2′ thick are possible and where winds have been loading up to 3 to 4+’ thick. 

Snow falling along the road and up to ~2,200′ is expected to turn to rain by this afternoon. This will saturate and melt much of the 6 to 10″ of wet snow existing in these mid-elevations (1,000′ to 2,500′). The worry at these elevations is being in the runout zone of an avalanche from above where debris can run. All this said, today is a day to leave the mountains alone and look ahead to the cooler weather on tap for the middle part of the week! 

 

 

Sunburst, above the alders – hard snow with suspected surface hoar on top existed before this storm rolled in yesterday (Saturday)

 

 

The pre-existing snow line was a little lower on Tincan, just above1500′

 

East facing slopes of Repeat Offender on Seattle Ridge on Friday, November 23. Snowline was around 2000′.

 

Looking into Lynx Creek drainage, Northern aspects before the storm on Friday.

Mountain Weather

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 31   9   1   13  
Summit Lake (1400′) 31   1 0.2 1  
Alyeska Mid (1700′) 31   4   0.5   4  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 25   NE   21   46  
Seattle Ridge (2400′) 30   E   11   33