ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday December 2nd, 2018
Posted by Wendy Wagner on 12/02/18 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger remains  MODERATE  above 2500′ in the Alpine. Human triggered slab avalanches up to 1-3′ thick remain possible due to a weak layer of snow under the Thanksgiving weekend storm snow. Additionally, watch for wind slabs that could form today, or did yesterday, with the continued southerly and easterly ridgetop winds.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1. Low
Treeline
/ 1,000'-2,500'
Avalanche risk
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk

Special Announcements

Hatcher Pass saw snowfall yesterday and is expecting more snow today/tonight – make sure to check out Hatcher Pass Avalanche Center’s forecast!!  

Avalanche Problem 1
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
  • TYPE
    Persistent Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Snowy skies are in the forecast for today and tonight. Although Turnagain Pass is not favored for this storm, up to 3″ could fall by this evening and an additional 3 to 6″ tonight. The Portage zone and areas on the southern Kenai should see rain at sea level but up to a foot of snow by tomorrow morning at the mid elevations. The rain line should hover near 1,000′ and possibly lower. Will this system start building back the mid elevation snowpack? It could be a start, but another warm system looks to move in Monday night. 

At the higher elevations, even small amounts of new snow will add weight to our existing snowpack. Sitting anywhere from 1 to 3′ below the snow surface is a thin layer of weak snow (buried surface hoar) . Friday’s earthquake gave this layer a good shake and triggering a large slide may becoming less and less likely, however it is still a concern. We can’t forget this layer is there in terrain above 2,500′, which can be easy as any obvious signs of instability are not likely to be seen. What we can do is listen for whumpfing, use safe travel protocol and choose lower angle slopes if we wish to avoid any uncertainty. 

Avalanche Problem 2
Wind Slabs
Wind Slab avalanches are the 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.
  • TYPE
    Wind Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Moderate to strong ridgetop winds from the south and east are again over the area. Wind slabs lurking in catchment zones from yesterday as well as new slabs formed today are possible to find and trigger in steep terrain. Keep an eye out for yesterday’s wind loading patterns, which could be obscured by last night’s one to two inches of snow. Snowfall should accompany the strongest winds today, which are forecast for the afternoon. 

Mountain Weather

Yesterday:   Mostly cloudy skies were over the area with light snow showers adding 1-2″ of snow in the evening. Winds were strong out of the northeast at Sunburst weather station, yet the main flow direction was more southerly. Temperatures were in the low 30’s F at most mid and low elevations and in the upper 20’s F along ridgelines.

Today:   Another round of light snowfall is expected later today (1 to 3″) that could continue through tonight (an additional 3 to 6″). Snow/rain line should hover around 1,000′. This system is coming in from the SW and more favorable for snowfall in the Anchorage area and Hatcher Pass. Associated winds will again be southerly and easterly in the 20-30mph range with stronger gusts. Temperatures will remain in the low 30’s near 1,000′ and the mid to upper 20’s along ridgetops.

Tomorrow:   A short break in weather systems will give us mostly sunny skies for Monday. Models are showing that Monday night clouds and precipitation move back in as another large low-pressure pushes a moist frontal band over Southcentral.  

*Seattle Ridge wind sensor is rimed over.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 32   2   0.2   12  
Summit Lake (1400′) 28   1   0.1   2  
Alyeska Mid (1700′) 31    0 0.15   0  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 23   NE   21   56  
Seattle Ridge (2400′) 27   *no data   *no data     *no data