ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Friday April 12th, 2019
Posted by Heather Thamm on 04/12/19 at 7:00 am.
The Bottom Line
Considerable Avalanche Danger
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

The avalanche danger is CONSIDERABLE today above 2500′ where strong winds and new snow have created touchy slabs 1-3′ thick. In the Treeline zone (1000′-2500′) the avalanche danger is MODERATE where the snowpack transitions from wet snow to dry snow. Within this zone triggering a wet loose avalanche or storm slab is possible. Any sign of sun or thin cloud cover today could quickly heat up this new snow and increase the potential for wet loose avalanches. Evaluate terrain and snow as you travel and keep your slopes angles below 35 degrees until this new snow has time re-freeze. Additionally – identify cornices and glide cracks and avoid travel below these unpredictable features.

PORTAGE VALLEY: Very little snow exists below 1000′, but summer trails with large steep slopes directly overhead, such as the Byron Glacier Trail, provide easy access for accidentally being in a dangerous place. Cornice fall or an avalanche from above has the potential to run to valley bottom. Travel in these areas is not recommended.

3. Considerable
Alpine
/ Above 2,500'
Avalanche risk
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
0. No Rating
Below Treeline
/ Below 1,000'
Avalanche risk
Avalanche Problem 1
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

A powerful storm yesterday dumped 1-3’ of new snow in the upper elevations, rain below 1000’, and blasted our region with very strong wind. Easterly ridgetop winds averaged in the 40s with gusts reaching 80-90 mph. A variety of storm related avalanche problem exist today and will require careful snowpack evaluation and cautious route finding and a flexible plan if the sun comes out. Snow totals could range from 1-2’ thick in Girdwood and Turnagain Pass, and 3’ in the upper elevations of Portage Valley. Rain/snow line fluctuated between 2000’ and 1000’ yesterday which means you will encounter wet heavy snow transitioning to drier snow with elevation. Keep the following in mind today:

WIND SLABS: In the alpine where strong winds have drifted new snow, slabs could be up to 2’ thick in heart of Turnagain Pass. Look for wind loaded features and avoid slopes steeper than 35 degrees. Larger and more connected slopes will have higher consequences if you trigger a slab. Look for obvious clues like shooting cracks and recent avalanches. Any additional blowing snow or radiation from the sun will make these slabs more touchy today.

STORM SLABS: Areas less affected by strong winds that are harboring dry snow – triggering a storm slab is possible. As you travel pay attention to where wet snow transitions to drier snow and how thick the slab becomes with elevation. Avoid steep features and terrain traps that could get a skier or rider in trouble.

CORNICES: Expect cornices to be large and unstable. Give these features lots of space – they often break further onto a ridge than expected. Natural cornice fall could trigger a slab below. 

Rain/snow line dropped to 1000′ yesterday afternoon at Turnagain Pass with 1cm of wet slushy snow accumulation. The upper elevations likely have 1-2′ of new snow.

 

Avalanche Problem 2
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
  • TYPE
    Wet Loose
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Below 2000’ where wet snow and rain fell yesterday, triggering a wet avalanche will be possible on steep terrain features. The more saturated the snow, the more potential for a small point release to entrain snow and build momentum. Daily warming and any sign of sun or thin cloud cover could increase the potential for wet loose avalanches. If you find your skis or machine sinking into wet saturated snow avoid slopes steeper than 35 degrees. Pay close attention to how the snow changes throughout the day and with elevation, and be prepared to avoid all South facing slopes later in the day if you feel heat from the sun. This includes all South facing slopes and all elevations.

**Tomorrow looks like our first sunny day following the storm with very little wind. Be aware that warming from the sun will elevate the avalanche danger and potential for triggering both wet loose and storm slabs in the alpine.

Rain and wet snow was falling along Turnagain Pass road level yesterday. Expect the snow surfaces at this elevation to be wet and punchy today. Note how quickly the snow is disappearing! There is little to no snow below 1000′.

Additional Concern
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches

Rain and new snow has added weight to the snowpack and a glide avalanche today would not be suprising. This is the entire snowpack gliding downhill and spontaneously releasing. Our last known glide avalanche was on Sunday near the Hope Wye. Cracks are present in a lot of popular terrain and require a keen eye – to locate and avoid travel below them. They can release at anytime, are completely unpredictable and can be very destructive.

Mountain Weather

Yesterday: A powerful storm yesterday brought a mix of rain, snow and strong wind. Heavy rain fell below 1000′ across the region and heavy snow to the alpine. Precipitation totals ranged from 5-10″ of wet snow (0.76″-1.0″ SWE) for Alyeska Midway and Turnagain Pass SNOTEL, and 2″ of rain for Bear Valley in Portage. Easterly winds averaged in the 40-50s mph with gusts to 80-90mph during the middle of the day. Temperatures at sea level were in the low-40Fs, temps at 1000′ were mid-30Fs and upper elevations were mid-20Fs. Rain/snow line fluctuated between 2000′ and 1000′. Overnight Easterly winds diminished to 10-20s mph.

Today: Expect overcast skies and a mix of rain showers below 1500′ and snow showers above, with 2-3″ (0.2″ SWE) of new snow possible. Areas further from the coast may experience broken skies later in the afternoon. Easterly ridgetop winds are expected to be 5-25mph, with gusty winds near Turnagain Arm in the afternoon. Temperatures at sea level will be in the mid-40Fs, temps in the mid elevations upper-30Fs, temps along ridge tops may reach the upper-20Fs.

Tomorrow: Partly cloudy skies are expected as high pressure moves into Southcentral AK. Winds are expected to be light and variable. Temperatures during the day will reach the 40-50Fs at sea level, 40Fs at 1000′ and low to mid 30Fs in the upper elevations.

*Seattle Ridge weather station was under-reporting wind data yesterday and stopped recorded at 9pm last night.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 33 10   1   70  
Summit Lake (1400′) 36   rain   0.1   18  
Alyeska Mid (1700′) 33   5   0.76   59  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24   ENE   36   90  
Seattle Ridge (2400′) 29   *N/A   *N/A     *N/A