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ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, January 23rd, 2019 - 7:00AM
Expires
Thu, January 24th, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is  CONSIDERABLE  at all elevations today. As strong winds and new snow/rain impact the region triggering a slab avalanche will be likely  on slopes steeper than 30 degrees and natural avalanches are possible. Cautious route-finding and conservative decision-making are essential today. Avoid travel under cornices and glide cracks. Pay attention to changing conditions during this active storm cycle. Precipitation intensity and strong winds may bump the danger up to  HIGH late in the day  and natural avalanches could become likely.    

GIRDWOOD: Warming temperatures and rain on old snow = ROOF AVALANCHES. Pay attention to children and pets and where you park your car.  

PORTAGE and PLACER VALLEY:    Heavier rain and snowfall rates are expected and large avalanches above treeline may send debris to sea level and over summer hiking trails such as Byron Glacier Trail.

SUMMIT LAKE / JOHNSON PASS:    A poor snowpack structure exists in this area, which is very different than Turnagain Pass. New snow/rain and wind in this series of storms over the next few days may overload weak layers in the snowpack and larger avalanches may occur. Look for signs of instability and pay attention to changing conditions.  

 

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Wed, January 23rd, 2019
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic (D4-5)
    Very Large (D3)
    Large (D2)
    Small (D1)
    Size
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.
Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

The first in a series of storms is over the region this morning bringing wind, snow and rain. Rain/snow line is forecast to be as high as 2500′ today. Water amounts/snow totals are quite different across the advisory area this morning. Keep this in mind if you go in the mountains. Turnagain has only received 0.1″ of water whereas Bear Valley (Portage side of the tunnel) is reporting 1.8″ of water weight and Alyeska top station is at 0.94″. For upper elevations that translates to the difference of approximately 1″ of snow at Turnagain and almost 2′ of snow in Portage. Sunburst weather station has been gusting as high as 70 mph and Max’s gusted into the 60s. Winds will remain strong throughout the day. The advisory area is forecast to pick up close to another inch of water today. Paying attention to changing conditions is crucial. There will be a variety of avalanche concerns associated with the storm.  Expect upper elevation areas where new snow is falling on weak surface snow (surface hoar and near surface facets) to develop tender storm slabs and/or wind slabs. Slab depth will depend on snow amounts and wind deposition. Look for recent avalanches, shooting cracks and collapsing. Heavy wet snow and wind will also build cornices that could easily be triggered. 

Wet loose avalanches: In places where rain is falling on snow wet loose avalanches may initiate. Watch for roller balls and steer clear of runout zones if these start to move. Eventually if lower elevation crusts start to deteriorate these may gouge into softer snow below. 

  Rain total forecast for today to Friday morning. 

Suface hoar and near surface facets, 1-21-19. Photo: Collin Atkinson.

Additional Concern
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

South of Turnagain – Johnson Pass/Summit Lake zone: A poor snowpack structure exists in these areas.  Multiple mid-pack weak layers of facets and buried surface hoar have been found as well as a facet/crust combination in the bottom of the snowpack. No recent avalanche activity and calm weather has allowed the pack to slowly adjust. However, as these weak layers get loaded over the next few days we may see avalanches stepping down to the older layers. This zone is not expected to get as much precipitation but incremental loading and strong winds may be enough to tip the balance. Look for signs of instability and choose terrain carefully. 

Weather
Wed, January 23rd, 2019

Yesterday:  Skies were cloudy and there was very light afternoon snow/rain showers. Temperatures were in the mid 20Fs to low 30Fs.   Northeasterly winds were 15-25 mph and started gusting into the 40s mid-day. Overnight temperatures rose into the high 20Fs and mid to high 30Fs. Winds increased gusting to 70 on Sunburst. Precipitation picked up overnight.  

Today: Cloudy skies with snow and rain likely throughout the day. 0.7″ of rain is forecast with potential for higher amounts closer to the coast. Rain/snow line around 2500′. Temperatures ranging from the 40Fs to 20Fs depending on elevation. Winds will be easterly 20-30 mph gusting into the 60s. Tonight another front is expected to move in. Click HERE for the Special Weather Statement from the National Weather Service.

Tomorrow:  Continued precipitation and strong winds. Temperatures are forecast to keep increasing with the warmest temperatures on Friday and yet another front that is forecast to be the strongest of the week.  

*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 34   1   0.1   50  
Summit Lake (1400′)  31      0     0   20  
Alyeska Mid (1700′) 33   4   0.5   39  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24    NE  22 70  
Seattle Ridge (2400′) 29   *N/A   *N/A   *N/A  
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.
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