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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, November 28th, 2018 - 7:00AM
Expires
Thu, November 29th, 2018 - 7:00AM
Forecaster
Aleph Johnston-Bloom
Avalanche risk The Bottom Line

The avalanche danger is CONSIDERABLE in the Alpine where triggering a slab avalanche is likely. Below 2500′ the danger is MODERATE. If there is any snow left triggering a wet loose avalanche will be possible and being in the runout of snow-covered slopes above should still be avoided today.

Special Announcements

The Friends of the CNFAIC have two scholarships dedicated to avalanche education. The funds generated to make these possible are in celebration of Rob Hammel and Amy Downing, their love and passion for the mountains, and to help others stay safe. We encourage you to read each one and apply if you fit the need, or pass along to someone who could benefit. Applications due on Dec 1st.

Rob Hammel Scholarship Fund   €“ For recreational users and professional avalanche workers.

Amy Downing Scholarship Fund   €“ For recreational users.

Wed, November 28th, 2018
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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

Yesterday was much like the previous few days will an additional inch of water falling in the advisory area. Rain/snow line was between 2500′ and 3000′.  In the Alpine there has been significant snow load. Today triggering an avalanche in this elevation band is likely and avalanches could be large. There has been almost no visibility to see what has happened above the rain/snow line over the past 4 days and we have limited data on the overall snowpack structure. We do know that there were conditions for surface hoar and a melt-freeze crust before the storm. If you do venture into the mountains this is a day to proceed with caution and choose terrain carefully. Be on the lookout for signs of instability: recent avalanches, shooting cracks, and whumpfing

Water totals at the mid-elevation snow stations for this storm cycle from Saturday through Tuesday: 

  • Girdwood Valley at 1,700′:  5.1″ of water equivalent 
  • Turnagain Pass at 1,880′:  4.1″ of water equivalent
  • Summit Lake at 1,400′: 1″ of water equivalent 

If there is any snow left at treeline (2500′ and below) triggering a wet loose avalanche in the saturated snowpack is also possible before the temperatures cool down later in the day. 

Magnum and Cornbicuit, November 26, 2018. Photo: Wendy Wagner

 West and NW aspects of Tenderfoot, November 26, 2018. Photo: Heather Thamm

Weather
Wed, November 28th, 2018

Yesterday:  Rain and snow throughout the day. Rain/snow line was between 2500′ and 3000′. An inch of water fell in advisory area. Winds were easterly 15-25 mph with gusts into the 40s. Temperatures were in the 40Fs at sea level and in the high 20Fs on Sunburst at 3800′.  

Today:  Light showers transitioning from rain to snow are forecast for today as cooler air moves into the area. Temperatures should slowly cool into evening starting in the mid 30Fs and dropping into the mid 20Fs. Chance of snow showers overnight. Winds will be easterly 5-15 mph with gusts in the 20s.  

Tomorrow:  Partly sunny with temperatures in the high 20Fs to low 30Fs. Light SW winds. There is some discussion about the next storm moving into the area over the weekend but timing, temperatures, and precipitation type are still uncertain.  

*Seattle Ridge weather station stopped collecting wind data at 10 pm.  

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′)  34 0    1.1  13
Summit Lake (1400′)  35 0    0.1  0
Alyeska Mid (1700′)  36 0   1.16    0

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 27   NE    21  40
Seattle Ridge (2400′)  32     E *  7 *   32*
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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.