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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Wed, March 20th, 2019 - 7:00AM
Expires
Thu, March 21st, 2019 - 7:00AM
Forecaster
Aleph Johnston-Bloom
High Avalanche Danger
Travel in avalanche terrain is not recommended. Avoid being on or beneath all steep slopes.
The Bottom Line

The avalanche danger will rise to HIGH today and large to very large natural avalanches will become likely. Very warm temperatures, heavy precipitation with a rising rain/snow line and strong winds will impact the region. Travel in avalanche terrain is not recommended. Avalanches have the potential to be very deep and dangerous.  

PORTAGE VALLEY:    Travel in runout zones from avalanches including venturing along and past the Byron Glacier Trail continues to be not recommended. Very large avalanches have been observed in Portage over the past few days and are expected to continue with this next round of active weather.

SUMMIT LAKE (& INTERIOR EASTERN KENAI MTS):    Extra caution is advised. Large human triggered avalanches remain likely. Avalanches have the potential to release in old buried weak layers.

SEWARD/LOST LAKE:    New snow, rain and wind will increase avalanche danger again in this region as well. Large and dangerous natural avalanches  have been observed in this area during the recent avalanche cycle.

 

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Wed, March 20th, 2019
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
High (4)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
High (4)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    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

Wow! Getting to see the extent of the St. Patty’s Day avalanche cycle was impressive and scary. With a window of good visibility yesterday large, deep avalanches that occurred 3.17 and 3.18 were observed across the advisory area. Additionally and notably two large avalanches were observed running naturally yesterday. The Five Sisters avalanche path in Portage ran in the morning with a 10+ foot crown and a slide in Main Bowl on the back side of Seattle Ridge was observed running in the afternoon. Today the next storm system is forecast to impact the area. Temperatures are already above freezing in the Alpine, rain is expected to fall to 2500′ and winds are already gusting into the 70s on Sunburst. There is a buffet of avalanche issues today and the potential for large to very avalanches makes travel in avalanche terrain including runouts not recommended. The crust that formed with cool temperatures Monday night could eventually break down and there is either colder snow that could be over loaded as it becomes saturated or at lower elevations already very saturated snow that could move. Above the rain/snow line new snow and wind will add stress to the snowpack and cornices are large and dangerous. Investigating the very large avalanche on Tincan yesterday showed that avalanches are breaking back to the March 8th new/old snow interface made up of small facets and buried surface hoar. Multiple feet of snow now rest on top of this layer and avalanches today and tomorrow may still break down at this weakness. The snowpack will need time to adjust to all the loading from the March barrage of storms. Give it some time and enjoy other activities away from the mountains!

 

Natural avalanche in Main Bowl on the backside of Seattle Ridge observed running yesterday afternoon, 3-19-19. Photo: Travis Smith

Five Sisters avalanche path in Portage Valley was observed running yesterday morning, 3-19-19.  

 

The 3.18.19 Tincan avalanche. Photo from Seattle Ridge: Travis Smith. 

Weather
Wed, March 20th, 2019

Yesterday: Partly sunny with temperatures in the 20Fs to 40Fs depending on elevation. Winds were light and easterly. Clouds moved in overnight and temperatures were in the 30Fs and 40Fs. Easterly winds increased blowing 30-40 mph with gusts into the 70s.

Today:  Rain and snow throughout the day with close to an inch of water in the forecast. Rain/snowline is expected to be around 2500′. Temperatures will be in the high 30Fs to high 40Fs, with Alpine temperatures already in the mid 30Fs. Winds will be easterly 30-40 mph with gusts into the 60s and 70s. Rain and snow continue overnight with unseasonably warm temperatures and strong winds.

Tomorrow: The warm wet weather continues into Thursday. Expect more of the same. This active pattern extends into the weekend with a brief break on Saturday and another storm Sunday.

*Seattle wind sensor was cleared and started reporting 4 pm yesterday, data is incomplete.

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

Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36 0 0.1 86
Summit Lake (1400′)  35 0 0 29
Alyeska Mid (1700′)  35 0 0.08 76

RIDGETOP 24-hour data (6am – 6am)

Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) 24  NE  18  78
Seattle Ridge (2400′) 30   SE*    4*     24*
Observations
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Riding Areas
Updated Fri, May 01st, 2020

Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

Area Status Weather & Riding Conditions
Glacier District
Johnson Pass
Closed
Placer River
Closed
Skookum Drainage
Closed
Turnagain Pass
Closed
Closed as of May 1. Thanks for a fun, safe season!
Twentymile
Closed
Seward District
Carter Lake
Closed
Lost Lake Trail
Closed
Primrose Trail
Closed
Resurrection Pass Trail
Closed
Snug Harbor
Closed
South Fork Snow River Corridor
Closed
Summit Lake
Closed

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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.