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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Fri, March 13th, 2020 - 7:00AM
Expires
Sat, March 14th, 2020 - 7:00AM
Forecaster
HPAC Staff
Conditions Summary

Avalanche Bulletin for 3/13

The Hatcher Pass Avalanche Center has issued a Special Avalanche Bulletin.

* Affected Area…For the Hatcher Pass State Park and Hatcher Pass surrounding areas including: Arkose Ridge, 16 mile, Gold Mint, Archangel, Marmot, IM Bowl, to Hatcher Pass.

* Avalanche Danger… Avalanche Danger is rated CONSIDERABLE. Significant rapid snowfall of 36” on 3/7-3/9 contributed to an avalanche cycle with a weak layer that continues to persist. An avalanche fatality occurred at low elevation on a S aspect on 3/9. On 3/12, a large avalanche on Marmot gully 3 crossed the road, closing the road. Hatcher Pass road will be closed at mile 12, on March 13th for avalanche mitigation.

Precautionary/Preparedness Actions… Careful snowpack evaluation, cautious route-finding and conservative decision-making skills are necessary for travel in the backcountry.

Similar avalanche danger may exist at locations outside the coverage area of any avalanche center.

 

 

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Fri, March 13th, 2020
Avalanche Problem 1
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
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.

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

You can get away with skiing/riding all kinds of terrain right now BUT triggering a very large avalanche is still possible, as we just saw with the one that hit the road. A large avalanche problem still exists with old weak layers lingers in the snowpack. The problem is challenging to find and stubborn to trigger in most locations. We saw evidence of this problem Thursday with a few recent avalanches. One avalanche is believed to have been remotely triggered, on a S aspect at approx: 3500 ft. in Marmot gully#3( lookers right of the standard Marmot uptrack).  This avalanche was large and crossed the road by 100 yards, and closed the road.

A second avalanche shortly after was reported on a W aspect of President’s ridge at a similar elevation. Trigger is unknown.

Snomachiners also reported a remotely triggered small slab on a S aspect at 2600′ near Idaho Peak. Although further away up Buffalo Mine, another incident occurred with a snomachine remotely triggered large slab avalanche.

We suspect that this persistent slab problem is easier to detect at low and mid elevation, on shallower slopes, most likely SE to W aspects where widespread whumping continues to be observed.  It will be more challenging, but not impossible, to trigger this slab on most leeward aspects where the January facets are buried 3-5 feet deep.

Spring time temps are beginning to warm up and affect snow surfaces.  If we see a rapid warm up, expect avalanche danger to increase.

Large recent avalanche that crossed the road on Marmot gully 3 on 3/12. S aspect, 3500′. Remote trigger is suspected.

Marmot avalanche that crossed the road 3/12.

Human triggered avalanche, resulting in fatality. 16 mile run. S aspect 2000′. 3/9

Weather
Fri, March 13th, 2020

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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