Issued: Sat, Mar 17, 2018 at 7AM

Expires: Sun, Mar 18, 2018

Above 3,500ft Moderate

2,500 to 3,500ft Moderate

Below 2,500ft Moderate

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

BOTTOM LINE

New snow, wind and warm temperatures will all add complexity to the avalanche hazard today. A Moderate Avalanche Hazard exists for Storm Slabs, Wind Slabs, and Persistent Slabs today. It will be possible to human trigger Storm Slabs on all aspects, Wind Slabs on west through north aspects, and Persistent Slab Avalanches on all aspects today, on slopes 35º and steeper. 

 The most likely scenario for triggering a persistent slab will be where a storm or wind slab overloads the old persistent slab problem, increasing the size and consequence of the avalanche. 

Expect the danger to increase for wet-loose slides on south through southwest aspects on slopes above 40º in the afternoon.

 


STORM SLAB AVALANCHE PROBLEM

A Moderate Avalanche Hazard exists for Storm Slabs today at mid and upper elevation, on all aspects, on slopes above 35º. Expect mostly small avalanches, D1 to D1.5 in size

Up to 14″ of new snow fell as temperatures increased throughout the storm, forming storm slabs on all aspects. Expect storm slabs to break within the new snow or on the old snow surface, up to 8-16″ deep. On southerly aspects (E>SW), most new storm slabs are sitting on old crusts that will provide a good bed surface.  These slabs will be stubborn to human trigger, increasing to touchy in the afternoon with the addition of solar radiation, on predominately south aspects, on slopes above 35º and steeper. Expect storm slabs instabilities to improve within 24-48 hours. Storm slabs have the potential to overload the deeper persistent weak layer, increasing the size and consequence of an avalanche.


WIND SLAB AVALANCHE PROBLEM

A Moderate Avalanche Hazard exists for Wind Slab avalanches today at upper elevations, on West through North aspects, on slopes 35º and steeper. 14″+ new snow this week combined with moderate winds have formed slabs 12-16″ thick slabs. Expect mostly small avalanches, D1 to D2 in size

Winds slabs will be touchy to trigger near and below ridgelines, and on side wall gullies at upper elevations. Visual clues will most likely not be evident as the wind subsided prior to the last couple inches of snowfall.

The danger lies in the details… It will be possible for wind slabs to step down into the deeper, weaker persistent grains under the Valentines Day layer. This could be 2-3 feet deep or deeper in isolated locations.

Expect wind slab instabilities to improve within 24-48 hours.


PERSISTENT SLAB AVALANCHE PROBLEM

A Moderate Avalanche Hazard exists for Persistent Slab avalanches at all elevations on all aspects, on slopes 35º and steeper.  Expect avalanches to be up to D2 –D2.5 in size, and 2-4 feet deep. Evidence of this layer exists in portions of terrain, but is not always obvious. This problem will most likely be stubborn to trigger. 

The Persistent Slab problems linger in the depth of the snowpack. In some locations it might be 3-4′ deep. The good news is that it will be hard to trigger a layer that deep. The bad news is it will be possible to trigger, carries high consequences, and it will probably kill you.  Storm slabs or wind slabs may step down to this deeper layer, and likely trigger points will be in shallow and/or rocky terrain.  A natural avalanche on Arkose Ridge, that occurred around 3/15-3/16, likely failed due to rapid wind loading and poor structure.  Although this avalanches appears to be an outlier, it clearly shows the persistent slab problem still exists.

This is a low probability/high consequence hazard. Evidence is not always obvious, and the lack of clues does not mean the problem does not exist. It will be difficult predicting where and when  persistent slab avalanches will occur.  This avalanche problem will allow multiple riders on the slope before letting you know its there.

Continuing to practice safe travel protocol and choose safer terrain that avoids funneling into terrain traps. Use communication devices, spread out, use safe and effective safe zones out of the runnout the the avalanche problem you are concerned with, and share your opinion. Carry a probe, beacon, shovel, airbag if possible- practice with and know how to use these tools.

 

Pit profile shows top 125 cm of 305 cm snowpack


WET LOOSE AVALANCHE PROBLEM

A Moderate Avalanche Hazard exists for Wet-Loose avalanches on steep southerly aspects today in the afternoon. Expect sluffs to be small to large and to be able to trigger slab avalanches that break into deeper layers. 

 

Recent Avalanche Activity

 For the most part, visibility was poor this week. It is likely that more natural avalanches occurred since the new snow and wind 3/14-3/16. Two naturals were observed 3/16.

1.Arkose Ridge- W/NW 4000′ SS-N-D2.5-O

2.Marmot Mid Rib- half way down SE 3600′ SS-N-D1-S

3.Several wet-loose sluffs were observed at low elevation along the roadway on the drive down Friday night.

 

 


Check out the photos below. The first one went naturally last week 3/9. The second photo (lookers right of this avalanche) went naturally 3/15 or 3/16. The same problem lingers.

3/9 Arkose Ridge NW 4000′ Natural wind slab appears to have stepped down to old persistent slab and triggered sympathetic avy below. Debris ran to top of icefall.

Arkose Ridge 4000′ W/NW Natural 3/15 or 3/16 Suspected wind slab that failed on old persistent weak layer

 

 

 

 

 

Arkose Ridge, same photo as above Notice how avalanche wrapped around gully. Poor widespread structure is the culprit.

 

 

 

 

 

 

 

 

 

Recent Weather

This week’s weather at 3550′:

Temps averaged 22ºF, with a low of 15ºF and a high of 34ºF.

IM reported about 14″ of new snow between 3/14 and 3/16 with 1.3″ of water (SWE).

Overnight at 3550′:

Temps averaged 27° F.

No new snow overnight.

This week’s weather at 4500′:

Temps averaged 19ºF, with a low of 12ºF and a high of 30ºF.

Winds averaged ESE 5 mph, max ESE 16 mph . Gusts averaged ESE 10 mph, max gusts SE 25 mph.

Overnight at 4500′:

Temps averaged  22ºF overnight.

Winds averaged SSW 3 mph overnight, with a max gust SSW 5 mph.

 


NWS recreational forecast for Hatcher Pass here


NWS point forecast here


State Parks snow report here

Additional Info & Media

Expect the avalanche hazard to remain at Moderate through the weekend.

NWS is calling for a temps 29º at 3000′, and winds SE 0-7 mph today.

Sun and warming temps Saturday could increase avalanche hazard on southerly aspects in the afternoon.

 

 

 

 

Posted in HPAC Forecasts.
Allie Barker

Forecaster: Allie Barker