Avalanche Advisory for Saturday, December 2, 2017 at 7:30 am

Issued: Sat, Dec 02, 2017 at 8AM

Expires: Sun, Dec 03, 2017

Above 3,500ft Considerable

2,500 to 3,500ft Moderate

Below 2,500ft Low

Degrees of Avalanche Danger ?

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

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

BOTTOM LINE

The avalanche hazard is moderate this morning. A couple inches of new snow accumulated overnight at 3000′. Today’s storm is tricky to predict. There is a chance that snowfall ends by noon, in which case new snow will not significantly increase the avalanche hazard. However, if snowfall continues through the afternoon the avalanche hazard will increase. There is 2″ of new snow at 3000′ this morning at 6 am. NWS is calling for 6-9″ inches of snow today at Hatcher Pass, with the chance for 1-3″ more tonight. Any new snow will not bond well to the old snow surface.

6 hours of light today with sunrise at 9:43 am and sunset at 3:42 pm.


STORM SNOW AVALANCHE PROBLEM

The avalanche hazard will increase throughout the day from moderate to considerable as today’s storm snow adds up.  Expect a considerable hazard, natural avalanches possible and human triggered likely, for up to 9-12″ deep storm slab avalanches by late afternoon on all aspects, at upper elevations, on slopes 35º and steeper. Expect storm snow to bond poorly to the old snow surfaces. Watch wind speeds carefully as consistent winds 15 mph+ will be able to transport new snow at upper elevations onto NW to NE aspects, increasing the likelihood and volume of human triggered slab avalanches.  At mid elevation, expect storm snow totals to be less and for the avalanche hazard to be moderate.

 

PERSISTENT SLAB PROBLEM

A moderate hazard exists for old, persistent, mid-pack, weak layers in the snowpack, at mid to upper elevations, on South to West aspects, on slopes 35° and steeper. Persistent slabs will be possible to human trigger, and natural avalanches will be unlikely. This could change depending on the amount of new snow and therefore the weight of the new load. The NWS forecast is predicting 0.4″ of water in 6-9″ of new, low density snow today which will have trouble overloading the persistent layers of concern. All bets are off if we get a much larger dump than anticipated, and in this case expect the avalanche hazard to rise.

 

LOOSE SNOW AVALANCHE PROBLEM

The avalanche hazard will be rising to considerable with more snow in the afternoon.  Expect small to medium loose snow avalanches (sluffs) on all aspects, on slopes 40° and steeper at mid to upper elevations. Natural avalanches possible, human triggered likely. Even a small sluff could wash you into a terrain trap, over a cliff or bury you in a deep ravine, significantly compounding the hazard.

 

We have low to moderate confidence for this avalanche advisory based heavily on the current weather forecast. Small changes in the position of the jet stream and other weather factors could radically change the total amounts of new snow today, and the avalanche hazard. More than 12″ of new snow in 12 hours will overload persistent weak layers deeper in the snowpack which will result in more sensitive, larger, and more numerous avalanches than we are predicting at this time. Pay close attention to the rate of accumulation today and make conservative decisions.

 

Recent snow profile here

Avalanche danger scale here


 

Recent Avalanche Activity

No recent avalanches.

Many small avalanches (D1-1.5) and one larger (D2) human triggered avalanche occurred between 11/20-11/23 due to a strong wind loading event on the 20th. This includes the avalanche accident on President’s Ridge of Marmot Mountain on 11/22.

A working theory on the pattern of avalanches from 11/20-23: The majority of observed avalanches appear to be concentrated between 3-4k’, the boundary between mid and upper elevation bands.  The upper elevation snowpack is more characteristic of a thicker winter snowpack, while low to mid elevations are more characteristic of a thinner ealry season snowpack. Typical avalanche forecasts tend to work top down, higher avalanche dangers at upper elevations and lower avalanche danger at lower elevations. However, in this season’s snowpack, it may be possible for the avalanche danger to be highest at the mid/upper elevation boundary due to a thinner, weaker snowpack and a pre-existing rain line at 4000′.

Recent Weather

This week’s weather at 3550′:

Temps averaged 20ºF, with a low of  4ºF and a high of  31ºF.

1-2 inches of new snow was recorded at IM snotel this week.

Overnight at 3550′:

Temperature averaged  21° F.

1″new snow overnight.

This week’s weather at 4500′:

Temps averaged  17ºF, with a low of  5ºF and a high of  26ºF.

Winds averaged SE 7mph, gusts averaging 12, max gusts 28 mph.

Overnight at 4500′:

Temps averaged  17ºF overnight.

Winds averaged  SE 9.5 mph overnight, gusting  SE 21 mph at 6:00 am.


NWS recreational forecast for Hatcher Pass here


NWS point forecast here


State Parks snow report here


NWS is predicting snow through the day, however there is the chance that precipitation stops around noon. Forecasted amounts vary from 6-9″ today depending on elevation, with 1-3″ tonight. Expect only a modest amount of water in this storm, producing low density powder today, but warming temperatures this evening may turn that nice powder into an upside down cake. 

Temperatures are forecasted to rise to 32°F tonight which will allow the new snow to become a more cohesive slab and an “upside-down cake”. Looking forward into Sunday the avalanche hazard will increase.  A good ‘ol rule of thumb: allow storm snow to sit for at least 24 hours, as most avalanches occur 24-48 hours after the storm. This will also give deeper instabilities in the snowpack time to adjust to the new load.

Factors to watch closely today, which could contribute to a spike in avalanche activity, are the following: rate of snowfall accumulation, the density of the new snow (temperature), and wind speeds which could transport snow, and build, sensitive, cohesive storm/wind slabs.  A heavy, rapid load may begin to tip the balance for persistent weak layers (persistent slab) in the mid-pack by this evening or into Sunday.

 

Additional Info & Media

A complete avalanche accident report for the 11/20 accident on Marmot Mountain should be available early next week.

Posted in Hatcher Pass Avalanche Center News, HPAC Forecasts.
Allie Barker

Forecaster: Allie Barker