Avalanche Advisory Archive 2016 – 2018

Date Issued:2017-03-07 06:44:00
Danger:3
Primary Trend:2
Primary Probability:3
Primary Likelihood:3
Primary Size:2
Primary Description:

With a 42cm super cold storm last week laid down on a bed surface of near surface facets... we built a weak slab... winds over the last week have also contributed to this danger...

We have seen natural and human triggered slides on all aspects. Yet windloaded areas remain the most questionable with continued loading.

Secondary Trend:2
Secondary Probability:4
Secondary Likelihood:2
Secondary Size:2
Secondary Description:

With these cold temps and the additional temperature gradients that often builds in the snowpack the weak layer under the storm slab may not be healing well...

Be aware the storm slab is now resting on a persistent weak layer that will not go away any time soon. Slides on this layer are still possible and we will have to watch it carefully as new load is applied in future storms.

Discussion:

The National Weather Service Forecasts-

Today- Mostly cloudy. Locally very windy. Highs around 18. northeast wind 15 to 25 mph. Near downtown juneau and douglas, northeast wind to 35 mph with gusts to around 70 mph decreasing to 30 mph with gusts to around 50 mph in the afternoon.

Tonight- Mostly cloudy. Locally windy. Lows 8 to 14. Northeast wind 15 to 25 mph. Near downtown juneau and douglas, northeast wind to 30 mph with gusts to around 50 mph.

Wednesday- Partly cloudy. Windy. Highs around 18. Northeast wind 30 mph with gusts to around 40 mph

Temperatures remain extremely cold this morning with Eaglecrest and Mt Roberts both touching season lows at -1f. Winds are primarily out of the North at 20-30mph gusting much higher along the channel near summit and ridgeline.

Forecast calls for increased wind speeds along the channel today.

We went through a fairly large avalanche cycle over the last week. We had several days of cold dry light snow that left 42+cm in our backcountry. Winds were light to moderate at the beginning. Much of this came to rest on a bed surface with near surface facets.

The snowfall amount did not immediately kick us into an avalanche cycle. We did see some loose snow sluffing early in the storm... but it was the winds that later built the current danger to where it is.

We have seen natural and human triggered avalanches on all aspects over the last 3-5 days.

Snow can stabilize over time and yet with these super cold temperatures it can take quite a bit longer... and if those cold temps build temperature gradients in the snowpack it can actually make the weak layer even weaker.

Recognizing so many slides occurred in the last few days... and recognizing these cold temps and continued windloading are not helping to stabilize the snowpack... Avalanche Danger Remains CONSIDERABLE Today.

Natural avalanches remain possible and human triggered avalanches remain likely in areas.

Mt Juneau had slides off of the Berhands path but Chop Gulley above town has still not slid. Due to this remaining hazard please avoid use of the Flume Trail.

Tip:

Air temperature not only determines the type of snow crystals that fall from the sky, but the metamorphic processes that occur within the snowpack.

You need to track:

Daily temperature values (highs and lows) and range

If air temperatures increase rapidly (more than 15?F or 8?C) in less than 12 hours, the rate of creep increases, which can lead to avalanching. This is most critical when temperatures are near or above freezing because the rate of creep increases exponentially with rising temperature.

If temperatures increase slowly over several days, settlement rates are slower. If air temperatures never rise above freezing for very long, the snowpack deforms slowly and creep rates are slower.

As temperatures decrease, settlement and creep rates also decrease.

Prolonged periods of cold temperatures promote the metamorphic processes that form weak layers.

Warm days and cold nights strengthen the pack, especially overnight and during early morning hours.

Several days of warmer temperatures followed by colder temperatures also strengthen the pack.

Key points:
Rapid rises in temperature can destabilize a pack, while slow rises stabilize it (especially if a cooling trend follows)

Long periods of cold temperatures build weaker snow, which isn?t a problem until the next significant snowfall

Temperature trend (increasing or decreasing) and rate and duration of change

Daily temperature values: Use daily high and low temperatures to determine freezing levels in the mountains as well as snow and rain levels during storms. Note the following.

Crusts may form if temperatures rise above freezing during the day and drop below freezing at night; the crust can create a potential sliding layer for the next storm

When temperatures remain above freezing for more than 24 hours, the upper snowpack becomes saturated from melt water and may cause wet snow avalanching

When temperatures stay well below freezing for long periods of time and high and low temperatures never rise above 15?F (-10?C) day and night, weak layers may develop on the surface or near the ground in a shallow snowpack; this can create a layer that fails with the next load of new snow

Key points:

Avalanches may occur when air temperatures rise above freezing and stay there

Weak layers often develop when temperatures get very cold and remain that way

Temperature trend: As temperatures increase, the snowpack undergoes settlement, deforming and becoming denser. Settlement on an incline results in creep, a slow downhill motion.

Forecaster:Tom Mattice