City and Borough of Juneau
155 S. Seward Street
Juneau, Alaska 99801
tel. 907-586-5240
fax 907-586-5385
http://www.juneau.org

Urban Avalanche Advisory

Current Advisory as of Friday, January 20, 2017

Expires 7:00am on Saturday, January 21, 2017
Issued by Tom Mattice


Avalanche Danger Level 1

Click here for danger definitions



Today's Discussion

The National Weather Service Forecasts-

Today- Decreasing clouds. Scattered snow showers early in the morning. Highs 27 to 33. Northwest wind 10 to 20 mph becoming east in the afternoon.

Tonight- Clear. Locally windy. Lows 16 to 24. East wind 15 mph...except northeast to 35 mph with gusts to 50 mph in downtown juneau and douglas.

Saturday- Partly cloudy. Locally windy. Highs 23 to 29. northeast wind 15 to 25 mph...except northeast to 35 mph with gusts to 55 mph in downtown juneau and douglas diminishing in the afternoon.

For the last week temperatures have been at or above freezing in our mountain zones. These temps come from short of summit so it has been a little cooler there... but in general the lower mountain was isothermal. The snowpack was one giant slurpee and very moisture saturated as we have seen 7.8" of precipitation in the last 7 days. This left only 14cm of new snow overall at Eaglecrest on the mid mountain gauge. That is a lot of rain. In the short run this places tremendous stress and load on the snowpack and also erodes the bonds that help with stability.

After long periods of warming and rain it becomes harder and harder to forecast wet avalanches. Yesterday even though I had the forecast set to low I cautioned of avalanche potential at mid to lower elevations with wet slabs. We saw exactly that in many places yesterday. I was surprised to see it and yet knew it could happen. Please use caution during periods of warm with isothermal snowpack... Continue to look for areas of glide opening up. Remember steep open faces with few or no anchors remain suspect until the cooling has had more time to take effect.

Its interesting that quite often glide and wet slab avalanches occur at the end of a warm rainy period during the cooling and drying out post storm... The slides yesterday did exactly that.

We only received about 2mm of precipitation in our mountain zones. Temps cooled off a great deal. Mt Roberts cooled from 33f yesterday morning to 21f this morning. Eaglecrest cooled even more from yesterday mornings 29f to this mornings 16f.

This moisture saturated snowpack has now drained and settled and is freezing solid in place. The snow is an insulator so it can take a little time for the deeper wet layers to cool enough to fully lock up. But in general with additional cooling today and no precipitation in the forecast avalanche danger is LOW.


Primary Avalanche Problem


GLIDE
Glide

AVALANCHE PROBLEM
Avalanche Size Level = LargeAvalanche Probability = UnlikelyAvalanche Danger Trend Next 24 Hours = Decreasing

Description

Be aware that until the snowpack freezes all the way back to the ground there is still a little concern for glide avalanches and wet slab avalanches at mid to lower mountain elevations.

Keep an eye out for glide cracks opening up as a tell tale sign of weakness...

This danger continues to decrease with cooling and time.

Secondary Avalanche Problem


Description



Today's Avalanche Tip

Glide

Release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

Predicting the release of Glide Avalanches is very challenging. Because Glide Avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide Avalanches.

How they form

Free water collects along the ground surface and lubricates the snowpack. The water can come from snowmelt or rainfall. As the water pools and reduces the friction holding the snowpack to the underlying surface. Wet snow is more viscous than dry snow, so a wet slab deforms and “flows” downhill easier. Glide is usually a slow process occurring over several days or weeks, but occasionally happens much faster. If the gliding snowpack breaks free of the surrounding snow it can avalanche. Glide cracks may open up in the snowpack, but do not necessarily indicate an avalanche is imminent.

Where they are

Glide Avalanches occur on smooth slopes. Grassy slopes, rock faces, and glacial ice or firn are common surfaces. The free water can spread evenly, and it is easy for the water to drown surface roughness. Very rough surfaces tend to channel water, rather than allowing it to spread out, and have very high friction.

Glide cracks often form where the ground surface changes from rough to smooth, and on convex rolls with a smooth surface. Glide Avalanches tend to happen on specific slopes.

Timing

Glide Avalanches require free water in the snowpack. In a continental snowpack, most free water comes from melting in the spring. Here the snowpack can be moisture saturated at any time.

Glide Avalanche release is challenging to predict. Surface conditions are not a good indicator, because it can take hours or days for free water to run through the snowpack and pool on the ground. Several days of warm temperatures can make glide avalanches more likely, but the avalanches sometimes release after the temperatures cool, but free water continues to run through the snowpack.

Recognition

Glide cracks are an indication that glide avalanches are possible. The cracks point to slopes with sufficient smoothness. Because the gliding occurs at the ground surface, the rate is hard to observe. There is not a direct correlation between weather events and Glide Avalanches, so those factors are not useful predictors.

Treatment and Avoidance

Predicting the release of Glide Avalanches is very challenging. Because Glide Avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide Avalanches.

Knowledge of the ground surface can be helpful. You can visit favorite areas in the summer and look for smooth grassy slopes and rock faces. Some of those slopes can be identified from air photos or satellite imagery.

This Advisory applies to the Mt. Juneau Avalanche Zones



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