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 Tuesday, April 15, 2014

Expires 7:00am on Thursday, April 17, 2014
Issued by Pat Dryer


Avalanche Danger Level 1

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Avalanche Trend Level 4Avalanche Size Level 1

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Today's Discussion

TODAY...CLOUDY. SLIGHT CHANCE OF RAIN. HIGHS AROUND 49. LIGHT WINDS BECOMING SOUTHWEST 10 MPH IN THE LATE MORNING.

TONIGHT...CLOUDY. CHANCE OF RAIN. LOWS AROUND 37. SOUTHEAST WIND 10 MPH.

WEDNESDAY...MOSTLY CLOUDY. CHANCE OF RAIN. HIGHS AROUND 49. SOUTHEAST WIND 10 MPH SHIFTING TO THE NORTHEAST IN THE AFTERNOON.

Yesterday temperatures reached 7.2C(45F) at both the Tram and Eaglecrest. The Tram weather station did not drop below freezing last night while Eaglecrest dropped to -1.1C (30F). Winds were light (less than 10 knots) and there was no precipitation at any of the stations over the past 24 hours.

Today is forecasted to be similar to yesterday with light rain (0.04”) and calm winds. I don’t anticipate the light rain to have much of an effect on the snowpack at this time. This is the second consecutive day at the Tram without a solid refreeze overnight. We are moving towards more of a spring isothermal snowpack as temperatures remain above freezing in our start zones. We may start to see some glide avalanches in the days to come as our spring cycle starts shape up.

The avalanche danger is LOW today. Remember to minimize exposure in avalanche runouts in the heat of the day when wet slides normally occur.

This is not a backcountry forecast. Backcountry travelers be sure to do your own terrain and snowpack evaluation as spatial variability exits. On days with strong diurnal warming it is best to travel through avalanche terrain in the morning when the snow has refrozen from the previous days solar warming.

Today's Avalanche Tip

As we move into our spring avalanche cycle start to think about what instabilities we may have lurking in the snowpack. Earlier this season you heard us talking about a persistent weak layer that failed led to several avalanches in the area. Here is an excerpt from powder magazine’s recent article on spring avalanches;

“There are two ways to trigger avalanches: stress the snowpack or weaken it. Dry slab avalanches, which are common in the colder winter months, occur when too much weight (or stress) is added to the snowpack. Layers can collapse under a heavy load of new snow, windblown snow, or skiers. In the spring, though, wet slab avalanches happen when the snowpack loses strength. Melting water breaks down snow crystals and provides lubrication between layers, which causes them to avalanche. Some types of layers are more vulnerable to this type of weakening, and this season the western U.S. is especially prone to one of the worst layers for wet slabs—depth hoar.

This spring, these faceted grains are the primary weak layer that can result in destructive wet slab avalanches. Sunny days are “solar storms” that when coupled with above freezing nighttime temperatures will raise the avalanche danger quickly. Typically, 48 to 72 hours of warm conditions will ripen the snowpack for wet slabs. Once water begins to flow, it weakens the internal structure of the snowpack, and when the liquid reaches the depth hoar crystals, they will crumble and avalanche rapidly.

Water flowing for a week or more will find paths of least resistance and create channels, which will act as pipes and flush the water out of the snowpack with only a minimal reduction in strength. The days before these channels form are the most dangerous. That’s when the entire snowpack is moist, weak, and primed to avalanche.”

The full article can be found at http://www.powdermag.com/avalanche-education/aware-depth-hoar/

This Advisory applies to the Mt. Juneau Avalanche Zones



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