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Logan area Avalanche advisory

Friday April 21, 2006

Hello, this is Toby Weed of  the Forest Service Utah Avalanche Center with your Logan area backcountry avalanche and mountain weather advisory. It's Friday April 21st at 9:30 pm.  This advisory is brought to you in part by the Friends of the Utah Avalanche Center in Logan with help from you.  We operate thanks to local public support.

Current Conditions

I've heard that some folks took advantage of this week's cold spell and found some nice supportable spring-like snow conditions.  Sadly, these stabilizing conditions didn't last long, and we'll be back into the melt-down mode this weekend.  Overnight temperatures will stay above freezing again in most locations, and cloud cover may prevent much long wave radiation cooling.  Any surface crusts that you may find tomorrow morning won't last very long as temperatures continue to climb.  

Snow and Avalanche Conditions:

 As the snow softens up with daytime heating this weekend, wet avalanches will be possible on steep slopes.  Isothermal or equal-temperature snow is solid when it has been refrozen between periods of melt.  But it more closely resembles a 7/11 Slushy on an incline when nighttime temperatures stay mild.  Melting destroys the layered winter snowpack, weakening the bonds between individual snow crystals from the surface down.  Melt-softened saturated snow tends to want to flow downhill with the pull of gravity.  Usually the natural down-slope movement occurs gradually, but a few different types of wet avalanches also become more likely during the annual spring melt-down.

Loose wet avalanches are easily triggered and can spontaneously release when the surface layers are saturated and sticky.  This fairly manageable type of slow moving avalanche, called a point release, starts narrow, (only the width of your turn or the size of a snowball rolling off a tree branch), but becomes increasingly wide in its descent.  This time of year, loose wet avalanches can entrain large piles of wet snow, especially on long, sustained slopes.  Occasionally, the weight of a smaller wet point-release avalanche overrunning a steep slope will trigger a larger, more dangerous slab avalanche.  

As melt-water seeps into the snowpack it may pool on crust layers or substantially weaken existing buried weak layers.  Solid winter slabs are also softened by the warming, sometime becoming sensitive to triggering.  Wet slab avalanches become more possible in these conditions.  This year, a common natural trigger has been cornice falls.  With this year's way above average snowfall, monstrous cornices formed along ridge-tops and above steep avalanche-prone slopes in the region.  Many of these are sagging ominously in the heat, and some could spontaneously collapse.  This is exactly what happened in the Wellsville Range last weekend, and a cornice-fall probably triggered a large natural wet slab avalanche on Easter at the top of the North Fork of Shumway Canyon.

If the entire snowpack starts to move down a slick underlying slope we say it's gliding, and glide cracks commonly form on steep slopes this time of year.  Sometimes this normally slow process speeds up drastically and a glide avalanche occurs.  Although these avalanches are common on certain slopes during the spring melt, they are not necessarily driven by warm daytime temperatures like the previously mentioned problems, and they can spontaneously avalanche any time of day or night.  It's wise to give glide cracks and giant cornices lots of room, and to plan a route that does not take you underneath these somewhat unpredictable dangers.

Bottom Line:

The danger will rise as slopes are heated and softened this weekend. Wet avalanches are possible on any slope steeper than about 35 degrees with melt-saturated snow.  Lack of overnight refreezes and warm daytime temperatures this weekend could cause avalanches to become probable in some areas.  You should leave any steep slope where you are sinking in over your boot-tops into mushy or sodden snow.

Mountain Weather:

 Cloud cover will keep mountain temperatures much warmer tonight, but the sun will be back in full force on Saturday.  Some cloudiness may trap heat during the day, and clouds will build in again tomorrow night.  Saturday night's low temperature at 9000' is forecast to stay up near 40 degrees, with thunder showers likely, and  convective activity will continue affect the region on Sunday.  Some thundershowers could produce significant rainfall.

General Information: 

If you're confused by some of our avalanche terms check out the cool new Avalanche Encyclopedia

For a list of recent avalanches in the regional backcountry go to Avalanche List.  Snow nerds, check out the new Snow Profiles page.  Check out our Images Page for pictures of recent local avalanches.

Please send backcountry observations to [email protected] or leave us a message at 755-3638, especially if you see or trigger an avalanche in the backcountry. We really want to hear from you, even if you think your observation is unimportant.    The information you provide may save lives...

  The information in this advisory is from the U.S. Forest Service, which is solely responsible for its content.  This advisory describes general avalanche conditions and local variations always occur.