Sun Valley Avalanche Bulletin

This bulletin is from the Sawtooth National Forest Avalanche Center in Idaho.

Sawtooth National Forest Avalanche Advisory

This information only applies to the backcountry and does not apply to highway programs or operating ski areas. Changing conditions and local variations may occur.

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This will be our last update for the 2011-12 season. It includes discussions of weather and snowpack conditions since our last advisory on Sunday April 15th and quick summaries of late-season avalanche concerns.

Our efforts this season would not have been possible without our advisory sponsors, our Friends' board, local avalanche professionals, and you - the backcountry riders and skiers who read our advisories, take our advice to heart, and provide us with observations. From Chris, Simon and I, thanks for a great winter! Weather Discussion: Since Sunday, snow showers and a freeze-thaw temperature cycle have dominated the weather in our advisory area. The snow showers have been concentrated in our northern zones, and it looks like the Sawtooth Mountains: and Salmon Headwaters have picked up 0.7-1.2" of SWE, and 4-8" of dense new snow at upper elevations. Galena Summit has picked up about half that, while stations to the south and east are showing minimal accumulations. Since early Tuesday, winds at Titus Ridge have been westerly - southwest through west-northwest - and averaging about 15 mph with gusts of 20-35 mph. Winds at Baldy haven't been as strong, and wind directions have been slightly more variable.

Mid- and upper-elevation stations are showing a distinct freeze-thaw temperature cycle, with mid elevation stations recording daytime highs in 40s and low 50s and overnight lows in the 20s. Temperatures have been warmest in our southern zones, and above about 10000 feet it doesn't look like daytime highs have regularly warmed above freezing.

Snowpack Discussion:

Outlook:

General Springtime Avalanche Discussion: Late-season conditions - snow surface and stability - are much less predictable than mid winter conditions. Late season conditions can also change very rapidly - in the span of an hour or two. So over the next few weeks, it'll be critical to be pay attention as you move from warmer to colder aspects, or vice versa, or climb or descend. While a different snow surface might make for better (or worse) skiing or riding, it can also mark a different avalanche concern and indicate a significant change in stability.

I've summarized some key aspects of different late-season avalanche issues in the geek-speak sections below: Wind slabs: These are typically not as frequent or widespread a concern as in mid winter but can be very dangerous nonetheless. Last season, numerous late season accidents in the western US, including several fatalities, involved parties triggering wind slabs in couloirs and other steep terrain. For wind slabs to form, dry snow has to be available for transport. It doesn't take much; moderate winds can redistribute just a few inches of new snow into 6-10 inch pillows and slabs in a matter of hours, so look for these after any late-season snowfall. Their stability depends on their bond with underlying snow surface - which in turn depends on temperature and nature of snow surface. The bond tends to be weaker when cold, hard slabs form on cold, weak facets or cold hard crusts, and they tend to stabilize quickly as they warm. Look for snow with a chalky, cakey or hollow feel near ridgelines and the tops of couloirs.

Storm snow: Late-season winter storms can drop enough new snow to create winter-like avalanche danger, particularly at upper elevations.

Be looking for this danger after any precipitation; rain in town is often new snow on peaks. The concern is typically just the bond between the new snow and the old snow surface; less often, late winter storms reactivate more deeply buried weak layers on colder, shadier slopes.

Storm snow instability gives itself away with shooting cracks and recent avalanche activity. Active tests like slope cuts, stepping above your partner's skin track and hand pits are a great way to check the bond between new and old snow. Direct sun can rapidly destabilize freshly-fallen snow, particularly with the sun higher in the sky and supplying more intense radiation, so pay attention to cloud cover and move off and out from under steep slopes if you see rollerballs, pinwheels and sluffs below rocks, cliffs and trees.

Wet Snow Avalanches: This instability results from liquid water draining through snowpack; warm temperatures and radiation produce the water by melting the snow surface. The instability decreases once drainage channels are established, but this process can take weeks, and isn't complete until late spring on shadier, upper elevation slopes.

While it's very difficult to determine how deeply and efficiently melt water is percolating through the snowpack on a given slope, it's easy to recognize the conditions that produce lots of water: warming temperatures and direct sun during the day, long periods (roughly 48 hours) without a surface refreeze of the snowpack, and less frequently, rain. A clear sign of instability is a mushy snow surface in which you or your sled are sinking more than 6-8".

Wet loose snow avalanches occur when the near-surface layers of the snowpack can't drain all the meltwater produced there. They're nothing to trifle with; the debris is dense and riding out of it is difficult.

If you're on steep slopes, they can carry you over or into terrain traps that lead to trauma even if you're not buried. Avoid this danger by venturing into steep,, consequential terrain during periods with overnight temperatures below freezing and riding or skiing early, before the snow surface softens.

Wet slab avalanches are unpredictable, because they fail when deeply buried weak layers are strained by the addition of melt water or by water changing properties of previously stable slabs. It takes a prolonged or dramatic warm-up for water to percolate deep enough in the snowpack for this to occur where the snow is deep, such as upper elevation, northerly slopes. Our snowpack presently contains one or more of these persistent weak layers - one or two early season basal weak layers, and a mid-pack layer buried February 17th. All seem to have consolidated and strengthened, but remain candidates for producing isolated wet slabs. If you venture onto steep, shady slopes, recognize that conditions and stability on such slopes can be much different than on warmer slopes. Don't travel during the conditions that increase melt at the surface, and use the same strategies to increase your odds that would with other deep slab concerns: avoid likely trigger points like abrupt changes in slope angle, and slopes with highly variable snow depths.

Regardless of conditions - and whether your wearing a Hawaiian shirt or a puffy - carry avalanche rescue gear and follow safe travel practices that minimize exposure and the consequences of misjudging stability.

Ride or ski one at at time, have an escape route, and keep your partner in sight at all times. Enjoy the mountains!!

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Reported Conditions

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• Titus Peak, Idaho, 10,080'
• Lower Titus Study Plot Idaho
• Sun Valley Bald Mountain, 9000'

Hazard Scale - [Full Size] [Pocket Reference]

www.avalanche-center.org