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ARCHIVED - Forecasts expire after 24 hours.
Tue, April 23rd, 2013 - 7:00AM
Wed, April 24th, 2013 - 7:00AM
Wendy Wagner
Avalanche risk The Bottom Line

There is a MODERATE avalanche danger today (Tuesday) below and above treeline. Below treeline, wet loose snow avalanches will be possible with the warm temperatures and light rain. These are likely to be easy to trigger and in the top foot of the pack. Above treeline, watch for shallow wind slabs to develop with 4-6″ of snow and moderate to strong easterly wind in the forecast.

Looking forward:

The main issue for Wednesday and into the remainder of the week+ will be the impending “melt down” or “shed cycle”. This is when the mountain snowpack literally falls off the slopes and very large and destructive avalanches can occur. This period usually lasts around 2 weeks and is on our doorstep. Once the cycle begins, travel in valley bottoms, and most places for that matter, are at risk as debris can be large, run quite far and bulldoze through just about anything. Stay tuned for Kevin’s forecast on Thursday to see where are sitting with this.

Special Announcements

We will be issuing advisories on Saturday, Sunday, Tuesday and Thursday through the end of the April. The next advisory will be Thursday, April 25th.

Tue, April 23rd, 2013
Above 2,500'
2 - Moderate
Avalanche risk
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Yep, we are finally getting our warm April temperatures and a little rain and snow today. We only have a small amount of precipitation forecast but, the main concern is how the snowpack is reacting to the warm temperatures and cloud cover. Below treeline, watch for the top 6-12+” of snow to lose cohesion and be easily pushed down any slope over 35 degrees. Mainly wet loose avalanches will be possible to trigger but there is also a potential for wet slabs around a foot deep to pull out. It is best to steer clear of any slope with soft wet snow deeper than 6″. Additionally, buried crusts in many areas can provide great bed surfaces allowing any slide triggered to run further than expected.

As for the larger and more dangerous slides: Once the entire pack warms up to a balmy 32F (0C) we call it isothermal and it looses much of its stability. This is the primary contributor to the shed cycle. Right now the snowpack at 2,000′ is not quite warm enough (see image below). However, snow can change rapidly and with that the avalanche danger. You can keep tabs on the evolution of the snowpack HERE but also keep in mind this is only one point (2,000′ on Center Ridge) and the snowpack varies considerably with elevation and aspect.

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
More info at Avalanche.org

Above treeline we can expect several inches of heavy snow today with moderate to strong east winds. The new snow is likely to stick rather well to the existing surface at the mid elevations but upper elevations should see dryer snow with more of a classic winter wind slab problem. Expect any fresh slab found at these upper elevations to be sensitive as they will be sitting on either crusts (wind or sun) or loose faceted snow.

Additional Concern
  • Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Cornices are likely to start peeling off any day now with our recent warm temperatures. They have the potential to trigger avalanches below, entrain large amounts of snow and run into valley bottoms. Again, with the springtime warm-up on the way, be very wary of traveling in drainage bottoms for the next several weeks.

Tue, April 23rd, 2013

Yesterday we had a brief return to the sunny skies that have characterized  April to date. Temperatures were quite warm, around 30F on ridgetops and the upper 40’s at 1,000ft. Ridgetop winds during the past 24 hours averaged ~15mph with gusts to 35mph from the east. The last snowfall ended 13 days ago, April 9th. Overnight, cloud cover has moved in and temperatures have only dropped 5-10F – it’s feeling like spring finally.

Today a warm, wet and windy, but fast moving, system is moving through from the Gulf. We should see around 4-6″ of dense snow above treeline and .3″ of rain below treeline throughout the day (rain/snow line ~1,500ft). Temperatures look to rise back to 30F on ridgetops and the mid 40’s at 1,000ft. Winds will remain from the east in the 15-20mph range with gusts to ~40mph.

Wednesday we should see partly cloudy skies as the system moves out and we return to a dry, but warm, period for the remainder of the week.

Kevin will issue the next advisory Thursday morning, April 25th.

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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.