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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, April 25th, 2013 - 7:00AM
Expires
Fri, April 26th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

The shot of rain and snow has passed, with sunny weather and colder nighttime temperatures back in the mix.  This morning will start with a  LOW  avalanche danger, which will increase to  MODERATE  as the daytime temperatures heat up the surface crusts.  

A lot of small wet avalanches will be visible on the mountains today, which mostly happened during the storm on Tuesday.  Wet avalanche problems are one of larger concerns, but the deeper snowpack still has quite a bit of strength as it remains colder.  

Higher up in the mountains you can find drier snow and some corresponding wind slab issues from new snow.

Special Announcements

We will be issuing advisories on Saturday, Sunday, Tuesday and Thursday through the final weekend of April. The next advisory will be Saturday, April 27th.

Thu, April 25th, 2013
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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

Wet avalanches will be a concern in the afternoon, mostly on sunny south facing slopes and primarily at lower elevations.  Overnight temperatures briefly dipped below freezing at sea level last night, which is colder than two nights ago.  

Sometime soon we expect to see a more dramatic warmup of the entire snowpack, at which point stability will become a major concern with larger avalanches a possibility.  I don’t think we are at that point today, but it can be a tough threshold to predict unless we actually see it happening.  Backcountry travel should be more careful late in the day as warmer temperatures cause stability to get worse.  If you start seeing spontaneous avalanche activity and you can sink your foot deep into the snow with ease, it’s time to be back at the car or another safe location.  

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

You can still find winter at higher elevations.  The recent storm dropped at least 5 inches of new snow in some areas, which will be deeper on wind loaded slopes.  Be aware that this new snow could be sitting on slick melt/freeze crusts, especially on south facing slopes.  

Additional Concern
  • Cornice
    Cornice
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

I continue to be impressed at the size of looming cornices this late in the season.  Colder than normal temperatures have been keeping ridgetop melting to a minimum.  Just like wet slab problems, cornices will lose strength as they heat up late in the day.  They will become increasingly unstable and may collapse spontaneously with warm enough temperatures.  Watch your exposure, especially late in the day when temperatures are highest.

Weather
Thu, April 25th, 2013

Center ridge gained at least half an inch of water equivalent at Turnagain Pass over the last couple days.  At higher elevations this means 5 or more inches of new snow.  

Temperatures last night dropped to 31 degrees at sea level, colder at higher elevations.  We can expect a thin surface crust on the snow this morning.

Today’s weather is expected to be sunny and warm.  High temperatures will reach the mid 40s at sea level.  A north wind is expected, which may channel into strong wind at Whittier and Seward.  

We will issue the next advisory on Saturday, April 27th.

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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.
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