Good morning backcountry travelers. This is Jon Gellings with the Chugach National Forest Avalanche Information Center on Wednesday, February 16th at 7am. This will serve as 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).
The avalanche danger today is MODERATE for deep slab avalanches in steep rocky terrain, as well as on slopes with a relatively shallow snowpack. Any triggered avalanche could potentially step down to deeper weak layers, creating a larger and more hazardous avalanche. There is also the possibility of triggering new wind slabs from our recent winds in upper elevation areas, where strong slabs have buried weaker layers of snow.
We have received one report of a human triggered avalanche within the past 3 days, and two reports within the past 5 days. We have more information about the Raggedtop avalanche, so I will discuss some of it here:
-SE aspect on Raggedtop on 2-14
-Wind slab that stepped down to a deeper slab in places
-No burial, but caught and carried 500 ft
More information can be accessed by looking up the following picture in our photo gallery.
There was also an observation that we received three days ago of a skier triggered deep slab avalanche near Bench Lake in the Johnson Pass area. This slide happened on Friday, Feb 11, and it failed in a weak layer of unconsolidated facets near the ground. The northwest facing slope was around 36 degrees steep, with the crown estimated at 30 inches.
Yesterday I called the Raggedtop avalanche a deep slab, but I was incorrect. Parts of it stepped down to our deeper weak layers, but the majority of the avalanche seems to have come from a failure in upper layers of the snowpack. Deep slab avalanches are still our most concerning type of avalanche, although the observed frequency has been slowly tapering off. One theory is that people have not been out travelling in areas where these avalanches are more likely, so we are not seeing as many potential triggers. These deep slabs can possibly be triggered in regions with a thinner snowpack, like in Johnson Pass and in Girdwood, as well as in Summit Lake and some areas of Turnagain Pass. Conditions show that this instability has historically been triggered from beneath steeper slopes, so try avoiding travels in terrain where slopes are looming overhead.
Our most recent windstorm has stopped transporting snow to the extent of two days ago in many of our areas, while some areas may still see active snow plumes in alpine terrain. These plumes have created new wind slabs, which buried a layer of cold, dry, and fluffy new snow, which could act as a significant weakness in the snowpack. Signs that you are travelling on a wind slab include seeing shooting cracks running away from you, and hearing hollow, drum-like sounds on hard snow. Thin areas near ridgelines and convex rollovers are spots where triggering a windslab would be the easiest.
The 7-12 inches of light fluffy snow on the surface has recently shown signs of new snow instability, but it is starting to tighten up and strengthen in many areas. Loose snow sluffs and shallow soft slabs have been failing within new snow density changes, as well as on the old snow/new snow interface. These types of releases are much less dangerous than the deep instabilities and hard wind slabs mentioned above, but they could still create a hazard to humans. Getting knocked down and buried does not happen very often, but it is still a possibility.
Encyclopedia of avalanche terms.
We currently have a temperature inversion in Turnagain Pass, which is creating a new layer of surface hoar at low to mid elevations. This could be our newest buried weak layer if NOAA forecasts for snow tomorrow night become verified. CNFAIC Staffwise, the alpine temps have been steadily in the teens. Winds have died down in many areas, and are not transporting as much snow as in the recent past. Precipitation is unlikely today, as radar, satellite, models and forecasts all show clear to partly cloudy weather.
The Friends of the CNFAIC (FCNFAIC) needs your thoughts! With a new staff of forecasters and a list of previously completed goals, the program is growing and potentially heading in new directions. The FCNFAIC wants to know what you have to say about YOUR avalanche center, so please complete the following anonymous survey by February 20th. Thank you in advance for taking it! Click here to take survey or cut and paste the address directly: https://www.surveymonkey.com/s/THWXVCD
Kevin will issue the next advisory tomorrow morning at 7am. If you get out in the backcountry give us a call at 754-2369 or send us your observations using the button at the top of this page. Thanks and have a great day.
The NWS weather forecast for:
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST WED FEB 16 2011
…STRONG WIND THROUGH THROUGH LATE TONIGHT NEAR WHITTIER…
.TODAY…PARTLY CLOUDY. HIGHS IN THE LOWER TO MID 20S. NORTHWEST
WIND 10 TO 20 MPH EXCEPT WEST WIND 25 TO 40 MPH WITH GUSTS TO 60 MPH
NEAR WHITTIER AND NORTH WIND 25 TO 35 NEAR SEWARD.
.TONIGHT…MOSTLY CLEAR. LOWS 5 TO 20 ABOVE. NEAR WHITTIER…WEST
WIND 30 TO 45 MPH WITH GUSTS TO 60 MPH DIMINISHING TO 15 TO 25 TOWARD
MORNING. NEAR SEWARD…NORTH WIND 25 TO 35 DIMINISHING TO 10 TO 20
TOWARD MORNING. ELSEWHERE NORTHWEST WINDS 10 TO 20 MPH.
.THURSDAY…INCREASING CLOUDS. HIGHS IN THE LOWER 20S TO LOWER 30S.
SOUTHWEST WIND 15 MPH.
.THURSDAY NIGHT…SNOW LIKELY. LOWS 10 TO 25 ABOVE…COOLEST
INLAND. SOUTH TO EAST WIND 15 MPH.
TEMPERATURE / PRECIPITATION
SEWARD 27 16 31 / 0 0 0
GIRDWOOD 24 15 30 / 0 0 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Temperature 13 F. Westerly wind 8 mph gusting to the mid teens.
-2600′ Seattle Ridge Wx Station-
Temperature 2 F. North wind 7 mph gusting into the mid teens.
-1800′ Center Ridge Wx Station-
Temperature 4 F. 0″ new snow. 85” total snow depth.