Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Saturday, December 20, 2008 at 7am. This will serve as a general backcountry avalanche advisory issued for the Turnagain Arm area (Turnagain Pass is the core advisory area). Local variations always occur.
MOUNTAIN WEATHER ROUND UP
In the last 24 hours…
-The Center Ridge weather station at 1800 feet in Turnagain Pass recorded 2 inches of new snow for a total snowpack depth of 53 inches. The temperature this morning is 27 degrees F (2 degrees warmer than yesterday).
-The Sunburst weather station at 3800 feet in Turnagain Pass recorded moderate winds out of the east averaging 15-25 mph with gusts in the 30’s. Max’s weather station at 3200 feet in the Girdwood Valley recorded light easterly winds averaging 10-15 mph with gusts in the 20’s and 30’s. Ridgetop winds are currently averaging 20mph up at the pass with temps ranging from 19-23 degrees F (1-2 degrees warmer than yesterday).
It has been a pretty uneventful week weather-wise with just 2 inches of snow since Dec. 10 and light winds up until yesterday afternoon. The snowpack at Turnagain Pass has settled 20 inches since our last big snowfall 10 days ago when the Girdwood Valley got 50 inches of snow and the pass got about 30 inches. Expect mostly cloudy skies today with easterly winds averaging 15-25 mph and ridgetop temps in the low 20’s. Skies will clear this afternoon through tomorrow with winds shifting to the north as high pressure returns to the area.
1. Pockets of sensitive windslab may exist on leeward westerly aspects in Turnagain Pass near ridgetops and along the sidewalls of subridges in channeled terrain.
2. Persistent Weak Layers of buried surface hoar and sugary October facets at the bottom of our snowpack continue to be a concern and will be for some time. These layers are responsible for most of the human-triggered avalanches so far this winter and are triggered from thin, shallow areas of the snowpack.
3. Glide cracks have appeared over the last few weeks and some of them have avalanched. Avoid traveling underneath them as it is impossible to predict when and if they will avalanche.
AVALANCHE AND SNOWPACK DISCUSSION
Starting yesterday afternoon, stronger winds in Turnagain Pass created pockets of unstable snow. There’s a lot of low density powder to be blown around, and moderate winds in the 20’s will easily drift this snow into touchy slabs. Ski cuts should be effective for dealing with pockets of this surface windslab.
It has been about a week since our last reported human-triggered avalanche when a skier was completely buried on the south face of Cornbiscuit. That same day a skier remotely triggered an avalanche on the SE Face of Superbowl Peak, and multiple parties reported widespread whumphing on N-E-S aspects in the same area. Both of these avalanches ripped out to the ground, failing on the sugary faceted snow and surface hoar that formed in October. They did not, however, propagate across the entire slope, staying confined to isolated terrain features.
We are still dealing with a shallow, early season snowpack in some areas, especially on the southern end of Turnagain Pass. The fact is our entire advisory area has weak faceted snow buried near the ground. In some areas it is buried under 10+ feet of snow and CNFAIC Staffs only 2-3 feet. It is in these shallow areas that human-triggered avalanches are most likely to occur because the weak layer is closer to the surface and thus more easily impacted. Although our snowpit results continue to show good stability on this layer, there are still pockets of instability that could catch someone off guard.
Likely areas to trigger an avalanche are:
-Higher elevation windswept aspects where the snowpack remains cold, shallow, and weak
-Shallow rocky slopes
-Wind-scoured ridges where the snowpack tapers from shallow to deep
Terrain management is the key to staying safe when dealing with the spacial variability of deep-slab instabilities and shallow areas of the snowpack. In CNFAIC Staff words, it is still too early for big exposed complex lines. Keep your angles below 35 degrees, use good travel habits, and evaluate the snow depth and terrain carefully.
This concludes today’s advisory; the next advisory will be Sunday, December 21st. If you are out in the backcountry and have the chance, please send us your observations. Simply click on “observations” next to the advisory page and fill in the blanks. Thanks and have a great day!
The weather forecast for:
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST SAT DEC 20 2008
.TODAY…SNOW SHOWERS IN THE MORNING…THEN SCATTERED SNOW
SHOWERS IN THE AFTERNOON. SNOW ACCUMULATION UP TO 1 INCH. HIGHS
IN THE UPPER 20S TO UPPER 30S. EAST WIND 10 TO 15 MPH. THROUGH
PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 15 TO 25 MPH IN THE
MORNING BECOMING LIGHT.
.TONIGHT…DECREASING CLOUDS. LOWS 10 TO 25 ABOVE…COOLEST INLAND.
LIGHT WINDS. NEAR SEWARD AND WHITTIER…LIGHT WINDS BECOMING NORTH
AND WEST 10 TO 20 MPH.
.SUNDAY…PARTLY CLOUDY. HIGHS IN THE MID TEENS TO LOWER 30S.
LIGHT WINDS. NEAR SEWARD AND WHITTIER…NORTH AND WEST 15 TO 25 MPH.
.SUNDAY NIGHT…MOSTLY CLOUDY. LOWS 10 TO 25 ABOVE. LIGHT WINDS
EXCEPT NORTH 15 MPH NEAR SEWARD.
TEMPERATURE / PRECIPITATION
SEWARD 35 24 28 / 80 0 0
GIRDWOOD 32 18 22 / 40 0 0