Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, March 22nd, 2009 at 7am. This will serve as a general backcountry avalanche advisory issued 5 days a week Wednesday through Sunday for the Turnagain Arm area (Turnagain Pass is the core advisory area). Local variations always occur. This advisory expires in 24 hours and does not apply to operating ski resorts or highways/railroads.
MOUNTAIN WEATHER ROUND UP FOR THE LAST 24 HOURS
-General Weather Observations-
Yesterday skies were sunny and clear in the morning, then the clouds moved in and it started snowing around 4pm. Mountain temperatures only warmed to the mid to high teens. Ridgetop winds were light and variable for most of the day but started to increase last night. 12-17 inches of new snow fell last night throughout our advisory area, and it is still snowing. As of 4am this morning, mountain temperatures ranged from 14-19 degrees F with winds in Turnagain Pass averaging 20 mph out of the east. The winds on Max’s in the Girdwood Valley picked up around midnight and are currently averaging 10-15 mph out of the east with gusts to 45. Sea level winds in Portage are averaging 20 mph out of the northwest. An additional 7-14 inches of snow is forecasted for today and tonight with mountain temperatures warming to the low 20’s. Later today, east winds will increase to 30-40 mph through Portage Valley and Turnagain Arm with 20-30 mph northeast winds elsewhere.
-The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass-
recorded 16 inches of new snow and 1 inch of water. The current temp is 14F (5 degrees warmer than yesterday) with a total snowpack depth of 92 inches.
-The Sunburst weather station at 3800 feet in Turnagain Pass-
recorded light and variable winds averaging 5-10 mph during the day yesterday. At 6pm, the winds started to increase and blow out of the east averaging 15-20 mph with gusts to 33….The current temp is 17F (9 degrees warmer than yesterday).
Primary avalanche concerns
-On southerly aspects, recently formed surface hoar on top of a 1-2 inch thick sun crust is now buried under 12-17 inches of new storm snow.
-From sea-level up to at least 3800 feet, a thin “glazed donut” freezing rain crust that formed 3/5/09 is now buried 2-3 feet deep. This layer was responsible for some of our most recent human-triggered avalanches.
-Below 3000 feet, the “January Hurricane” rain crust is now buried 3-6 feet deep. There is a layer of weak sugary faceted snow surrounding this hard rain crust. Stability tests on this layer continue to show clean shears on top of this crust, a sign of lingering instability.
AVALANCHE AND SNOWPACK DISCUSSION
The avalanche hazard has INCREASED since yesterday. Today, a HIGH avalanche danger exists on all wind-loaded slopes, especially on southerly aspects where buried surface hoar is sitting on a sun crust under 12-17 inches of new and windblown snow. The addition of over a foot of new snow in the last 12 hours, and winds strong enough to move the snow onto the lee side of ridges and gullies, means that human-triggered avalanches are likely on wind-loaded slopes. Non wind-loaded slopes have a CONSIDERABLE danger. Please remember that the majority of human-triggered avalanches occur during or within 24 hours of a storm.
Dangerous avalanche conditions exist today, and things will only get worse as the winds increase and the snow keeps falling. Stay out from under recently sun-affected southerly slopes where all of this new snow is sitting on buried surface hoar on top of a slick sun crust. There’s a lot of powder to be blown around, especially on northerly aspects that were not baked by the sun. The snow will be sensitive to human triggers with natural avalanches certainly possible on actively loading steep slopes. Remotely triggered slides are also a concern on southerly aspects where the buried surface hoar is sitting on a sun crust. This feathery crystal is famous for producing long-running fractures, and it’s even more dangerous when it forms on a slick crust and is buried intact standing upright.
Besides today’s danger from wind-loading and buried surface hoar, further instability is associated with a thin “glazed donut” freezing rain crust that formed 3/5/09 and is now buried 2-3 feet deep. Some of our most recent human-triggered avalanches ran on this layer this past Wednesday. We also received a report of a recent, possibly skier-triggered slide on the south face of Cornbiscuit at 3200 feet that failed just underneath the 2 inch thick surface sun crust (see photo gallery). This small, shallow slab was 30-40 feet wide and slid about 600 feet nearly to the valley floor.
And of course this advisory would not be complete without mentioning the “January Hurricane” rain crust/facet layer. Last week we had widespread collapsing and shooting cracks at the lower elevations (in addition to clean fast Q1 shears in numerous stability tests on this layer), but for some reason 1-2 feet of new snow on St. Paddy’s Day was not enough to tip the balance. Maybe this storm will. Extra caution is advised at the lower elevations as this storm progresses and adds a significant load to our snowpack.
This concludes today’s advisory; the next advisory will be Wednesday, March 25th. If you are out in the backcountry and have the chance, please send us your observations. Simply click on “Submit a snow observation online” at the top of the advisory page and fill in the blanks. Thanks and have a great day!
The weather forecast for:
WESTERN PRINCE WILLIAM SOUND-
500 AM AKDT SUN MAR 22 2009
.TODAY…SNOW. SNOW ACCUMULATION 3 TO 6 INCHES. HIGHS IN THE MID
20S TO LOWER 30S. NORTH TO EAST WIND TO 15 MPH.
.TONIGHT…SNOW. SNOW ACCUMULATION 4 TO 8 INCHES. LOWS IN THE
20S. NORTH TO EAST WIND 20 TO 30 MPH EXCEPT EAST 30 TO 40 MPH THROUGH
PORTAGE VALLEY AND TURNAGAIN ARM.
.MONDAY…SNOW. SNOW ACCUMULATION 2 TO 4 INCH. HIGHS 30 TO 35. NORTH
TO EAST WIND 15 TO 25 MPH EXCEPT NORTH 35 MPH NEAR SEWARD.
.MONDAY NIGHT…SNOW LIKELY. LOWS IN THE 20S. NORTH TO EAST WIND 10
TO 20 MPH.
.TUESDAY…MOSTLY CLOUDY WITH SCATTERED SNOW SHOWERS. HIGHS IN
THE 30S. EAST WIND 10 TO 15 MPH.
TEMPERATURE / PRECIPITATION
SEWARD 32 28 33 / 100 100 80
GIRDWOOD 27 25 35 / 80 80 60