Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, February 15, 2008 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.
MOUNTAIN WEATHER ROUND UP
In the last 24 hours…
-The DOT weather station near the crest of the highway in Turnagain Pass at 1000 feet-
is recording a current temp of 29F (1 degree colder than yesterday) with NE winds averaging 9-12 mph.
-The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass-
recorded 2 inches of new snow and 0.1” water. The current temp is 25F (1 degree colder than yesterday) with a total snowpack depth of 67 inches.
-The Sunburst weather station at 3800 feet in Turnagain Pass-
recorded light easterly winds averaging 10-15 mph with gusts in the teens and 20’s during the day yesterday. The current temp is 18F (same as yesterday)
-General Weather Observations-
This past week 7-10 inches of snow fell in Turnagain Pass and 12-16 inches fell in the Girdwood Valley. Temps warmed up significantly midweek as southerly flow returned to the area along with strong easterly winds Wednesday through Friday.
Early this morning, ridgetop temperatures ranged from 18-23 degrees F with increasing winds blowing 20 mph out of the East and gusts in the 30’s and 40’s. Portage is currently 33F and Girdwood 31F. A strong frontal trough ahead of a 980 mb low will be the main weather maker today. Mountain temperatures will remain in the 20’s with up to 10 inches of new snow at the higher elevations forecasted by Monday night. The “R” word is possible at sea level this afternoon as temps warm up to the mid 30’s. Expect gale force winds today out of the East averaging 35-50 mph along Turnagain Arm increasing to 45-60 mph late this afternoon and tonight. Winds will be moderate elsewhere averaging 10-25 mph out of the East.
Primary avalanche concerns
New snow, strong winds, and warm temps will combine to create unstable slabs on top of an already weak, upside-down surface snowpack.
AVALANCHE AND SNOWPACK DISCUSSION
The avalanche hazard will increase today and tomorrow with the arrival of this decent sized storm. Human-triggered avalanches will be probable on slopes steeper than 35 degrees, and natural avalanches will be possible on actively loading slopes. Please remember that the majority of human-triggered avalanches occur during or within 24 hours of a storm.
Avalanche conditions will change rapidly with the arrival of this storm. New snow and strong winds will add energy to the snowpack and increase the avalanche danger. Red flags include shooting cracks, whumphing sounds, and hollow drum-like hard snow. During the storm the new snow will be sensitive to human triggers with natural avalanches possible, especially in higher elevation wind-affected terrain. As the new and wind-blown snow accumulates to depths of 6 inches or more, it will be possible to trigger surface soft slabs on steep slopes. Rain on new snow will cause almost instant avalanching, so be especially cautious if the rain line bumps up in elevation.
Below 3000 feet, layers of weak faceted snow surround a thick rain crust now buried 1-2 feet deep on all aspects. At the higher elevations above 3000 feet, a hard dense slab of snow exists inplace of the rain crust but still provides a slick bed surface for the overlying snow. In the last 2 weeks, numerous small to medium-sized skier-triggered avalanches below 3000 feet ran on this rain crust. Avalanches also fractured on lighter density snow underlying recently formed wind slabs at the higher elevations. The combination of warm dense storm snow on top of lighter density cold snow on top of slick bed surfaces equals dangerous avalanche conditions.
Yesterday I saw a fairly recent natural avalanche in one of the northern chutes on Cornbiscuit (2nd one back from the highway). The crown face was 6-10 inches deep, and the debris ran all the way to the valley floor. What caught my eye was the long vertical propagation…the crown face extended from the ridgetop down approximately 500 feet on both sides of the chute. One of our observers noted lots of whumphing and shooting cracks yesterday on Eddies below treeline. Even though the obvious signs of instability stopped once they got above the trees, they still decided to ski a conservative line in the north bowl. AnCNFAIC Staff one of our observers reported 2 skier-triggered and 2 remotely-triggered avalanches in the upper Girdwood Valley yesterday between 1700 and 2000 feet elevation. Collapsing on wind loaded convexities created these Class 1 and 2 avalanches. The bed surface was the January monsoon crust with faceted snow as the weak layer.
This concludes today’s advisory; the next advisory will be Wednesday, February 18th. 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 AKST SUN FEB 15 2009
…STRONG WIND THROUGH MONDAY MORNING FOR PORTAGE VALLEY AND
.TODAY…SNOW…MIXING WITH RAIN NEAR SEA LEVEL IN THE AFTERNOON.
SNOW ACCUMULATION UP TO 3 INCHES. HIGHS IN THE 30S. SOUTH TO EAST
WIND 10 TO 25 MPH EXCEPT EAST WIND INCREASING TO 35 TO 50 MPH
THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.
.TONIGHT…SNOW AND RAIN. SNOW ACCUMULATION UP TO 4 INCHES…MAINLY
AT HIGHER ELEVATIONS. LOWS IN THE UPPER 20S TO MID 30S. NORTH TO
EAST WIND 10 TO 25 MPH EXCEPT EAST 45 TO 60 MPH THROUGH PORTAGE
VALLEY AND TURNAGAIN ARM.
.PRESIDENTS DAY…SNOW AND RAIN. SNOW ACCUMULATION UP TO 3 INCHES.
HIGHS IN THE 30S. NORTH TO EAST WIND 10 TO 20 MPH. THROUGH PORTAGE
VALLEY AND TURNAGAIN ARM…EAST WIND 30 TO 45 MPH DECREASING TO
15 TO 25 MPH IN THE AFTERNOON.
.MONDAY NIGHT…SNOW. LOWS IN THE 20S. NORTH WIND TO 15 MPH.
.TUESDAY…MOSTLY CLOUDY WITH A SLIGHT CHANCE OF SNOW. HIGHS
AROUND 30. NORTH TO WEST WIND TO 15 MPH.
TEMPERATURE / PRECIPITATION
SEWARD 36 33 36 / 100 100 80
GIRDWOOD 35 32 35 / 100 100 80