Good morning backcountry travelers this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Monday, March 15th 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).
AnCNFAIC Staff 5 inches of new snow fell in the last 24 hours in Turnagain Pass, bringing our snowfall total for the last day and a half to around 7 inches of snow and 0.5 inches of water. It snowed pretty consistently in the Girdwood Valley yesterday, Turnagain Pass was off and on, and Summit Lake was sunny. Girdwood got 10-14 inches out of this system while Grandview got 9 inches. Ridgetop winds averaged 10-20mph out of the east yesterday with gusts to 36mph while mountain temperatures warmed up to the mid 20’s to low 30’s. As of 4am this morning, ridgetop winds are averaging 10-15mph out of the east while temperatures range from 24F@3800′ to 30F@1800′ to 29F@sealevel. Waves rotating around the gulf low will continue to produce snow showers today tapering off by tonight. Expect moderate easterly ridgetop winds averaging 15-25mph and mountain temperatures in the mid 20’s to mid 30’s. We may get a couple more inches of snow today before a ridge of high pressure builds in by midweek.
Today the avalanche danger is rated MODERATE with pockets of CONSIDERABLE hazard on windloaded slopes steeper than 35 degrees. Our main concern today is soft windslab and sluffing from new and wind-drifted snow, especially on southerly aspects where the new snow is sitting on a slick sun crust that formed this past Saturday. This new snow also came in upside-down, starting off cold and light and finishing warm and heavy, adding further to the instability. Large human-triggered sluffs are likely on big slopes steeper than 35 degrees. Also watch for point release natural sluffs on steep southerly aspects warmed by daytime heating, especially if the sun comes out for any period of time. Rollerballs, pinwheels, and loose sluffs are all indications that the surface snow is unstable and that it is time to move to a different aspect.
AnCNFAIC Staff concern is higher elevation windswept aspects where the snowpack remains shallow and weak. Human-triggered avalanches are possible near rock outcrops and scoured ridges where older weak layers are closer to the surface and thus more easily impacted by skis, boards, or snowmachines. Four days ago, a large skier-triggered avalanche on Cornbiscuit was triggered near some exposed rocks on the high point of the ridge while the group was skinning. The slope had already been skied 10 times that day with no obvious signs of instability observed. It propagated 300-500 feet wide and ran 1500 feet down and 100 ft. up the north side of Lipps. The crown face ranged from 1 foot deep at the trigger point to 6 feet deep in the windloaded areas.
Some more thoughts on this slide…The weak layer that failed was possibly a small layer of near-surface facets that formed the previous Sunday (March 7) when skies were sunny and clear. The bed surface was a crust in some areas and hard dense storm snow in CNFAIC Staff areas. Judging by the long propagation, it is also possible that a very small indiscernible layer of surface hoar formed the evening of March 7 on top of the sun crust before it started snowing early the next morning.
Matt 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 AKDT MON MAR 15 2010
.TODAY…NUMEROUS SNOW AND RAIN SHOWERS. SNOW ACCUMULATION UP TO 2
INCHES. HIGHS IN THE 30S. EAST WIND 10 TO 25 MPH EXCEPT NORTH 10 TO
20 MPH NEAR SEWARD.
.TONIGHT…MOSTLY CLOUDY WITH ISOLATED SNOW SHOWERS. LOWS IN THE
MID 20S TO LOWER 30S. VARIABLE WIND 10 MPH EXCEPT NORTH 10 TO
15 MPH NEAR SEWARD.
.TUESDAY…PARTLY CLOUDY IN THE MORNING…THEN MOSTLY CLOUDY WITH
ISOLATED SNOW SHOWERS IN THE AFTERNOON. HIGHS IN THE MID 30S TO
LOWER 40S. VARIABLE WIND 10 MPH.
.TUESDAY NIGHT…MOSTLY CLEAR. LOWS IN THE 20S. VARIABLE WIND TO 10
MPH. NEAR WHITTIER…WEST WIND 10 TO 15 MPH.
.WEDNESDAY…MOSTLY SUNNY IN THE MORNING THEN BECOMING PARTLY
CLOUDY. HIGHS IN THE 30S. LIGHT WINDS EXCEPT WEST 10 TO 15 MPH
TEMPERATURE / PRECIPITATION
SEWARD 38 29 39 / 80 30 20
GIRDWOOD 37 28 39 / 60 20 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
recorded light to moderate winds yesterday averaging 10-20mph out of the east with gusts to 36mph. The current temp is 24F (10 degrees warmer than this time yesterday) with winds averaging 15mph out of the east.
-2600′ Seattle Ridge Wx Station-
recorded light winds yesterday averaging 10-15mph out of the southeast with gusts to 24mph. Winds are currently averaging 10mph out of the southeast.
-1800′ Center Ridge Wx Station-
recorded 5 inches of new snow and 0.4 inches of water in the last 24 hours. The current temp is 30F (15 degrees warmer than this time yesterday) with a total snowpack depth of 130 inches.