Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, March 15th, 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-
This past week Turnagain Pass received 6 inches of new snow and the Girdwood Valley 8-10 inches. Most of this snow fell in 1 inch increments, except on Monday when the Girdwood Valley got 5 inches. Yesterday a trace of new snow fell in Turnagain Pass and the Girdwood Valley. Ridgetop winds were light and variable, and mountain temperatures climbed to the high teens to low 20’s. Sea level winds near Portage, however, averaged 15-20 mph out of the northwest all day yesterday. A dense band of fog lingered at the mid to upper elevations and finally burned off late in the afternoon.
Mountain temperatures really took a dive and currently range from 4-8 degrees F. Skies are currently clear, and ridgetop winds are light and variable. Sea level winds in Portage continue to average 15-25 mph out of the northwest this morning. A stationary ridge of high pressure over most of mainland Alaska will bring unseasonably cold temperatures and sunny skies today. Winds will average 10-25 mph out of the northwest with much stronger, gustier winds closer to the coast. A complex low in the gulf may cloud up our skies tomorrow and Tuesday.
-The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass-
recorded no new snow. The current temp is 6F (12 degrees colder than yesterday) with a total snowpack depth of 68 inches.
-The Sunburst weather station at 3800 feet in Turnagain Pass-
recorded light winds averaging 3-8 mph out of the northwest all day yesterday. The current temp is 4F (10 degrees colder than yesterday)
AVALANCHE AND SNOWPACK DISCUSSION
For today, pockets of CONSIDERABLE avalanche hazard exist on steep unsupported slopes and convex rollovers below 3000 feet elevation on all aspects. A layer of weak faceted snow on top of a thick rain crust buried 1-4 feet deep below 3000 feet continues to show signs of DECREASING stability. All CNFAIC Staff slopes steeper than 35 degrees have a MODERATE danger. Point release natural avalanches may occur on steep southerly aspects warmed by the sun.
The avalanche danger will increase if northwest winds pick up in the alpine and start actively loading lee aspects. Sensitive windslab will develop on leeward southerly aspects, but watch for localized winds and wind channeling off the ridgelines loading different aspects.
There’s all sorts of funny business going on in our snowpack right now as we continue to get reports of wide spacial variability. Some folks dig down in the snow and find perfectly stable conditions while CNFAIC Staffs are finding highly unstable areas with collapsing and shooting cracks. For the most part these areas of instability are confined to the lower elevations below 3000 feet where the layer of weak faceted snow on top of the January rain crust persists. This thick crust is generally buried 1.5 to 2.5 feet deep in Turnagain Pass and up to 4 feet deep in the Girdwood Valley. Most of our action has been occurring lately in the 1000-2000 ft. elevation band, so extra caution is advised on steep slopes in these areas.
Yesterday a group from the Alaska Avalanche School toured up Tincan and dug a pit on a steep rollover at 1400 feet on a northwest aspect. The entire slope collapsed before they started digging, and their stability tests produced clean fast Q1 shears on the layer of facets above the crust (CTE4Q1, RB2Q1, ECTP6Q1 @55cm). This is a HUGE red flag that indicates this layer will easily propagate fractures. AnCNFAIC Staff group of students several hundred feet higher on the same aspect found nothing alarming in their stability tests on the facet/crust layer and had a totally different take on the general snowpack stability. Several of our observers reported widespread collapsing on these facets between 1200 and 1800 feet elevation on Pete’s South earlier this week in addition to 70′ long shooting cracks and clean shears in their stability tests. AnCNFAIC Staff one of our observers reported collapsing of the snowpack at the lower elevations of the Bertha Creek drainage yesterday, most likely on this layer of facets above the crust. Although it’s been three weeks since we’ve had any avalanches on this layer, all signs point to this layer waking up, like a bear coming out of its den. Extra caution is advised on steep unsupported slopes at these lower elevations.
There are a number of instabilities at the higher elevations within the top 1-2 feet of snow that could be sensitive to human triggers. The snow is a mix of powder, windslab, melt-freeze crust on southern aspects, thin freezing rain crust on all aspects, and facets. The skiing and riding conditions are challenging in some areas with variable breakable crusts and slabs, and excellent in CNFAIC Staff areas with powder on supportable crust. I would expect any avalanches on these surface layers to be small and not propagate very far, but this could change if strong northwest winds materialize in the alpine areas and load lee aspects. Visibly drifting snow, shooting cracks, and hollow drum-like hard snow are the most obvious clues to wind slab formation.
This concludes today’s advisory; the next advisory will be Wednesday, March 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 AKDT SUN MAR 15 2009
…STRONG WIND THROUGH THROUGH MONDAY MORNING…
.TODAY…SUNNY. HIGHS IN THE UPPER TEENS TO MID 20S. SEWARD AND
WHITTIER…NORTH TO WEST WINDS 20 TO 35 MPH WITH GUSTS TO 45 MPH.
ELSEWHERE…NORTHWEST WINDS 10 TO 25 MPH.
.TONIGHT…INCREASING CLOUDS WITH A CHANCE OF SNOW TOWARD MORNING.
LOWS ZERO TO 10 ABOVE. SEWARD AND WHITTIER…NORTH TO WEST WINDS 20
TO 35 MPH WITH GUSTS TO 45 MPH. WINDS DIMINISHING NEAR WHITTIER AFTER
MIDNIGHT. ELSEWHERE…NORTHWEST WINDS 10 TO 25 MPH.
.MONDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW. HIGHS IN THE UPPER
TEENS TO MID 20S. NORTH TO EAST WINDS 10 TO 25 MPH SEWARD….NORTH
WINDS GUSTING TO 45 MPH IN THE MORNING DIMINISHING TO 35 MPH IN THE
.MONDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF SNOW. LOWS 5 TO
15 ABOVE. NORTH TO EAST WINDS 10 TO 25 MPH. GUSTS TO 40 MPH NEAR
.TUESDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW. HIGHS IN THE
UPPER 20S TO MID 30S. NORTH WIND 15 MPH. NEAR SEWARD…NORTH WIND
30 MPH DECREASING TO 15 TO 20 MPH IN THE AFTERNOON.
TEMPERATURE / PRECIPITATION
SEWARD 24 11 25 / 0 0 40
GIRDWOOD 21 -1 21 / 0 0 40