Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, March 1st, 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 does not apply to operating ski resorts or highways/railroads.
MOUNTAIN WEATHER ROUND UP FOR THE LAST 24 HOURS
-General Weather Observations-
February left with a flourish and surprised everyone with quite a bit more snow than expected. Since early yesterday morning the Girdwood Valley received about a foot of new snow and Turnagain Pass 5-7 inches. The snow stopped falling around 8 pm last night, and skies are currently clear. Prior to yesterday, only 2 inches of snow fell this past week throughout the advisory area.
Early this morning, mountain temperatures were in the single digits to low teens and winds were increasing in most locations. At 43 Mile Peak they picked up last night and are currently averaging 25-50 mph out of the NW with gusts to 79. The winds on Max’s in the Girdwood Valley are currently averaging 20-25 mph out of the east with gusts in the 30’s and 40’s. A weak ridge of high pressure will bring sunny skies today and continued cool mountain temperatures in the single digits. Prevailing winds will blow out of the west averaging 15-30 mph with much stronger, gustier winds through channeled terrain near the coast.
-The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass-
recorded 4 inches of new snow and 0.1 inch of water (for a total of 5” snow/0.2” water from this storm). The current temp is 14F (6 degrees colder than yesterday) with a total snowpack depth of 71 inches.
-The Sunburst weather station at 3800 feet in Turnagain Pass-
recorded light to moderate SE winds averaging 10-15 mph with gusts in the 20’s until 2pm yesterday. Winds subsided for a bit then slightly increased and switched direction around midnight last night averaging 10 mph out of the NW with gusts in the 20’s. The current temp is 6F (8 degrees colder than yesterday)
AVALANCHE AND SNOWPACK DISCUSSION
The avalanche hazard has increased since yesterday. The addition of 5-13 inches of new snow, and winds strong enough to move the snow onto the lee sides of ridges and gullies, means that human-triggered avalanches are probable on wind-loaded slopes. Besides today’s danger from wind-loading, further instability is associated with a layer of weak faceted snow surrounding a thick rain crust buried 1-4 feet deep below 3000 feet elevation. For today, the avalanche danger is rated CONSIDERABLE on all wind-loaded slopes steeper than 35 degrees. Slopes not affected by the wind have a MODERATE danger.
The snow that fell yesterday was of the feather-light fluff variety. The 10-13 inches in the Girdwood Valley only contained 0.3 to 0.4 inches of snow water equivalent…that’s 4-5% density snow. The bonding of the new snow to the old snow surface will initially be poor because this low-density new snow fell on a variety of slick surfaces that formed this past week, including widespread hard windslabs and sun crusts on southern aspects. Today’s winds will easily transport the new snow to leeward aspects where human-triggered avalanches are certainly possible on the soft snow underlying recently formed wind slabs. On non wind-loaded slopes, I would not expect the new snow to be cohesive enough yet to readily fracture. Instead, it will be possible to trigger loose snow sluffs on steep slopes, especially those approaching 40 degrees or steeper.
No new natural or human-triggered avalanches were reported this past week. This could be partially due to the fact that Turnagain Pass was unusually empty because of the less than ideal snow conditions. The big wind event on 2/20 really hammered our snowpack above treeline. In general our stability has improved within the old snow, but we still have a significant weak layer buried 1-4 feet deep below 3000 feet elevation. This buried rain crust surrounded by weak faceted snow was the culprit in the majority of our natural and human-triggered avalanches the third week of February. Avalanches have been common and widespread on this lower elevation instability. Yesterday Ben and I toured up Sunburst and found enough spacial variability on this layer to keep us scratching our heads. There is still a lot of uncertainty as to how this layer will react to the next big snow load.
Hard wind slabs above treeline, now buried under 5-13 inches of fluff and even more on wind-loaded slopes, range from 2 inches thick to 2 feet thick. In areas where the wind slab was less than 4 inches thick, we noticed collapsing and cracking under our skis yesterday. We are still getting clean fractures in our snowpits, a sign of lingering instability.
Loose snow point releases on steep southerly aspects below rock outcrops may also occur with daytime heating from the sun. These sluffs could possibly trigger a slab avalanche if given the chance to run on a steep slope with buried weak layers.
This concludes today’s advisory; the next advisory will be Wednesday, March 4th. 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 MAR 1 2009
…BLIZZARD WARNING IS CANCELED…
.TODAY…PARTLY CLOUDY. AREAS OF BLOWING SNOW WILL REDUCE VISIBILITY
TO AROUND ONE MILE NEAR WHITTIER. HIGHS IN THE LOWER TO MID 20S. WEST
WIND 15 TO 30 MPH EXCEPT NORTH 25 TO 35 MPH NEAR SEWARD.
.TONIGHT…MOSTLY CLEAR. LOWS 5 BELOW TO 10 ABOVE…COOLEST
INLAND. NORTHWEST WIND 10 TO 20 MPH EXCEPT NORTH 15 TO 25 MPH
.MONDAY…MOSTLY SUNNY IN THE MORNING THEN BECOMING PARTLY
CLOUDY. HIGHS IN THE LOWER TO MID 20S. VARIABLE WIND TO 10 MPH.
.MONDAY NIGHT…PARTLY CLOUDY IN THE EVENING THEN BECOMING MOSTLY
CLOUDY. LOWS 5 TO 15 ABOVE. VARIABLE WIND TO 10 MPH.
.TUESDAY…MOSTLY CLOUDY IN THE MORNING THEN BECOMING PARTLY
CLOUDY. A CHANCE OF SNOW. HIGHS IN THE LOWER 20S TO LOWER 30S.
VARIABLE WIND 10 MPH.
TEMPERATURE / PRECIPITATION
SEWARD 26 12 24 / 0 0 0
GIRDWOOD 26 3 25 / 0 0 0