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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, March 14th, 2009 - 7:00AM
Expires
Sun, March 15th, 2009 - 7:00AM
Forecaster
CNFAIC Staff
The Bottom Line

GENERAL ANNOUNCEMENTS

Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Saturday, March 14th, 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 1-2 inches of new snow fell in Turnagain Pass and the Girdwood Valley. Winds were light and variable, and mountain temperatures climbed to the mid 20’s. Early this morning, mountain temperatures ranged from 13-18 degrees F with light northerly winds at all locations. An isolated snow shower may spit out a trace of snow this morning, but skies will clear this afternoon as a huge blocking ridge of high pressure moves into our area for the foreseeable future. This means cold, dry, and windy conditions through the week with a possible snow shower on Monday from a complex low in the gulf. Expect slightly cooler mountain temperatures today topping out in the teens. Northwest winds will increase and blow 10-20 mph with much stronger gusts closer to the coast.

-The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass-

recorded 1 inch of new snow and 0.1 inches of water. The current temp is 18F (4 degrees colder than yesterday) with a total snowpack depth of 69 inches.

-The Sunburst weather station at 3800 feet in Turnagain Pass-

recorded light northerly winds averaging 1-7 mph all day yesterday. The current temp is 13F (3 degrees colder than yesterday)

AVALANCHE AND SNOWPACK DISCUSSION

Bottom Line

For today, the avalanche danger is rated MODERATE on all slopes steeper than 35 degrees. Human-triggered avalanches are possible as pockets of instability may exist within the top 1-2 feet of snow. Also, a layer of weak faceted snow on top of a thick rain crust buried 1-4 feet deep below 3000 feet elevation is showing signs of decreasing stability. Point release natural avalanches will also be possible on steep southerly aspects warmed by the sun.

The avalanche danger will increase later this afternoon as northwest winds pick up and start actively loading lee aspects. Pockets of shallow sensitive windslab will develop on leeward southerly aspects, but watch for localized winds and wind channeling off the ridgelines loading different aspects.

Discussion

The top 1-2 feet of snow is a veritable matrix of powder, windslab, melt-freeze crust, freezing rain crust, plain-old rain crust, and facets. So undoubtedly there are a number of instabilities within this top 1-2 feet of snow that could be sensitive to human triggers. 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. On Thursday we received a report of a small human-triggered slab avalanche in a gully near the snowmachine up-track on Seattle Ridge. Unfortunately bad visibility yesterday prevented us from taking a closer look at this slide. One of our observers found several surprisingly clean shears within the top 1-2 feet of snow on a west aspect of Magnum at 2300 feet. He felt that a failure on the upper layer of intact stellars (or surface hoar?) could possibly step down and trigger the deeper layer of facets. There is also a widespread freezing rain crust buried 4-12 inches deep that formed March 4th. Numerous small to medium-sized natural avalanches last occurred on this layer during our wind event on March 6. On Magnum, Matt and Ben found stiff slabby snow on top of this crust, which formed on top of lighter density powder, which in turn created some challenging skiing conditions.

We have a significant weak layer buried 1-4 feet deep below 3000 feet elevation that is showing signs of decreasing stability. 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. This thick crust from the January hurricane is generally buried 1.5 to 2.5 feet deep in Turnagain Pass and up to 4 feet deep in the Girdwood Valley. I have been monitoring this layer the last 3 weeks at a study plot on Tincan at 1700 feet. My compression test scores are getting lower, and the facets above the crust are growing larger and becoming less cohesive like sugar. 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, their stability tests produced clean shears that propagated across the entire 3 ft. wide column, anCNFAIC Staff obvious sign of instability. Yesterday, Ben and I toured up Pete’s North and dug a pit at 2200 feet on a SW aspect. Although we did not get any collapsing or shooting cracks, our stability tests produced the lowest scores so far on the facets above the crust (CTM12&14Q2), yet anCNFAIC Staff sign of decreasing stability.

This concludes today’s advisory; the next advisory will be Sunday, March 15th. 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-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKDT SAT MAR 14 2009

…STRONG WIND TONIGHT THROUGH SUNDAY NIGHT NEAR SEWARD AND

WHITTIER…

.TODAY…PARTIAL CLEARING THIS AFTERNOON. SCATTERED SNOW SHOWERS.

HIGHS IN THE 20S TO MID 30S. NORTH TO WEST WINDS 10 TO 20 MPH EXCEPT

GUSTS 25 TO 35 MPH NEAR SEWARD AND WHITTIER.

.TONIGHT…MOSTLY CLEAR. LOWS 5 TO 20 ABOVE EXCEPT 5 BELOW TO

5 ABOVE INLAND. WARMEST IN WINDY AREAS. NORTH TO WEST WINDS 10 TO 20

MPH EXCEPT GUSTS 40 TO 50 MPH NEAR SEWARD AND WHITTIER.

.SUNDAY…MOSTLY SUNNY. HIGHS IN THE UPPER TEENS TO MID 20S.

NORTH TO WEST WINDS 10 TO 20 MPH EXCEPT GUSTS 40 TO 50 MPH NEAR

SEWARD AND WHITTIER.

.SUNDAY NIGHT…MOSTLY CLEAR. LOWS ZERO TO 10 ABOVE EXCEPT

5 BELOW TO 5 ABOVE INLAND. NORTH TO WEST WINDS 10 TO 20 MPH EXCEPT

GUSTS 40 TO 50 MPH NEAR SEWARD AND WHITTIER.

TEMPERATURE / PRECIPITATION

SEWARD 32 20 24 / 40 0 0

GIRDWOOD 31 10 22 / 40 0 0

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This is 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.