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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, February 9th, 2010 - 7:00AM
Expires
Wed, February 10th, 2010 - 7:00AM
Forecaster
CNFAIC Staff
Avalanche risk The Bottom Line

Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Tuesday February 9th at 7 am. 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).

WEATHER ROUNDUP

There is a major storm over the Aluetians that will affect the snowpack today. The NWS has issued a BLIZZARD WARNING around the Turnagain Pass area. The radars show moderate precip moving north over the southern end of the Kenai Peninsula. The snotel sites consistently recorded only 0.1 inch of water in the past 24 hours, and there is only about 1 inch of new snow on my truck this morning. Ridgetop winds in the past 24 hours have averaged calm to extreme up to 39mph with extreme gusts up to 57mph. Temps have increased at all wx stations by 6-9 degrees F compared to yesterday. Temps range from 32 degrees F at sea-level and 14 degrees F at 3800′.

AVALANCHE DISCUSSION

Today’s avalanche danger for the Turnagain Pass area will increase to CONSIDERABLE this morning due to wind. The danger could increase again to HIGH this afternoon if this storm starts to dump lots of new snow in combination with the strong winds.

There are two main weak layers in our snowpack.

1.A thin breakable melt-freeze crust with surface hoar on top of it. We have seen this weak layer up to 3000′ on multiple aspects on both sides of highway at Turnagain Pass. As of Monday 2/8/2010, this weak layer was buried about 1-1 ½ feet deep. Of course it will be buried deeper in areas where wind slabs formed. We were getting moderate failures with clean fast shears on this layer along Seattle Ridge yesterday, but this weak layer was not reactive to human-triggers yesterday (see forecasters video link or copy http://www.youtube.com/watch?v=26BwPKClsPk). This layer will become more dangerous with 2-3 feet of snow on top of it. This is the same weak layer that was responsible for the skier-triggered avalanches on Tincan on Saturday 2/6/2010. Plus I suspect this was the same weak layer that failed creating a natural avalanche on Widowmaker on Saturday 2/6/2010 as well (see photo gallery). This particular weak layer will be the likely culprit for human-triggered wind slabs today.

2.The Jan 7 rain crust is generally buried 3-5 feet deep and it qualifies as a deep slab instability. As of yesterday 2/8/2010, it was still showing moderate failures with clean smooth shears. This weak layer is still showing significant signs of instability, but the question is what kind of trigger could create an avalanche on this weak layer? It appears to require a large trigger because the last storm that hit us Friday night 2/5/2010 created medium sized natural avalanches in places like Widowmaker and Lipps, and these natural avalanches did not appear to step down to this rain crust. This weak layer is widespread on all aspects between 2000-3000 feet. Maybe a bigger storm will start to trigger this deep instability???

WEATHER FORECAST (National Weather Service)

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST TUE FEB 9 2010

…BLIZZARD WARNING REMAINS IN EFFECT UNTIL 6 PM AKST THIS EVENING

THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…

.TODAY…SNOW IN THE MORNING…THEN RAIN AND SNOW IN THE AFTERNOON.

BLOWING SNOW WILL REDUCE VISIBILITIES TO LESS THAN A QUARTER MILE AT

TIMES THROUGH PORTAGE VALLEY AND TURNAGAIN ARM. SNOW ACCUMULATION 6

TO 12 INCHES OVER THE HIGHER ELEVATIONS. HIGHS IN THE 30S. NORTH TO

EAST WIND 15 TO 30 MPH EXCEPT EAST 50 TO 65 MPH THROUGH PORTAGE

VALLEY AND TURNAGAIN ARM.

.TONIGHT…RAIN AND SNOW. SNOW ACCUMULATION 4 TO 8 INCHES OVER THE

HIGH ELEVATIONS. LOWS IN THE MID 20S TO MID 30S. NORTH TO EAST WIND

15 TO 25 MPH. THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 50

TO 65 MPH DECREASING TO 30 TO 45 MPH AFTER MIDNIGHT.

.WEDNESDAY…SNOW AND RAIN IN THE MORNING…THEN RAIN IN THE

AFTERNOON. LITTLE SNOW ACCUMULATION. HIGHS IN THE MID 30S

TO LOWER 40S. NORTH TO EAST WIND 15 TO 25 MPH EXCEPT EAST 25 TO 35

MPH THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.

Temperature / Precipitation

SEWARD 37 30 38 / 100 80 80

GIRDWOOD 37 35 43 / 100 80 80

Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass

Sea-level: temps are forecasted between 35-37 with up to .75” of water today (up to 3″ of water is forecasted out in Prince William Sound)

3000′: temps are forecasted between 14-23 degrees F with winds 25-35 mph

6000′: temps are forecasted between 5-14 degrees F with winds 35-40 mph

(The NAM model shows temps warming up and bringing the freeze line up high near 3000′ later today and tomorrow)

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Wednesday February 10th.

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Tue, February 9th, 2010
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
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Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

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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.