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Issued
Wed, January 20th, 2010 - 7:00AM
Expires
Thu, January 21st, 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 Wednesday January 20th 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

Hindcast (Last 24 hours)

3800′ -Sunburst Wx Station-

Current temp is 23 (2 degrees warmer than yesterday) Winds have been light to moderate averaging 7-23 mph out of the east with a strong max gust of 35 mph

2400′-Seattle Ridge Wx Station-

Winds have been light averaging 4-15 mph out of the ESE with a moderate max gust of 22mph

1800′-Center Ridge Wx Station-

Current temp is 27 (6 degrees warmer than yesterday) Zero to 0.1 inches of water and 1-2 inches of new snow has fallen.

Nowcast

Temps are warmer at all wx stations this morning by 1-6 degrees compared to yesterday. The precip amounts are consistent at all snotel sites this morning with probably 1-2 inches of new snow. Winds are currently light at all ridgetop wx stations this morning. Both the Middleton and Kenai radars shows very scattered light precip moving W toward the southern Kenai Mountains.

Forecast

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

550 AM AKST WED JAN 20 2010

.TODAY…CLOUDY WITH SCATTERED RAIN AND SNOW SHOWERS…MAINLY ALONG

THE COAST. DECREASING CLOUDS IN THE AFTERNOON. AREAS OF FOG REDUCING

VISIBILITY TO ONE HALF MILE OR LESS IN THE MORNING. HIGHS IN THE MID

20S TO MID 30S. LIGHT WINDS EXCEPT NORTH 10 MPH NEAR SEWARD.

.TONIGHT…PARTLY CLOUDY. LOWS 15 TO 25. NORTH TO WEST WIND 10 TO 15

MPH WITH GUSTS TO 25 MPH NEAR SEWARD AND WHITTIER AFTER MIDNIGHT.

.THURSDAY…MOSTLY CLOUDY. HIGHS IN THE LOWER 20S TO LOWER 30S.

NORTH TO WEST WIND 10 TO 15 MPH WITH GUSTS TO 25 MPH NEAR SEWARD AND

WHITTIER.

.THURSDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF SNOW. LOWS 15 TO

25. NORTH TO WEST WIND 10 TO 15 MPH WITH GUSTS TO 30 MPH NEAR SEWARD

AND WHITTIER.

Temperature / Precipitation

SEWARD 35 18 27 / 40 0 0

GIRDWOOD 30 20 27 / 20 0 0

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

Sea-level: GFS shows 0.05 inches of water forecasted today

3000′: there is a bubble of above freezing temps that gets real close to Turnagain Pass today, but for the most part, temps are forecasted between 23-32 degrees F with winds 5 mph

6000′: temps forecasted between 23 and 32 degrees F with winds 10-20 mph

AVALANCHE DISCUSSION

Today’s weather should not contribute to the avalanche danger today at Turnagain Pass.

The main avalanche concern today is that slippery rain crust underneath the 1.5-2.5 feet of newer snow especially between 2000′-3000′. This slippery rain crust formed on Thursday Jan 7 and is widespread on all aspects from Girdwood to Seward.

For the past 3 days we have been digging and testing the snow at Tincan, Sunburst, and above the snowmachine uptrack on Repeat Offender. The rain crust has looked different at all three locations, but there has been one common feature at all locations. I keep getting the new snow to fail about ½ inch above the top rain crust with hard failures in the lower 20 range on compression tests with clean shears (Q2) where the shear plane is smooth but does not slide readily. According to the text books, this “generally indicates good stability (in these particular test pit locations) but a compact group of skiers/snowboarders, a snowmachine, or explosives may trigger an avalanche. Natural avalanches are rare”.

These test scores, however, are on the low end of good stability and the structure of the snowpack is poor right now because it has the three main ingredients for an avalanche: a slab on top of a weaker layer of snow on top of a slippery bed surface. It’s a little bit alarming that we have been finding the same results at three different mountains. This could indicate that this weak snow immediately above the crust is widespread across Turnagain Pass; which, makes sense based on the snowpack history from the last two weeks.

Actual data is always better for making decisions than opinions; but, for whatever it is worth, I’m not willing to risk my life on big terrain when the stability is on the low end of good. I like to wait till the weak layers don’t react in my snowpits before I ramp my slope angles and get out into steeper terrain. It’s going to take at least a couple more days before this weak layer starts to heal itself. The danger level needs to stay at MODERATE until we see some improvement.

On a side note…One of our observers took a closer look at those Tincan human-triggered avalanches (see photo gallery). He said they were surface snow slabs only a couple inches deep; so, that means these did not fail on the rain crust. Plus there are some rotten facets underneath the crust in some spots, but we have not found enough consistency or failures with this weak layer to say it’s a major problem yet.

Always remember that safe backcountry travel requires training and experience. You control your own risk by choosing where, when, and how you travel.

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Thursday January 21st.

Wed, January 20th, 2010
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
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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.