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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, December 28th, 2008 - 7:00AM
Expires
Mon, December 29th, 2008 - 7:00AM
Forecaster
CNFAIC Staff
Avalanche risk The Bottom Line

GENERAL ANNOUNCEMENTS

Good morning backcountry travelers, this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, December 28, 2008 at 7am. This will serve as a general backcountry avalanche advisory issued for the Turnagain Arm area (Turnagain Pass is the core advisory area). Local variations always occur.

MOUNTAIN WEATHER ROUND UP

In the last 24 hours…

-The Center Ridge weather station at 1800 feet in Turnagain Pass is still down this morning.

The Grandview weather station at 1100 feet along the railroad tracks recorded no new snow and a temperature this morning of 2 degrees F (6 degrees colder than yesterday).

-The Sunburst weather station at 3800 feet in Turnagain Pass and Max’s weather station at 3200 feet in the Girdwood Valley both recorded light and variable winds yesterday.

-Mile 43 Peak at 3300 feet above the railroad tracks recorded northerly winds averaging 10-20 mph with a few gusts in the 20’s. Plumes of windblown snow were visible on the ridgetops in Portage Valley and Placer River Valley for most of yesterday.

Mountain winds are currently light and variable at all three locations with temps ranging from -3 to 1 degrees F (same as yesterday).

This past week Turnagain Pass and the upper elevations of the Girdwood Valley received a ½ inch of water in the form of 10-14 inches of new snow, with most of it falling on Christmas Day. Moderate to strong winds Dec 24-26 loaded N-S-E aspects. Expect partly to mostly cloudy skies today with NW winds averaging 10-20 mph and mountain temps near or below 0 F. Stronger gustier winds will be found closer to the coast. Cold, clear weather looks to be settling in for the duration as the main jet stream remains well to the south of us.

BOTTOM LINE

1. Pockets of sensitive wind slab are still a concern today and are likely to be triggered on steep leeward aspects and cross-loaded gullies greater than 35 degrees. Visibly drifting snow, shooting cracks, and hollow drum-like hard snow are the most obvious clues to wind slab formation.

2. Weak snow at the bottom of our snowpack continues to be a concern and will be for some time. Buried surface hoar and sugary October facets are responsible for most of the human-triggered avalanches so far this winter, the most recent ones occurring 2 weeks ago on Cornbiscuit in Turnagain Pass.

3. Many of the glide cracks are now covered with a thin layer of snow and may be difficult to see. A skier reported accidentally dropping into one on the south face of Cornbiscuit a few days ago. Watch your partners carefully, especially on south and west aspects where most of the glide cracks seem to have formed.

AVALANCHE AND SNOWPACK DISCUSSION

So far our snowpack at Turnagain Pass has survived the winds mostly unscathed with wind hammered snow mostly confined to the ridgetops, passes, and cross-loaded subridges. Some southern aspects were wind-affected and CNFAIC Staffs were not. Folks generally found great skiing and riding conditions on western aspects and in the back bowls off Seattle Ridge. I talked to a few skiers who found hollow, drum-like wind slab on convex rollovers, but it supported their weight and didn’t fracture. I would expect some lingering instabilities, however, underneath these hard wind slabs due to the cold temperatures and the fact that they formed on top of weak snow.

Our most recent batch of surface hoar formed in mid-December and is now sitting underneath this week’s new windblown snow on protected leeward aspects. The good thing is that this surface hoar formed on top of a foot of powder before it was buried, and not on a slippery bed surface like a crust. My snowpit stability tests from yesterday, however, produced easy shears underneath these newly formed 4”-10” wind slabs. Cold temps will slow down the settlement and strengthening process, so these instabilities will hang around for a while.

One of our observers from the Summit Lake area reported strong winds the past few days loading S and E aspects. He also noticed recent natural avalanche activity on leeward aspects due to the wind loading, with one stepping down to the weak snow near the ground. In his snowpit stability tests, surface wind slabs sheared easily at the new snow/old snow interface. (Thanks Alex!)

We are still tracking the weak snow at the bottom of our snowpack. This facet/crust/surface hoar combo was reactive 2 weeks ago when a skier was caught in an avalanche on Cornbiscuit. Despite our snowpit tests consistently showing good stability on this layer and big lines getting skied everywhere, I still think these persistent weak layers warrant our attention. Large triggers like cornice breaks or CNFAIC Staff avalanches may cause these layers to fail, as well as skiers and riders hitting a shallow weak spot in the snowpack. Likely areas to trigger an avalanche are on shallow rocky slopes and steep rollovers. Terrain management is the key to staying safe when dealing with the spacial variability of deep-slab instabilities and shallow areas of the snowpack. Use good travel habits and evaluate the snow depth and terrain carefully.

We have noticed in the past that glide cracks tend to release during cold snaps. Be careful not to find yourself underneath one in the runout zone, especially now that they are difficult to see.

This concludes today’s advisory; the next advisory will be Wednesday, December 31st. If you are out in the backcountry and have the chance, please send us your observations. Simply click on “observations” next to 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 AKST SUN DEC 28 2008

…STRONG WIND THROUGH THIS AFTERNOON NEAR SEWARD…

.TODAY…MOSTLY CLOUDY. HIGHS ZERO TO 15 ABOVE…COLDEST INLAND.

NORTH TO WEST WIND 10 TO 20 MPH WITH GUSTS TO 35 MPH NEAR WHITTIER

AND 45 MPH NEAR SEWARD.

.TONIGHT…MOSTLY CLOUDY. LOWS ZERO TO 15 BELOW EXCEPT ZERO TO 10

ABOVE ALONG THE COAST. NORTH TO WEST WIND TO 15 MPH WITH GUSTS TO

35 MPH NEAR SEWARD.

.MONDAY…MOSTLY SUNNY. HIGHS 5 BELOW TO 15 ABOVE…COLDEST INLAND.

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

.MONDAY NIGHT…CLEAR. LOWS ZERO TO 20 BELOW EXCEPT 5 TO 10 ABOVE

NEAR SEWARD AND WHITTIER. LIGHT WINDS EXCEPT NORTH 15 TO 25 MPH

NEAR SEWARD.

.TUESDAY…SUNNY. HIGHS 5 BELOW TO 15 ABOVE…COLDEST INLAND.

VARIABLE WIND 10 MPH EXCEPT NORTH 15 TO 25 MPH NEAR SEWARD.

TEMPERATURE / PRECIPITATION

SEWARD 15 9 14 / 0 0 0

GIRDWOOD 6 -1 4 / 0 0 0

Sun, December 28th, 2008
Alpine
Above 2,500'
0 - No Rating
Avalanche risk
Treeline
1,000'-2,500'
0 - No Rating
Avalanche risk
Below Treeline
Below 1,000'
0 - No Rating
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.