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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, December 18th, 2008 - 7:00AM
Expires
Fri, December 19th, 2008 - 7:00AM
Forecaster
CNFAIC Staff
Avalanche risk The Bottom Line

GENERAL ANNOUNCEMENTS

Good morning backcountry travelers, this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Thursday, December 18, 2008 at 7am. This will serve as a general backcountry avalanche advisory issued for the Turnagain Arm (Turnagain Pass is the core advisory area). Local variations always occur. Note: We are now issuing regular advisories 5 days a week Wednesday-Sunday.

I forgot to give anCNFAIC Staff HUGE THANKS to Snow Safaris for helping with the avalanche rescue training last weekend.

MOUNTAIN WEATHER ROUND UP

In the last 24 hours…

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

Recorded zero new precip. Current temperature is 23 degrees F (3 degrees warmer than yesterday morning). Total snowpack depth is 54 inches. with a total of 14 inches of settlement since last Tuesday’s storm.

-Sunburst weather station at 3800 feet in Turnagain Pass-

Recorded light winds averaging 0-12 mph mostly from the east with a light max gust of 16 mph. Current temperature is 19 degrees F (4 degrees colder than yesterday)

-Surface Analysis Maps-

From 3 am Wednesday to 9pm last night, the maps show a storm building (994-988) over the Aleutians, but it is kind of staying in one place. A new high pressure just formed to our SE.

-Jet Stream-

The analysis from 9pm last night showed the main flow streaming from north to south from the Yukon Territory toward Southeast Alaska.

The forecast predicts shift over the next 5 days, where it will flow west to east but way to our south. It is predicted to aim the firehose at the lower 48 along the Washington, Oregon, and Northern California coasts.

-Satellite-

As of 6:00 am this morning…shows that storm over the Aleutians spinning around in one place. It does not show much over us.

-Radar-

The Middleton radar shows nothing.

The Kenai radar is clear as well.

-General Weather Observations-

Temps have flip-flopped. They are warmer at all weather stations below 2700 feet (by 3-10 degrees) F, and colder (by 2-4 degrees F) on the ridgetops. Winds have still been very calm in the past 24 hours.

PRIMARY AVALANCHE CONCERNS

-Terrain Management

-Shallow Snow

-Chocolate Chip Rocks

-Cliff Bands

-October Facets

-Buried Surface Hoar

SECONDARY AVALANCHE CONCERNS

-Glide Cracks

AVALANCHE AND SNOWPACK DISCUSSION

I was blathering a bunch yesterday; so, I will try to summarize the facts and observations that I was talking about.

No new information has come to light, but there are some things worth talking about again…

-During the storm last week, large natural avalanches propogated across large alpine bowls.

-Since then, there have been three significant human-triggered avalanches on isolated terrain features that did not propogate across entire slopes. One of these resulted in the burial of a skier 5-6 feet deep who was rescued by her companions due to their EXCELLENT RESCUE SKILLS. AnCNFAIC Staff avalanche involved a snowmachiner who deployed his ABS airbags. (We are still trying to find information on that avalanche. Please call or email us if know some good information to share with CNFAIC Staff people)

-Everywhere we dig pits we find sugary October facets on the ground with buried surface hoar on top of that deeper layer, and a cohesive slab (ranging in thickness of 1-6 feet) on top of those two weak layers

-Our stability tests show that it is very difficult to get those weak layers to react or fail.

-People snowmachined/skied/snowboarded on all kinds of terrain over the past weak, including big complex lines like Tincan Proper.

Here are my avalanche speculations for today…

-Dangerous avalanche conditions still exist on some terrain features. Evaluate the snow and terrain carefully and use good travel habits.

-Shallow snow seems to be where most of the human-triggered avalanche activity is happening.

-The general snowpack gets more and more shallow the further south you drive; so, places like Cornbiscuit almost always have less snow than places like Tincan. Summit Lake is even thinner, colder, and weaker.

-There is a possibilty of triggering small to medium sized avalanches on isolated terrain features like: steep rollovers near rocks or cliffs, wind scoured ridges where the snow depth tapers from shallow to deep

It is still early season, and we have an early season intermountain style snowpack. This is not the best time of year for big steep Alaskan lines. There will be opportunity for that later in the season. The best way to stay safe right now is to manage your terrain, and stick to simple slopes and enjoy the great powder.

Future avalanche problems will be glide cracks, which are very difficult to predict. Treat them like cornices and realize that they could avalanche at any time for wierd reasons; so, don’t travel on or underneath those things. They are very easy to avoid because you can see them!

The next big problem will be the surface hoar and rimed surface snow that has been forming all over the place.

WATCH OUT SITUATIONS

-Rocky terrain with shallow snow connected to deeper snow (photo gallery)

-Steep rollovers (photo gallery)

-If you hear any “whumpfing” or feel any collapsing, then get to safe terrain immediately and call it a day.

WEATHER FORECAST

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST THU DEC 18 2008

.TODAY…CLOUDY WITH ISOLATED SNOW SHOWERS. HIGHS IN THE 20S TO MID

30S. VARIABLE WIND 15 MPH OR LESS.

.TONIGHT…CLOUDY WITH SCATTERED SNOW SHOWERS. LOWS IN THE 20S.

VARIABLE WIND 10 MPH EXCEPT EAST WIND 10 TO 20 MPH THROUGH PORTAGE

PASS AND TURNAGAIN ARM.

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

AFTERNOON. SNOW ACCUMULATION UP TO 2 INCHES. HIGHS IN THE LOWER

TO MID 30S. EAST WIND 10 TO 25 MPH.

.FRIDAY NIGHT…SNOW…MIXED WITH RAIN ALONG THE COAST. LOWS IN THE

20S TO LOWER 30S. EAST WIND 15 TO 20 MPH.

TEMPERATURE / PRECIPITATION

SEWARD 30 28 33 / 20 40 80

GIRDWOOD 27 24 34 / 0 20 20

Thu, December 18th, 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.