Turnagain Pass Avalanche Forecast

GENERAL ANNOUNCEMENTS

Good morning backcountry travelers, this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Wednesday, December 3, 2008 at 7am. This will serve as a general backcountry avalanche advisory issued for the Turnagain Arm area (Turnagain Pass). Local variations always occur. Note: We are now issuing regular advisories 5 days a week Wednesday-Sunday.

INTERAGENCY AVALANCHE RESCUE TRAINING

On the weekend of December 13-14, there will be an avalanche rescue training taking place at Turnagain Pass. Please be aware of rescue workers, helicopters, and areas set up for rescue training drills.

MOUNTAIN WEATHER ROUND UP In the last 24 hours…

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

As of the last report time at midnight, this weather station was recording a temperature of 25 degrees F (19 degrees warmer than yesterday morning). There was .1 inches of water (wierd reading). Total snowpack depth was 46 inches after 2 inches of settlement.

-Sunburst weather station at 3800 feet in Turnagain Pass-

As of the last report time of 3am, this weather station has recorded light winds averaging 1-16 mph out of the east/south/east with a strong gust of 34 mph. Temperature is 18 degrees F (4 degrees colder than yesterday morning).

-Surface Maps-

Since Monday morning, a new storm has been building (969Mb-960Mb) and heading very slowly in our direction. As of 3am today, it was centered south of the Aluetians.

-Satelite-

As of 5:30 am today, the satellite image shows that this new storm is pulling a bunch of moisture up from the south. This outer arm of the storm is directly over the top of the Kenai Peninsula.

-Radar-

Shows very light scattered precip along the outer coast of Prince William Sound. It looks like most of the moisture from this storm is still over the Alaska Peninsula

-Jet Stream-

The analysis shows the main flow going south the north from Hawaii to Alaska. It is forecasted to shift back to a west to east flow in 2 days.

-General Weather Obs-

Temperatures are warmer this morning than yesterday by 7-28 degrees at most weather stations from sea-level to 3000 feet. Winds are increasing in Portage.

PRIMARY AVALANCHE CONCERNS

-Thin snow attached to deep snow

-buried surface hoar (about 3 feet deep)

AVALANCHE AND SNOWPACK DISCUSSION

We are getting closer to that good stable snowpack that allows us to ski the best terrain in the world, but we are not there yet. Its still early season. We continue to find a thin weak snowpack. Right now, most places look like an intermountain snowpack like what you might find in the Tetons. People ski that kind of snowpack all the time without consequences, but I wanted to remind everybody that this is not the same snowpack that we typically have in the spring time on our big Alaskan terrain. With that being said, I think it is safer to stay away from big lines right now, and stick to mellow stuff. We will get plenty of chances later in the season for those types of runs. The snowpack has had a couple days to mellow out since the last storm; so, there is not much energy in it right now. The last storm really put some of our persistant weak layers to the test and created a lot of big avalanches, but we need one, two, or maybe three more big storms before all these weak layers become less of a problem.

Yesterday, Lisa and I went up Sunburst to take a closer look at that large avalanche that was triggered remotely from a group of skiers down below in the flats. That avalanche was an example of VERY unstable snow; so, we wanted to wait for a couple days before putting ourselves up on those slopes. We finally got a chance, and here is what we found…

The fact that it was a remote trigger screamed of buried surface hoar, and we found that weak layer about 3 feet deep on a WSW aspect at about 3500 feet. The total snowpack depth at that elevation was about 4 feet deep. My compression tests found some moderate to hard failures (CTM19Q235cm, CTM22Q238cm) on density changes in the top foot of the surface snow, but I could not get that layer of buried surface hoar to fail until I hammered on my shovel. At that point it broke clean and the surface crystals were large an in tact and visible to the naked eye (~4mm). Lisa’s compression test got that layer of surface hoar to fail with hard failures (CTH22-23Q295cm). Her extended column test show full propogation on this layer with the same hard failure (ECH28Q295cm).

Basically, we found a slab of snow about 3 feet thick on top of a layer of depth hoar about 1.5 feet thick. The depth hoar is getting denser (4 Finger) and it is rounding out. That layer of surface hoar is still an issue, but I don’t think it will show itself untill the next big storm comes in. Watch out for wind tapered slopes where thin snow attaches to deeper snow. This will be the most likely area to trigger an avalanche today.

-Similar observations have been reported from Cornbiscuit and Magnum.

-A large natural avalanche was report from the Summit Lake area.

WEATHER FORECAST

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST WED DEC 3 2008

…STRONG WIND THIS AFTERNOON THROUGH EARLY THURSDAY MORNING

THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…

.TODAY…SNOW…MIXING WITH RAIN IN THE AFTERNOON. SNOW ACCUMULATION

2 TO 6 INCHES. HIGHS IN THE UPPER 20S TO MID 30S. EAST WIND 20 TO 35

MPH THROUGH PORTAGE VALLEY AND TURNAGAIN ARM. EAST WIND 10 TO 15 MPH

ELSEWHERE.

.TONIGHT…SNOW…MIXING WITH RAIN NEAR THE COAST. SNOW ACCUMULATION

4 TO 10 INCHES. LOWS IN THE MID 20S TO LOWER 30S.

EAST WIND 5 TO 15 MPH EXCEPT EAST WIND 40 TO 50 MPH THROUGH PORTAGE

VALLEY AND TURNAGAIN ARM.

.THURSDAY…SNOW AND RAIN. SNOW ACCUMULATION UP TO 4 INCHES. HIGHS IN

THE 30S. EAST WIND 20 TO 35 MPH DECREASING TO 10 TO 15 MPH IN THE

AFTERNOON. NEAR SEWARD…NORTH WIND 25 MPH.