Turnagain Pass Avalanche Forecast

GENERAL ANNOUNCEMENTS

Good Morning backcountry travelers, this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Wednesday, March 18, 2009 at 7am. This will serve as a general backcountry avalanche advisory issued 5 days a week Wednesday-Sunday for the Turnagain Arm area(Turnagain Pass is the core advisory area). Local variations always occur. This advisory expires in 24 hours and does not apply to operating ski resorts or highways/railroads.

MOUNTAIN WEATHER ROUND UP FOR THE LAST 48 HOURS

-General Weather Observations-

By far, the biggest weather event to hit us since the January Hurricane. A lot of changes in the past couple days. The St. Paddy’s storm brought 1-2 inches of water and 8-24 inches of snow to Summit, Turnagain Pass, Grandview, Portage, and Girdwood. Summit had the least amount of precip observed as 8-12 inches of snow, and Grandview had the most precip according to the SNOTEL sites with 24 inches of new snow. Winds were moderate to strong during the storm on Monday 3/16/09, and became strong again yesterday averaging as high as 37 mph. Temps have decreased by a couple degrees at most weather stations compared to yesterday, but Granview and Portage are showing much larger drops in temps by 10-19 degrees.

–The NRCS Center Ridge weather station at 1800 feet in Turnagain Pass–

Has a temp of 19 degrees (4 degrees colder than yesterday). 48 hour storm totals: 1.2 inches new water and 16-18 inches of new snow.

–The Sunburst weather station at 3800 feet in Turnagain Pass–

Is recording a temp of 20 degrees (4 degrees colder than yesterday). Winds were light to strong averaging 1-37 mph with a max gust of 48mph.

-Surface Analysis Maps-

Between 3am and 9pm yesterday….Show those low pressures that brought us the snow spinning around in some sort of eddy in the Gulf of Alaska. They got weaker except for a new low that looks like it is heading toward SE AK.

-Radar/Satellite-

Radar is down again this morning, but I don’t think there is much going on out there according to the NOAA forecast at the bottom of this page. The satellite jives with the surface maps.

AVALANCHE HAZARDS

Primary avalanche concerns

-Below 3000 feet. The “Janurary Hurricane” rain crust buried under 2-5 feet of surface snow. There is a layer of weak sugary faceted snow on top and below this hard rain crust. Stability tests on this layer continue to show clean fast Q1 shears on top of this rain crust. This weak layer is still showing signs of life, and probably will become very dangerous if it gets any sort of rapid load, especially rain.

-Sea-level up to at least 3800 feet (Probably higher). The thin “glazed donut” freezing rain crust that formed 3/5/09 has been observed in Girdwood, Turnagain Pass and Summit Lake. This layer formed almost everywhere on all aspects from the parking lots to the ridgetops. This layer is now buried under about 2 feet of snow

AVALANCHES AND SNOWPACK

Bottom Line

Elevated Caution is advised at all elevations and aspects. This moderate avalanche hazard is due to a significant change in our snowpack. We just had the biggest load since the January Hurricane. There is also a moderate level of uncertainty with how this new snow will react to human triggers because we were unable to get much data above treeline yesterday.

Don’t take any chances today.

Discussion

Yesterday, we went to as many steep rollover test slopes on Tincan that we could find. We got away from the main skin track and zig-zagged around in 16-18 inches of light density powder. We did not feel any collapsing. We were not able to find any sensitive slabs. We did not see any shooting cracks with wide propogation. Our ski cuts found no suprises. I dug several hand pits and isolated several columns with my ski poles. A hasty compression test found a moderate failure (CTM18Q3) with an uneven shear Q3 above the “glazed donut” rain crust. I was also able to get the facets above the Janurary Hurrinane layer with a hard failure (CTH28Q2). It was total greybird when we were there; so, we turned around at treeline. We did not get any observations above 3000 feet, but the surface snow started getting a lot stiffer and slaby before we turned around.

We got a report of a snowboard triggered slab along Seattle Ridge on a steep rollover above a small cliff. It was reported as 50 yards wide with a 2 foot deep crown face. AnCNFAIC Staff observation came in from Eddy’s with anCNFAIC Staff report of anCNFAIC Staff clean shear Q1 that “zipped off” above the Januray Hurricane rain crust. This test was the third isolated column in this observers test pit and was a hard failure (CTH29Q1). AnCNFAIC Staff report came in from Summit Lake with a 12-14 inch slab that was cut out on a test slope that was 33 degrees.

The weak layers are there, but the question remains…. What is going to tip the balance? We don’t know, and need more info. We do know that we have some deep instabilities that could be triggered by some of this new snow if it slabs up. We all need to slowly ease back into the mountains. The new snow yesterday was not very slabby, but that can change quickly with sun. Follow strict travel rituals today like:

-watch your slope angles. It would be wise to stay on slopes less than 35 degrees till we get a chance to understand the snowpack better

-descend and ascend one at a time from safe spot to safe spot, watching your partners

-scale back if the snow changes and watch out if the sun starts cooking the slopes

Eventually, the balance will get tipped and that January Hurricane layer will most likely create big avalanches. The question is where and when will this happen? The Seattle Creek Bowls and Placer/Skookum Valley slopes will be likely areas for that January Hurricane rain crust to avalanche once that balance is tipped. Natural avalanches on this weak layer are unlikely today, but human-triggered avalanches are possible in the right spot. Watch out for super steep pockets below 3000 feet. Keep in mind that there is a a certain level of uncertainty with when this weak layer will avalanche, but we know that it has had decreasing stability with very clean shears in spots.

WEATHER FORECAST

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKDT WED MAR 18 2009

.TODAY…PARTLY CLOUDY. HIGHS IN THE UPPER 20S TO MID 30S.

NORTH AND WEST WIND TO 10 MPH EXCEPT GUSTS TO 20 MPH NEAR SEWARD AND

WHITTIER.

.TONIGHT…PARTLY CLOUDY IN THE EVENING THEN BECOMING MOSTLY

CLOUDY. LOWS IN THE UPPER TEENS TO MID 20S. VARIABLE WIND 10 MPH.

.THURSDAY…MOSTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE

AFTERNOON. HIGHS IN THE UPPER 20S TO MID 30S. LIGHT WINDS EXCEPT

NORTH 15 MPH NEAR SEWARD.

.THURSDAY NIGHT AND FRIDAY…PARTLY CLOUDY. LOWS IN THE UPPER

TEENS TO MID 20S. HIGHS IN THE UPPER 20S TO MID 30S. NORTHWEST

WIND 15 MPH.

TEMPERATURE / PRECIPITATION

SEWARD 36 23 32 / 0 0 20

GIRDWOOD 37 17 35 / 0 0 0

This concludes today’s avalanche advisory which will expire in 24 hours. The next advisory will be on Thursday, March 19th. Thanks and have a great day.