Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Friday, March 4th at 7am. 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).
Today’s avalanche danger rating is Low with pockets of MODERATE. Old aging wind slab is overlying persistent weak layers. Human triggered avalanches are possible in isolated areas.
The possibility of triggering an avalanche is slowly diminishing over time since the last wind storm event. It’s still worth keeping a heightened level of caution on steeper terrain in shallower areas.
We got a close look at the old avalanches in Seattle creek yesterday. A couple details jumped out as being noteworthy and definitely do not indicate a Low danger rating, hence the pockets of Moderate.
The entire area is covered with a firm layer of winded snow. Sometimes that wind slab is supportable and thick, sometimes you punch through in the thin areas. Everywhere underneath that firm layer is a soft layer of weaker snow. Strong snow over weak snow is exactly the recipe needed to create an avalanche.
The larger avalanche, farther back in this photo was probably natural during the windstorm on Friday, 2-25. At the crown we found Moderate failures in isolated columns and when it did fail it collapsed on a layer of facets above the Thanksgiving Rain Crust. Take a look at a pit profile here
For the technically minded folks out there, this crown profile showed 5 lemons within the structure. This indicates characteristics that are statistically shown to be contributors to avalanches. You can read the technical paper here www.snowpit.com/articles/lemons%20reprint%20copy.pdf
The closer avalanche was triggered by a high marking snowmachine. You can see tracks going right through the bed surface and just to the left in this photo. The interesting detail is the snowmachine tracks easily penetrated the slab, into the weak layer, and sometimes through the rain crust. With a steep angle of 40 degrees and recent wind loading, the slope was ripe for a trigger.
The specific hazard today is a combination of the wind slab and the deeper facets. It is unlikely to trigger in an area of deeper snow (6ft or more). Shallow areas deserve extra caution, especially where it’s recently been wind loaded with a hard slab. Areas to avoid are steep, wind loaded, shallow, rocky, and unsupported terrain.
High pressure is holding us tight. There has been no significant snowfall in 11 days. The last major wind event was on Saturday, 2-26. A diurnal temperature swing is in effect now meaning cold temperatures at night, increasing during the day. Wind yesterday was light with the highest gusts into the 20s. Mostly sunny skies are expected today with light wind and temperatures in the 20s.
ANNOUNCEMENTS An APU student is conducting a survey on backcountry skiing group dynamics. Click on the link to participate in research on avalanche safety. Click here to take survey
The Forest Service is installing winter use bridges across Granite creek in Turnagain Pass. Be aware of an ice road from the Lyon creek bridge south towards Johnson Pass on the West side of the highway. Heavy equipment will also be used in the area.
I will issue the next advisory tomorrow morning at 7am. If you get out in the backcountry give us a call at 754-2369 or send us your observations using the button at the top of this page. Thanks and have a great day.
The NWS weather forecast for:
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST FRI MAR 4 2011
.TODAY…PARTLY CLOUDY. HIGHS IN THE 20S. LIGHT WINDS EXCEPT
NORTH 10 TO 15 MPH NEAR SEWARD.
.TONIGHT…CLEAR IN THE EVENING THEN BECOMING PARTLY CLOUDY. LOWS
ZERO TO 20 ABOVE…COOLEST INLAND. VARIABLE WIND 10 MPH EXCEPT
NORTH 15 TO 30 MPH NEAR SEWARD.
.SATURDAY…MOSTLY CLOUDY. A SLIGHT CHANCE OF SNOW IN THE
AFTERNOON. HIGHS IN THE MID 20S TO LOWER 30S. VARIABLE WIND TO 10
MPH. NEAR SEWARD…NORTH WIND 15 TO 30 MPH.
.SATURDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF SNOW. LOWS 10 TO
25 ABOVE…COOLEST INLAND. NORTHEAST WIND 10 TO 15 MPH EXCEPT
NORTH 10 TO 20 MPH NEAR SEWARD.
.SUNDAY…MOSTLY CLOUDY. A SLIGHT CHANCE OF SNOW IN THE MORNING…
THEN ISOLATED SNOW SHOWERS IN THE AFTERNOON. HIGHS IN THE UPPER
20S TO MID 30S. NORTHEAST WIND 15 MPH.
.SUNDAY NIGHT…PARTLY CLOUDY WITH ISOLATED SNOW SHOWERS. LOWS IN
TEMPERATURE / PRECIPITATION
SEWARD 28 17 30 / 0 0 20
GIRDWOOD 23 10 24 / 0 0 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Temperature 10 degrees. Light wind shifted overnight, currently from the West.
-2600′ Seattle Ridge Wx Station-
Temperature 12 degrees. Light wind from the Southeast.
-1800′ Center Ridge Wx Station-
Temperature 13 degrees. No new snow recorded.