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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Thu, February 11th, 2010 - 7:00AM
Expires
Fri, February 12th, 2010 - 7:00AM
Forecaster
CNFAIC Staff
The Bottom Line

Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Thursday February 11th at 7 am. 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).

WEATHER ROUNDUP

That storm over the Aluetians has started to fizzle out, but it definitely left an impact on our snowpack.

-The winds have backed off at all ridgetop weather stations and are currently light to moderate with average wind speeds between 7-18mph. In the past 24 hours, however, Sunburst had moderate to extreme winds averaging 17-44 mph with an extreme max gust of 58mph.

-The radar shows scattered precip currently moving west towards us.

-In the last 24 hours (4am-4am), the snotel sites recorded .7 inches of water at Turnagain Pass, .6 inches at Grandview, and .1 at Summit Creek. Snowfall totals range 1-9 inches of new snow based on elevation.

-Temps have increased again at all wx stations by 1-2 degrees F compared to yesterday. Temps range from 35 degrees F at sea-level and 24 degrees F at 3800′.

AVALANCHE DISCUSSION

Due to lack of information and a questionable weak layer underneath a fresh load of heavy snow, the avalanche danger needs to stay at CONSIDERABLE. Not enough new information has come to light, in order, to drop the danger yet.

We still have some uncertainty with the snowpack. We tried to get some higher elevation observations yesterday on Tincan, but the visibility was very poor. We only made it up to 2500′; so, we don’t have much information about the snowpack in the steeper starting zones along the ridges. The only information we have about these upper elevations is that AK RR shot along the tracks south of Girdwood yesterday, and we did not see very much new avalanche debris in any of those paths between 20 Mile and Girdwood, except, for a medium sized pile in the path closest to Kern Creek.

Up to 2500′ on Tincan, we did not see any obvious signs of instability yesterday. We left the skin track in spots to jump around on steep test slopes, and we dug some pits on a steep roll. We felt no collapsing, I had one shooting crack that was only about 4 feet long in the top 2 inches of the snow, and the apprx 12” of storm snow did not appear to have any energy. The surface snow was like heavy tooth paste up to 1800′ and lighter density snow above 1800′.

The biggest danger for a human triggered avalanche today will be a new storm snow slabs on top of the thin breakable melt-freeze layer that has surface hoar on top of it on steep rollovers greater than 35 degrees. Since we have seen surface hoar on top of a crust in several locations, we have to give this type of weak layer lots of respect. We always have multiple layers of buried surface hoar in our snowpack. A lot of times they don’t do much of anything, but this layer has my attention. This is the type of weak layer you hear old-timers talking about because the surface hoar is on top of a crust. This combination is probably responsible for the majority of surprises and fatalities. It is easy to forget about this type of weak layer, especially when the snowpack was showing very good stability in spots on Monday (see “backcountry artillery” in the forecasters video or on YouTube).

My general impression yesterday was that the snowpack was adjusting well to its new load, but now there is an additional .7 inches of water weight and apprx 9 inches of new snow. That brings the storm total up to 1.5 inches of water weight and 14” of new snow at Center Ridge. As usual, this is a general number and there will be more snow at the northern end of Turnagain Pass and less snow on the southern end.

There are two main weak layers in our snowpack.

1.A thin breakable melt-freeze crust with surface hoar on top of it. We have seen this weak layer up to 3000′ on multiple aspects on both sides of highway at Turnagain Pass. This weak layer is generally buried 1 ½ to 2 ½ feet deep. Of course it will be buried deeper in areas where wind slabs formed. We were getting moderate failures with clean fast shears on this layer along Seattle Ridge on Monday 2/8/2010, but this weak layer was not reactive to human-triggers in certain locations on Monday (see forecasters video link for “backcountry artillery). Surface hoar has been observed on top of this layer in several pits. Not all pits show the surface hoar, and these feathery crystals get smaller as you gain elevation. This is the same weak layer that was responsible for the skier-triggered avalanches on Tincan on Saturday 2/6/2010. Plus I suspect this was the same weak layer that failed creating a natural avalanche on Widowmaker on Saturday 2/6/2010 as well (see photo gallery). This particular weak layer will be the likely culprit for human-triggered avalanches today.

2.The Jan 7 rain crust is generally buried 3-5 feet deep and it qualifies as a deep slab instability. As of yesterday 2/10/2010, it was still showing moderate failures with clean smooth shears. This weak layer is still showing significant signs of instability, but the question is what kind of trigger could create an avalanche on this weak layer? It appears that this weak layer will require a big trigger or more weight on top of it. This weak layer is widespread on all aspects between 2000-3000 feet.

WEATHER FORECAST (National Weather Service)

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST THU FEB 11 2010

…STRONG WIND LATE TONIGHT THROUGH FRIDAY AFTERNOON THROUGH PORTAGE

VALLEY AND TURNAGAIN ARM…

.TODAY…RAIN SHOWERS DIMINISHING THIS AFTERNOON. PATCHY FOG.

HIGHS IN THE MID 30S TO LOWER 40S. NORTH TO EAST WIND 10 TO 20 MPH.

.TONIGHT…A CHANCE OF RAIN IN THE EVENING…THEN RAIN LIKELY

AFTER MIDNIGHT. LOWS IN THE MID 30S. NORTH TO EAST WIND 10 TO 20 MPH.

THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 10 TO 20 MPH

INCREASING TO 30 TO 45 MPH AFTER MIDNIGHT.

.FRIDAY…RAIN LIKELY. HIGHS IN THE MID 30S TO LOWER 40S.

NORTH TO EAST WIND 15 TO 30 MPH BECOMING SOUTH TO EAST IN

THE AFTERNOON. THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…EAST

WIND 35 TO 50 MPH.

Temperature / Precipitation

SEWARD 39 34 38 / 40 60 80

GIRDWOOD 38 33 40 / 80 60 60

Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass

Sea-level: temps are forecasted between 33-38 with up to 0.1” of water today

3000′: temps are forecasted in the range of 23-32 degrees F with winds 15-20 mph

6000′: temps are forecasted in the range of 23-32 degrees F with winds 30 mph

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Friday February 12th.

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Thu, February 11th, 2010
Alpine
Above 2,500'
Considerable (3)
Avalanche risk
Treeline
1,000'-2,500'
Considerable (3)
Avalanche risk
Below Treeline
Below 1,000'
Considerable (3)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
Considerable (3)
Treeline
1,000'-2,500'
Avalanche risk
Considerable (3)
Below Treeline
Below 1,000'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
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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.