Turnagain Pass

Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Saturday, December 18th 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).

ANNOUNCEMENTS

The Lost Lake and Primrose trails are open to motorized use.

BOTTOM LINE

Today the avalanche hazard is MODERATE. Natural avalanches are unlikely, but small human triggered avalanches are possible in specific areas. Steep wind loaded pockets at upper elevations are the primary concern. Persistent weak layers are widespread through the region, but most areas lack a sufficient slab to cause avalanches.

AVALANCHE DISCUSSION

The overall danger rating is remaining steady. My own sense of caution is dropping ever so slightly on a daily basis but the continued internal weakening of the snowpack caused by faceting is preventing me from dropping the danger rating. The faceting is unlikely to produce spontaneous issues on its own and the slab that might break under a human trigger is relatively small and weak. Our main goal right now is to watch the slow faceting process and think about how it’s going to react in the future when we do get a heavy storm load on top. If you choose to ride on larger committing terrain this is a good time to use common sense and safe travel protocol. Expose only one person at a time, watch your partner, stop in safe spots, and have an escape plan.

Our recent snow pits have shown facet producing temperature gradients with obvious facets above and below the Thanksgiving rain crust. Weak layers around a crust can be especially dangerous because the crust can act as a slick sliding surface or support the load above before it collapses all at once on the weak layers below. Our column tests are showing moderate to hard force needed to cause failure, low propagation, and irregular failure planes. My prediction is that you really need to look hard to find the areas with built up slabs to cause avalanching right now.

Many of you may be wondering why we keep rambling on about these insidious weak layers but the reality is very few avalanches are happening. The reason lies in the structure of the snow and the related balance between stress and strength.

If you think about the ingredients for producing an unstable structure, we have two of the three pieces of the puzzle. The current structure consists of many potential weak layers over some harder layers. The puzzle piece we are lacking is a stiffer, stronger, and heavier slab on top of the junky snow. Those isolated areas near the ridges holding wind deposited pillows have proven reactive but they are only found in small pockets. When we do get a significant snowfall it will bring that final piece of the puzzle. A hard, fast storm deposit will build the dangerous slab and create stress on top of the weak layers. If we keep getting small amounts of snow at long intervals, which allows the snowpack to adjust in between, then we might not see any reaction for quite some time. Keep this in mind when our current high pressure gives way to more snow.

The key here is the critical balance between the stress and strength of the snowpack. Right now strength is low, but stress is also low. Slab avalanches happen when the stress exceeds the strength of the snowpack. At some point the stress ramps up during and after a major storm. The tipping point will be reached and avalanches will happen.

CNFAIC Staff noteworthy conditions include the obvious surface hoar forming right now. We would love to hear reports of where people are finding this most recent layer. Also the glide cracks on Seattle ridge near the motorized uptrack are slowly opening up. The snow is still gliding (moving slowly at the ground interface).

Check out an encyclopedia of terms here:

www.fsavalanche.org/Encyclopedia.aspx

WEATHER ROUNDUP

The temperature inversion continues today in the Chugach and Kenai regions. The lowest temperatures at the valley floors have come up slightly since yesterday. Wind has increased a bit in the last 24 hours. Yesterday you could see wind picking up vortexes of snow at the ridge-tops and moving it around a bit. The highest recorded gust yesterday was 36mph at the Fresno ridge station. No precipitation and clear skies are expected to continue today. Temperatures at the ridge tops have come up a little to the low 20s in some places.

Lisa 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:

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST SAT DEC 18 2010

…STRONG WIND THROUGH SUNDAY AFTERNOON NEAR WHITTIER…

.TODAY…PARTLY CLOUDY IN THE MORNING THEN BECOMING MOSTLY SUNNY.

HIGHS 10 ABOVE TO NEAR 30…COOLEST INLAND. WEST WIND 40 TO 55 MPH

NEAR WHITTIER. NORTH WIND 25 TO 35 MPH NEAR SEWARD. NORTHWEST WIND 10

TO 15 MPH ELSEWHERE.

.TONIGHT…MOSTLY CLEAR. LOWS 5 BELOW TO 5 ABOVE EXCEPT AROUND

15 NEAR SEWARD AND WHITTIER. WEST WIND 40 TO 55 MPH NEAR WHITTIER.

NORTH WIND 25 TO 35 MPH NEAR SEWARD. NORTHWEST WIND 10 TO 15 MPH

ELSEWHERE.

.SUNDAY…PARTLY CLOUDY. HIGHS 15 TO 25…COOLEST INLAND. NORTH WIND

10 TO 20 MPH. NEAR WHITTIER…WEST WIND 30 TO 45 MPH.

.SUNDAY NIGHT…PARTLY CLOUDY. LOWS 5 BELOW TO 20 ABOVE…COOLEST

INLAND. VARIABLE WIND TO 10 MPH. NEAR WHITTIER…WEST WIND 15 TO 30

MPH.

.MONDAY…PARTLY CLOUDY IN THE MORNING THEN BECOMING MOSTLY

SUNNY. HIGHS IN THE MID TEENS TO LOWER 30S. NORTHWEST WIND 10 TO

15 MPH EXCEPT WEST 20 TO 35 MPH NEAR WHITTIER.

.MONDAY NIGHT…PARTLY CLOUDY. LOWS IN THE TEENS.

.TUESDAY THROUGH WEDNESDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW.

HIGHS IN THE TEENS. LOWS 5 TO 15 ABOVE.

.WEDNESDAY NIGHT…PARTLY CLOUDY. LOWS ZERO TO 10 ABOVE.

.THURSDAY…PARTLY CLOUDY. HIGHS IN THE TEENS.

.THURSDAY NIGHT AND FRIDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW.

LOWS ZERO TO 10 ABOVE. HIGHS 15 TO 25.

TEMPERATURE / PRECIPITATION

SEWARD 22 15 24 / 0 0 0

GIRDWOOD 14 4 19 / 0 0 0

WEATHER STATION SUMMARY for Turnagain Pass:

-3800′ Sunburst Wx Station-

Temperatures in the low 20s. Wind ranging 8-16mph from the west.

-2600′ Seattle Ridge Wx Station-

Current temperature 19. Wind light and variable.

-1800′ Center Ridge Wx Station-

No new snow in the last week. Temperature is 5 degrees.