Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Friday, December 17th 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).
The Lost Lake and Primrose trails are now open to motorized use.
Today the avalanche hazard is MODERATE overall. 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.
The overall danger rating is remaining steady. Upper elevation wind slabs seem to be stubborn to trigger but we have gotten occasional reports of small isolated avalanches in the last week. The clear and cold weather is changing the snowpack in slow and subtle ways. We have facet producing conditions right now with cold temperatures and a relatively shallow snowpack.
Taking a quick look at recent temperatures, it’s clear that the temperature gradient within the snow is exceeding the 1 degree C per 10 cm threshold for facet formation. Take a look at the photo gallery for a recent snow pit with a temperature profile. We are concerned about the Thanksgiving rain crust and its effect on faceting. Thick impermeable crusts such as this one have a tendency to nurture facet formation. It can be especially dangerous because the crust can also act as a slick sliding surface or support the load above before it collapses all at once on the weak layers below. Recent snow pits have confirmed that well developed facets can be found above and below the rain crust and the crust is one of the primary failure points in compression tests.
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 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 and allow 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 and stress is low. At some point as the stress ramps up during and after a major storm the tipping point will be reached and avalanches will happen.
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The temperature inversion continues today in the Chugach and Kenai regions. Portage is reading -14 compared to Sunburst at +16. Wind has not been a major player in the last 24 hours. Max recorded gust in our region was 22mph at the 43 mile station. No precip and clear skies are expected to continue today. If you are brave enough to go out in negative temps at the parking lot it looks to be fairly pleasant higher up on the ridges today.
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 DEC 17 2010
…STRONG WIND THROUGH SATURDAY NIGHT NEAR WHITTIER…
.TODAY…SUNNY. HIGHS 5 TO 10 ABOVE EXCEPT IN THE LOWER 20S TO
LOWER 30S NEAR SEWARD AND WHITTIER. WEST WIND 45 TO 60 MPH NEAR
WHITTIER. NORTH WIND 20 TO 35 MPH NEAR SEWARD. NORTHWEST WIND
10 TO 15 MPH ELSEWHERE.
.TONIGHT…CLEAR. LOWS ZERO TO 10 BELOW EXCEPT 10 TO 20 ABOVE
NEAR SEWARD AND WHITTIER. WEST WIND 40 TO 55 MPH NEAR WHITTIER.
NORTH WIND 20 TO 35 MPH NEAR SEWARD. NORTHWEST WIND 10 TO 15 MPH
.SATURDAY…MOSTLY SUNNY. HIGHS IN THE MID TEENS TO LOWER 30S…
COOLEST INLAND. WEST WIND 45 TO 60 MPH NEAR WHITTIER. NORTH WIND
20 TO 35 MPH NEAR SEWARD. NORTHWEST WIND 10 TO 15 MPH ELSEWHERE.
.SATURDAY NIGHT…CLEAR. LOWS 5 BELOW TO 5 ABOVE EXCEPT AROUND
15 NEAR SEWARD AND WHITTIER. WEST WIND 30 TO 45 MPH NEAR WHITTIER.
NORTH 15 TO 30 MPH NEAR SEWARD. NORTHWEST WIND 10 TO 15 MPH
.SUNDAY…SUNNY. HIGHS 15 TO 25…COOLEST INLAND. VARIABLE WIND
10 MPH. NEAR WHITTIER…WEST WIND 20 TO 30 MPH. NEAR SEWARD…NORTH
WIND 15 TO 25 MPH.
.SUNDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF SNOW. LOWS 5 TO 15.
.MONDAY THROUGH TUESDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF
SNOW. HIGHS IN THE 20S. LOWS 5 TO 15.
.WEDNESDAY THROUGH THURSDAY…PARTLY CLOUDY. HIGHS 15 TO 25.
LOWS 5 TO 15 ABOVE.
TEMPERATURE / PRECIPITATION
SEWARD 22 12 24 / 0 0 0
GIRDWOOD 8 1 18 / 0 0 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Temperatures are dropping from the high of 28 yesterday. Currently 16 degrees. Wind was light from the WNW yesterday.
-2600′ Seattle Ridge Wx Station-
Temperatures dropping. Last recorded temp at 0200 was 13 degrees. Wind was light and variable and on an increasing trend.
-1800′ Center Ridge Wx Station-
No new snow in the last several days. The temperature just dipped below zero in the last hour.