Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Thursday, December 16th 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 opened yesterday to motorized use.
Today the avalanche hazard will remain MODERATE overall. Natural avalanches are unlikely, but small human triggered avalanches are possible in specific areas. My primary concern is steep wind loaded pockets near ridges sitting on top of multiple layers of buried surface hoar. Use extra caution in the upper elevation terrain with bad consequences.
The overall danger rating is remaining steady. Upper elevation wind slabs aren’t going anywhere and the structure underneath is slowly getting weaker as it facets out in some areas. Clear and cold weather doesn’t cause quick change to the snowpack, but it does produce change. 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. This is textbook facet growing conditions. 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. Snow pit tests yesterday confirmed that well developed facets can be found around 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.
Try to think about the main ingredients for producing unstable structure in the snow. You need a weak layer (we have several in the forms of buried surface hoar, and facets.) You need a bed surface, or a layer below the weak layer that is stronger (we have different possibilities but focus on the rain crust which is as hard and smooth as layers can get.) You need the slab on top (our current slab is shallow, flimsy, and weak). Our prediction is that when we get a real slab on top of the CNFAIC Staff layers that avalanches will be imminent. The big question now is when that storm will happen and how fast will it come in. A hard, fast loading will create unstable conditions. If we keep getting small bits of snow at infrequent intervals and allow the snowpack to adjust in between then we might not see any reaction for quite some time. Stay tuned. We’ll let you know when we get concerned. The key here is that 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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Clear skies and cold temperatures have been ongoing the last few days. Temperatures were consistently in the single digits yesterday, but have risen at upper elevations quickly in the last few hours creating a significant inversion. For example Portage is currently –10 while Sunburst is 20 above. The clear and cold has produced surface hoar and promoted faceting within the snowpack. Today is forecasted as the same with clear skies and slowly diminishing wind.
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 THU DEC 16 2010
…STRONG WIND THROUGH FRIDAY NIGHT NEAR SEWARD AND WHITTIER…
.TODAY…MOSTLY SUNNY. HIGHS 10 TO 20 ABOVE. WEST WIND 50 TO 65 MPH
NEAR WHITTIER. NORTH WIND 30 TO 50 MPH NEAR SEWARD. NORTHWEST WIND 10
TO 15 MPH ELSEWHERE.
.TONIGHT…CLEAR. LOWS 15 BELOW TO 15 ABOVE…COOLEST INLAND.
WEST WIND 35 TO 50 MPH NEAR WHITTIER. NORTH WIND 30 TO 45 MPH NEAR
SEWARD. NORTHWEST WIND TO 10 MPH ELSEWHERE.
.FRIDAY…SUNNY. HIGHS 5 TO 25 ABOVE…COOLEST INLAND. WEST WIND 35
TO 50 MPH NEAR WHITTIER. NORTH WIND 30 TO 45 MPH NEAR SEWARD.
NORTHWEST WIND TO 10 MPH ELSEWHERE.
.FRIDAY NIGHT…CLEAR. LOWS 15 BELOW TO 15 ABOVE. WEST WIND 35 TO 50
MPH NEAR WHITTIER. NORTH WIND 30 TO 45 MPH NEAR SEWARD. NORTHWEST
WIND TO 10 MPH ELSEWHERE.
.SATURDAY…SUNNY. HIGHS 15 TO 25…COOLEST INLAND. WEST WIND 20 TO
35 MPH NEAR WHITTIER. NORTH WIND 20 TO 35 MPH NEAR SEWARD. NORTHWEST
WIND TO 10 MPH ELSEWHERE.
.SATURDAY NIGHT…INCREASING CLOUDS. LOWS ZERO TO 10 ABOVE.
.SUNDAY…PARTLY CLOUDY WITH A CHANCE OF SNOW SHOWERS. HIGHS
15 TO 25.
.SUNDAY NIGHT THROUGH MONDAY NIGHT…PARTLY CLOUDY. LOWS 5 TO 15.
HIGHS 15 TO 25.
.TUESDAY…PARTLY CLOUDY WITH A CHANCE OF SNOW SHOWERS. HIGHS IN
.TUESDAY NIGHT AND WEDNESDAY…MOSTLY CLOUDY WITH A CHANCE OF
SNOW SHOWERS. LOWS IN THE TEENS. HIGHS 15 TO 25.
TEMPERATURE / PRECIPITATION
SEWARD 20 12 25 / 0 0 0
GIRDWOOD 12 3 18 / 0 0 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Recorded light wind from the NE with a max gust of 15. Temperatures are rising and currently 20 degrees.
-2600′ Seattle Ridge Wx Station-
Light wind averaging 2-6 in the last 24 hours. Temperatures on a rising trend and currently 13.
-1800′ Center Ridge Wx Station-
No new snow in the last several days. Generally cold temperatures. Station hasn’t updated since last night.