Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Thursday, December 30th 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 avalanche hazard rating is at least CONSIDERABLE today, possibly with pockets of HIGH. The difficulty is gauging the amount and intensity of new snow coming in and limited field observations. The danger is certainly increasing as this storm passes through with significant new snow and high wind. Currently radar is showing snowfall across the Kenai and Turnagain arm. All this new snow is bonding poorly to the old surface. Natural and human triggered avalanches are likely today.
I’m expecting widespread avalanche activity today with 7 inches yesterday, new snow falling currently and wind gusting into the 40s. Yesterday we had excellent visibility through Turnagain pass and saw limited avalanche activity in the form of many point releases and a few isolated, small, mid-slope slabs. That was with 7 inches of light density 5% snow. Today I’m expecting that avalanche activity to ramp up significantly if the weather service snowfall prediction is correct.
Primary concern: Wind slab. With high wind and easily transportable snow, wind slabs are going to build quickly and avalanche spontaneously. The bonding to the old surface layer is not good, so I’m expecting a lot of this snow to flush out easily on its own. If you get near a ridge-top today, expect extremely tender wind slab near the ridges and on upper slopes.
Secondary concern: New Storm Snow. If we get enough snow today the wind might not be necessary to build a slab. Pay attention to the depth and reactivity of the new storm layer on lower elevation steeper slopes. Quick pits, and informal shear tests will be a good indicator of how easily this storm snow might slide. Rising temperatures during the storm are also a concern, building a denser slab on top of yesterday’s light density powder.
Secondary concern: Persistent weak layers. This is the wild card today. We know that the new snow is a concern in itself. We don’t know how much weight is needed to affect the weak layers within the older snow. If and when this starts to happen it will be a game changer. These weak layers, including facets around the Thanksgiving rain crust, and buried surface hoar, have the potential to produce less predictable and far more dangerous avalanche conditions. These layers will potentially hold a large amount of snow before being triggered from a shallow spot by a person. If anyone starts seeing activity into the old snow layers, please send us your observation.
Check out an encyclopedia of terms here: www.fsavalanche.org/Encyclopedia.aspx
This setup started weeks ago after the last significant snowfall. Clear and cold weather since early December has built surface hoar and facets. Yesterday 7 inches at Turnagain pass and 14 inches in Portage with light to moderate wind didn’t produce much avalanche activity. Today more predicted snowfall and increased wind. The Weather Service is calling for heavy snow today at higher elevations. 6-12 inches today, east wind 30-45 increasing to 50-60 this afternoon.
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 30 2010
…BLIZZARD WARNING NOW IN EFFECT UNTIL 3PM AKST THIS AFTERNOON NORTH
OF MOOSE PASS…
…HIGH WIND WATCH IN EFFECT FROM THIS EVENING THROUGH EARLY FRIDAY
MORNING THROUGH PORTAGE VALLEY AND ALONG TURNAGAIN ARM…
.TODAY…SNOW HEAVY AT TIMES THIS MORNING AT HIGHER ELEVATIONS AND
INLAND. SNOW BECOMING MIXED WITH RAIN ALONG THE COASTS THIS MORNING
THEN SPREADING INLAND THIS AFTERNOON. BLOWING SNOW REDUCING
VISIBILITIES TO ONE QUARTER MILE OR LESS AT TIMES NORTH OF MOOSE
PASS…THROUGH PORTAGE VALLEY…TO ALONG TURNAGAIN ARM. VISIBILITIES
WILL IMPROVE THIS AFTERNOON. SNOW ACCUMULATIONS 6 TO 12 INCHES INLAND
AND 2 TO 4 INCHES ALONG THE COASTS. HIGHS IN THE MID 20S TO MID 30S.
NORTH TO EAST WINDS 15 TO 35 MPH. THROUGH PORTAGE VALLEY AND
TURNAGAIN ARM…EAST WIND 30 TO 45 MPH INCREASING TO 50 TO 60 MPH IN
.TONIGHT…RAIN AND SNOW. SNOW ACCUMULATION 1 TO 4 INCHES AT HIGHER
ELEVATIONS AND INLAND. LOWS IN THE MID 20S TO MID 30S. SOUTH TO EAST
WINDS 20 TO 45 MPH. THROUGH PORTAGE VALLEY AND ALONG TURNAGAIN
ARM…SOUTHEAST WIND 40 TO 65 MPH WITH POSSIBLE GUSTS TO 80 MPH.
.FRIDAY…RAIN AND SNOW LIKELY. SNOW ACCUMULATION UP TO 1 INCH.
HIGHS IN THE UPPER 20S TO MID 30S. SOUTHEAST WIND 15 TO 30 MPH
DECREASING TO VARIABLE 10 MPH IN THE AFTERNOON. THROUGH PORTAGE
VALLEY AND TURNAGAIN ARM…EAST WIND 40 TO 55 MPH DECREASING TO
10 TO 20 MPH IN THE AFTERNOON.
.FRIDAY NIGHT…SNOW LIKELY. LOWS IN THE LOWER 20S TO LOWER 30S.
SOUTHEAST WIND 10 TO 25 MPH INCREASING TO EAST 30 TO 45 MPH AFTER
.NEW YEARS DAY…SNOW LIKELY IN THE MORNING…THEN RAIN AND SNOW
LIKELY IN THE AFTERNOON. HIGHS IN THE LOWER TO MID 30S. EAST WIND
35 TO 50 MPH.
.SATURDAY NIGHT…SNOW AND RAIN LIKELY. LOWS IN THE 20S.
.SUNDAY THROUGH TUESDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF
SNOW. HIGHS 25 TO 35. LOWS 15 TO 25.
.WEDNESDAY…MOSTLY CLOUDY. HIGHS 25 TO 35.
TEMPERATURE / PRECIPITATION
SEWARD 34 34 34 / 90 90 80
GIRDWOOD 28 25 31 / 80 90 80
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Temperatures slowly increasing to the current 19. Wind increasing. Average in the high 20s, gusting into the 40s from the East.
-2600′ Seattle Ridge Wx Station-
Temperature 23 degrees. Wind steady in the 20s gusting into the 40s from the SE.
-1800′ Center Ridge Wx Station-
Recorded about 0.3 inches of water yesterday, estimated at 7 inches snow. Since last night anCNFAIC Staff 0.2 inches water, currently snowing. Temperature 26 degrees.