Turnagain Pass

Good morning backcountry travelers this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Monday, December 21 at 7 am. This will serve as a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (advisory does not apply to highways, railroads, or operating ski areas).

ANNOUNCEMENTS

Placer River Drainage is open to snowmachining. Twentymile remains closed.

WEATHER ROUNDUP

First off and most importantly, at 8:47am this morning the Winter Solstice will occur, and tomorrow we will actually gain 9 seconds of daylight. A trace to one inch of new snow fell yesterday in Turnagain Pass, and ridgetop winds averaged 15-25mph out of the southeast and east with gusts in the 30’s. Winds were a little lighter at Summit Lake averaging 10-20mph out of the southeast with gusts in the 20’s and 30’s. Mountain temperatures ranged from the low to high 20’s yesterday.

As of 4 am this morning, ridgetop winds have increased and are currently averaging 25-35mph out of the east to southeast with gusts in the 40’s and 50’s in Turnagain Pass and Summit Lake. It is lightly snowing/raining in Girdwood right now with temperatures ranging from 38F at sea level to 22F at 3800 feet. Temperatures will continue to rise through Tuesday as strong southerly flow brings tropical air into the area. The winds are forecasted to ramp up even higher this afternoon through Portage Valley and Turnagain Arm, averaging 40-65mph out of the east. Ridgetop winds in Turnagain Pass and Summit Lake will continue to average 25-35mph (and possibly higher) out of the east and southeast, with even stronger winds through passes and CNFAIC Staff terrain gaps. Expect mountain temperatures in the mid 20’s to lower 30’s today with maybe an inch or two of new snow at the higher elevations.

-The Center Ridge Wx Station at 1800 feet/Turnagain Pass-

recorded 1 inch of new snow in the last 24 hours. The current temp is 31F (5 degrees warmer than yesterday) with a total snowpack depth of 56 inches (same as yesterday).

-The Sunburst Wx Station at 3800 feet/Turnagain Pass-

recorded moderate easterly winds yesterday averaging 15-25mph. The current temp is 22F (3 degrees warmer than yesterday) with winds averaging 25-35mph out of the east.

-The Summit Lake Wx Station at 1200 feet-

recorded no new snow in the last 24 hours. The current temp is 31F (8 degrees warmer than yesterday) with a total snowpack depth of 29 inches (same as yesterday).

-The Fresno Ridge Wx Station at 3400 feet/Summit Lake-

recorded light to moderate southeasterly winds yesterday averaging 10-20mph. The current temp is 25F (5 degrees warmer than yesterday) with winds averaging 25pmh out of the southeast.

The weather forecast for:

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST MON DEC 21 2009

…STRONG WIND TODAY AND TONIGHT THROUGH PORTAGE VALLEY AND ALONG

TURNAGAIN ARM…

.TODAY…SNOW AND RAIN. SNOW ACCUMULATION 1 TO 4 INCHES…MAINLY

HIGHER ELEVATIONS AND PORTAGE VALLEY. HIGHS IN THE LOWER 30S TO LOWER

40S. SOUTH TO EAST WINDS 15 TO 25 MPH WITH GUSTS TO 35 MPH. THROUGH

PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 30 TO 50 MPH INCREASING

40 TO 65 MPH THIS AFTERNOON.

.TONIGHT…RAIN AND SNOW. SNOW ACCUMULATIONS 1 TO 4 INCHES…MAINLY

HIGHER ELEVATIONS. LOWS IN THE UPPER 20S TO MID 30S. SOUTH TO EAST

WINDS 20 TO 35 MPH DIMINISHING TO 10 TO 20 MPH AFTER MIDNIGHT.

THROUGH PORTAGE VALLEY AND ALONG TURNAGAIN ARM…EAST WINDS 40 TO 65

MPH DIMINISHING 25 TO 45 MPH AFTER MIDNIGHT.

.TUESDAY…SNOW AND RAIN. LITTLE SNOW ACCUMULATION. HIGHS IN THE MID

30S TO LOWER 40S. EAST WIND 15 TO 30 MPH.

TEMPERATURE / PRECIPITATION

SEWARD 39 36 38 / 100 80 80

GIRDWOOD 32 29 36 / 70 70 70

AVALANCHE DISCUSSION

Human-triggered avalanches are still likely today on recently formed windslabs above treeline and any CNFAIC Staff actively loading slopes. These sensitive windslabs vary in thickness from 6 inches to 2 feet and were reactive to human triggers on Saturday. I would expect the same today as ridgetop winds continue to transport snow. For today, the avalanche danger is rated CONSIDERABLE on all actively loading slopes steeper than 35 degrees. Non wind-affected slopes below treeline have a MODERATE avalanche hazard. If the rain line creeps higher today, watch for natural avalanches on lower elevation steep slopes where rain is falling on dry snow.

At the higher elevations yesterday snow was observed pluming off the ridgetops actively loading lee aspects and adding energy to the snowpack. I’m sure a skier or rider could have triggered an avalanche on those higher elevation slopes. The same holds true for today.

Jon toured down by the old ski hill at Summit Lake yesterday and found that the windslab that formed Saturday was no longer reactive to ski cuts. The winds were light yesterday down there, and the warm temperatures certainly helped bond the windslabs to the old snow surface. The 6 inch thick slab at this location did not crack or collapse like it did the previous day on Manitoba where 40 to 50 ft. long shooting cracks up to 2 feet deep and whumphing were observed. That being said, there is still weak snow underlying these recently formed windslabs, including surface hoar up to 2800 feet elevation.

On Saturday we easily found the huge surface hoar that formed during the inversion earlier this month. The biggest, most well-preserved surface hoar is below 2000 feet on protected slopes below treeline. It is currently buried under 10-12 inches of dry powdery snow, but was not reactive in any of our stability tests. On these lower elevation non wind-affected slopes, the surface snow was not cohesive enough yet to readily fracture nor was there enough of a load yet to cause problems. If rain falls on these slopes, however, watch for natural avalanches on this dry surface snow.

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.