Turnagain Pass

Good morning backcountry travelers this is Lisa Portune with the Chugach National Forest Avalanche Information Center on Sunday, December 20 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

No new snow fell in the last 24 hours, but temperatures shot up by about 15 to 20 degrees yesterday. Mountain temperatures started off in the single digits but ended the day in the upper teens to low 20’s. The most dramatic temperature swing goes to Portage and Moose Pass where temps went from -20F to +30F in 24 hours. Ridgetop winds in Turnagain Pass averaged 15-30mph out of the east to southeast yesterday with gusts in the 30’s and 40’s. Summit Lake winds averaged 15-25mph out of the south with gusts in the 20’s and 30’s.

As of 4 am this morning, ridgetop winds continue to average 15-20 mph out of the east to southeast in Turnagain Pass and Summit Lake. It is lightly snowing in Girdwood right now, and temperatures currently range from 34F at sea level to 19F at 3800 feet. Temperatures will continue to rise through Monday as southerly flow brings a warmer air mass into the area. The winds are forecasted to ramp up even higher this afternoon, averaging 25-40mph out of the southeast. Expect mountain temperatures in the low to mid 20’s today with maybe a few inches of new snow.

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

recorded no new snow in the last 24 hours. The current temp is 26F (16 degrees warmer than yesterday) with a total snowpack depth of 56 inches (2 inches of settlement since yesterday).

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

recorded moderate to strong easterly winds yesterday averaging 15-30mph. The current temp is 19F (18 degrees warmer than yesterday) with winds averaging 20mph 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 24F (23 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 moderate southeasterly winds yesterday averaging 15-25mph. The current temp is 20F (16 degrees warmer than yesterday) with winds averaging 12pmh out of the southeast.

The weather forecast for:

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST SUN DEC 20 2009

…STRONG WIND THIS EVENING THROUGH MONDAY AFTERNOON…

.TODAY…SNOW SHOWERS BECOMING MIXED WITH RAIN IN THE AFTERNOON. SNOW

AREAS OF BLOWING SNOW THROUGH PORTAGE VALLEY AND ALONG TURNAGAIN ARM

THROUGH THIS MORNING. SNOW ACCUMULATION 1 TO 4 INCHES. HIGHS IN THE

MID 20S TO MID 30S. SOUTHEAST WIND 10 TO 25 MPH. PORTAGE VALLEY TO

ALONG TURNAGAIN ARM…EAST WINDS 25 TO 35 MPH. GUSTS TO 45 MPH THIS

AFTERNOON.

.TONIGHT…SNOW MIXED WITH RAIN. SNOW ACCUMULATION 1 TO 5 INCHES.

LOWS IN THE LOWER 20S TO MID 30S. SOUTHEAST WIND 25 TO 40 MPH EXCEPT

EAST 35 TO 55 MPH THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.

.MONDAY…SNOW AND RAIN. SNOW ACCUMULATION 1 TO 4 INCHES. HIGHS IN

THE 30S. PORTAGE VALLEY AND ALONG TURNAGAIN ARM…EAST WINDS 30 TO 55

MPH. SOUTHEAST WINDS 15 TO 30 MPH ELSEWHERE.

TEMPERATURE / PRECIPITATION

SEWARD 33 33 34 / 80 80 80

GIRDWOOD 26 22 30 / 60 80 80

AVALANCHE DISCUSSION

Human-triggered avalanches are likely today on recently formed windslabs above treeline and all CNFAIC Staff actively loading slopes. These sensitive windslabs vary in thickness from 1-2 feet and were reactive to human triggers yesterday. I would expect the same today as the winds will continue to be strong. For today, the avalanche danger is rated CONSIDERABLE on all wind-loaded slopes steeper than 35 degrees. All CNFAIC Staff slopes have a MODERATE danger.

The winds were ripping at the higher elevations yesterday in Turnagain Pass and Summit Lake, actively loading lee aspects and increasing the avalanche danger. Weak snow is underlying these recently formed windslabs, including surface hoar up to 2800 feet elevation. During his tour up Manitoba yesterday, our intern Jon got 40 to 50 ft. long shooting cracks at 3000 feet elevation. The wind was actively loading the slope at the time, and the cracks went down 2 feet to the new snow/old snow interface. He also heard the snowpack collapse in between wind gusts. Shooting cracks and whumphing are obvious red flags that the snow is unstable. Check out Jon’s great photo in our photo gallery. AnCNFAIC Staff party reported triggering a small avalanche between 2500 and 3000 feet elevation on Tincan on a slope below the main bowl. This slide was 1.5 feet deep, 20 feet wide, and ran 30 feet long. While not that big, it is certainly indicative of the instability that exists at these higher elevations.

I was with an avalanche class yesterday on the lower reaches of Tincan, and we easily found the huge surface hoar that formed during the inversion earlier this month. It is currently buried under 10-12 inches of snow, but was not reactive in any of our stability tests. On these lower elevation non wind-affected slopes, the new snow that fell Dec 14-16 is not cohesive enough yet to readily fracture. Watch out though when we get more of a load on this layer.

We also received a report of a skier triggered avalanche yesterday in the bowl below Peak 2 by Flattop Mountain. It was triggered on the lee side of the ridge where winds had loaded the slope. Within 3 – 4 turns a crack began just below his skis and propagated to the ridge, ripping out a surface slab ranging from 4-12 inches. The skier was fortunately not caught in the slide.

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.