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Issued
Thu, March 18th, 2010 - 7:00AM
Expires
Fri, March 19th, 2010 - 7:00AM
Forecaster
CNFAIC Staff
Avalanche risk The Bottom Line

Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Thursday March 18th, 2010 at 7 am. 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).

WEATHER ROUNDUP

-The winds are increasing this morning with moderate average speeds in the 20mph range and strong to extreme gusts up to 40mph. Winds have CNFAIC Staffwise been light to moderate in the past 24 hours.

-In the last 24 hours (5am-5am), the snotel sites recorded: 0.0 inches of water and 1 inch of snow melting/settlement at Turnagain Pass, 0.0 inch of water and 1 inche of snow melting/settlement at Grandview, and 0.0 inches of water and 0 inches snow melting/settlement at Summit Creek.

-The current radars show scattered precip moving west over Prince William Sound heading toward the Kenai Mountains.

-Temps made it as high as 34 degrees at 3300′ and 42 degrees at 1800′. Valley floors are warming up this morning by 5-16 degrees compared to yesterday. Current temps range from 34 degrees F at sea-level and 21 degrees F at 3800′.

AVALANCHE DISCUSSION

Today’s danger level is MODERATE with pockets of CONSIDERABLE. The sun has been the main contributing factor to recent avalanches. Natural avalanches will be less likely today since the sun is forecasted to go away. The weather forecast is calling for snow and rain today; so, our stability situation will be quite a bit different today with wind and new precip. There have been several alarmingly dangerous human and natural triggered avalanches since Monday. Terrain management will still be key today, to give recent avalanche activity a chance to adjust.

We have heard through the grapevine that there were 2 more skier triggered avalanches on Tuesday. Neither party reported any details to the Avalanche Center; so, we can not confirm any information. Sounds like one avalanche occurred Big League near Girdwood Valley, and the CNFAIC Staff one somewhere near the Turnagain/Johnson Pass/Grandview area. We have no further information about these human-triggered avalanches. Thank you to everyone that has been submitting observations lately. Your information could save somebody’s life.

A natural slab avalanche was reported yesterday 3/17/2010 at North Gilpatrick in the Summit Lake area yesterday. It was unclear exactly which day this avalanche occurred. It was described as South-Southeast aspect, 3500′, 33 degree slope, and 300′ wide (see photo gallery).

A human-triggered slab avalanche was reported 3/16/2010. The slab was triggered under the foot of the second person skinning up a South East aspect. The party heard a couple of “whumphs” before the avalanche a ripped out a few hundred feet wide (see photo gallery).

It ranged in depth from 1′-4′. It failed on top of a recent sun crust (potentially with facets on top of the crust) on a sun baked slope at about 2:20pm on Tuesday 3/16/2010. No injuries or burial reported.

Also see the photo gallery for pictures of the human-triggered slab avalanche that was reported on Monday 3/15/2010 at 3600′ on the west side of the Seward Highway across from Lower Summit Lake yesterday. Estimates from the reporting party describe the avalanche as 300′ wide, 1′ deep, which ran 2000′ and sympathetically triggered anCNFAIC Staff avalanche 150′ wide. The bed surface/failure layer was a recent crust. The slope angle at the trigger point was estimated to be 35 degrees on shallow rocky terrain that was slightly wind loaded. The snowboarder that triggered the avalanche started to get partially buried on the way down, but luckily was spit out on top of the debris at the bottom. No injuries reported.

There is a slippery surface that formed on Saturday 3/13/2010 with 5-7” of new snow ontop that fell on Sunday 3/14/2010. We have not been able to confirm that any facets exist above this slippery layer closest to the surface. Pits and hand pits on Sunburst and Seattle Ridge have been finding Q2-Q3 (uneven to relatively clean shears) with broken stellar on top. There were also winds associated with Sunday’s snowfall; so, there are areas where the surface snow has tapered depths from 1-4 feet deep. The recent intense sun has been warming this relatively shallow surface snow, causing it to become a slab that has resulted in wide propagation and fairly long running human-triggered avalanches on Monday and Tuesday from Lower Summit Lake to Turnagain Pass.

There is a lot of spatial variability out there right now. I found thick suncrust on Seattle Ridge on Tuesday, but found more of a windcrust with pockets of suncrust on Sunburst yesterday. The suncrust varies in thickness due to aspect and slope angle. We have observed large surface hoar on top of the suncrust at lower elevations, and medium sized surface hoar on top of multiple surfaces on the ridgetops on both sides of the highway at Turnagain Pass. This will be the next weak layer problem that might become responsible for some new avalanches out of the new storm that will start hitting us today.

Glide cracks are secondary concern today. These crevasse like features that are cracks that go all the way to the ground appeared to have opened up a bit more over the past 2-4 days. Glide cracks are like cornices in that they are wild cards that are very difficult to predict. Stay far away from these cracks today since they are actively moving right now.

WEATHER FORECAST (National Weather Service)

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKDT THU MAR 18 2010

…STRONG WIND THROUGH TONIGHT…

.TODAY…RAIN AND SNOW. SNOW ACCUMULATIONS UP TO 2 INCHES. HIGHS IN

THE MID 30S TO LOWER 40S. EAST TO NORTH WINDS 15 TO 30 MPH. PORTAGE

VALLEY AND ALONG TURNAGAIN ARM…EAST WINDS 15 TO 30 MPH. GUSTS TO 45

MPH THIS AFTERNOON.

.TONIGHT…RAIN AND SNOW. SNOW ACCUMULATION 1 TO 4 INCHES. AREAS OF

BLOWING SNOW. LOWS IN THE UPPER 20S TO MID 30S. SEWARD…NORTH WINDS

15 TO 30 MPH. ELSEWHERE…EAST WINDS 20 TO 30 MPH WITH GUSTS TO 50

MPH.

.FRIDAY…RAIN AND SNOW LIKELY IN THE MORNING…THEN A CHANCE OF RAIN

AND SNOW IN THE AFTERNOON. SNOW ACCUMULATION 1 TO 4 INCHES. HIGHS IN

THE UPPER 30S TO MID 40S. NORTH TO EAST WIND 15 TO 30 MPH DECREASING

TO 10 TO 20 MPH IN THE AFTERNOON.

TEMPERATURE / PRECIPITATION

SEWARD 43 34 43 / 70 90 70

GIRDWOOD 42 32 44 / 40 70 60

Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass

Sea-level: temps are forecasted between 32-42 and between 0.1”-0.25” of water forecasted

3000′: temps are forecasted in the range of 23-32 degrees F with winds 20-25 mph

6000′: temps are forecasted in the range of 14-23 degrees F with winds 20-40 mph

WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS

3800′-Sunburst Wx Station

Current Temp: 21 (3 degrees colder than yesterday morning)

Winds: Are increasing this morning. In last 24 hours winds have been light to moderate averaging 2-22mph with an extreme max gust of 40mph

HIGHEST TEMP: 28 @ 5pm

2600′-Seattle Ridge Wx Station

Current Temp: 25 (1 degree warmer than yesterday morning)

Winds: Are increasing this morning. In last 24 hours winds have been light to moderate averaging 3-24mph with a strong max gust of 38mph

HIGHEST TEMP: 29 @ steady between 1-6pm

1800′-Center Ridge Wx Station

Current Temp: 28 (6 degrees warmer than yesterday morning)

Precip: 0 new precip. 1” of melting/settlement

HIGHEST TEMP: 42 @ 1pm

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Friday March 19, 2010.

Thu, March 18th, 2010
Alpine
Above 2,500'
2 - Moderate
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
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This is 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.