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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 30th, 2010 - 7:00AM
Expires
Wed, March 31st, 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 Tuesday March 30th, 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 have been averaging light in the 2-10mph range at all ridge top wx stations with strong gusts up to 31mph.

-In the last 24 hours (5am-5am), the snotel sites recorded:

0.2 inches of water with minus 5” of snow due to melting/settlement at Turnagain Pass

0.4 inches of water 2 inches new snow then minus 5” of snow due to melting/settlement at Grandview

0.0 inches of water and no change to total snowpack depth Summit Creek.

-The current (5am) radars show scattered precip over Prince William Sound Kenai Peninsula

-Temps have increased slightly at all weather stations this morning by 1-5 degrees compared to yesterday. Temps at 5am range from 34 degrees F at sea-level and 25 degrees F at 3800′. Temps made it above freezing up to 3300′ yesterday. The wx stations recorded the warmest temps between 1-4pm. That is the same time frame that natural avalanches occured.

AVALANCHE DISCUSSION

Today’s avalanche danger will remain at CONSIDERABLE, due a heavy slab on top of a variety of crusts and weak layers of buried surface hoar. Terrain management is the key right now. Keep your slope angles below 35 degrees. AVOID traveling on any sunny slopes IF the sun comes out (the sun is not forecasted today, but this is the critical “watch out” situation with our current snowpack)

The new snow that fell on Saturday has become a slab. This slab is sitting on multiple crusts and weak layers of buried surface hoar on all aspects. It has been showing signs of natural and human-triggered activity in the past 24-48 hours:

-Large natural avalanches occurred yesterday 3/29/2010 due to the intense sun and warm temps. An avalanche on Sunnyside across from the motorized parking lot at Turnagain Pass was clearly big enough to injure, bury or kill a person.

-I was able to easily trigger a medium sized slab avalanche that failed on a crust on a steep rollover on Tincan 2 days ago on 3/28/2010. If this slab would have occurred on bigger terrain it would have been big enough to injure, bury or kill a person.

-Several natural avalanches occurred 3/27 or 3/28/2010 on steep slopes like Tincan Proper and along the Library on Tincan Ridge

Before this new snow formed the current heavy slab, the snowpack was already having problems with the weak layers that we are worried about. Now there is a significantly bigger slab sitting on top of these weak layers. Recent stability tests have been finding clean shears below this new slab, but it seems to require very steep slopes to trigger this slab.

Recent avalanches include:

– North aspect of Granddaddy Chute (Bertha Creek drainage), 2.5 feet deep, 150-200 feet wide (probably buried surface hoar).

-North aspect of Magnum at Taylor Pass between Sunburst and Magnum, triggered while skinning, 3400 feet elevation, 80-90 feet wide, 1.5-2 feet deep, ran 300 feet on layer of (buried surface hoar).

-South face of Sunburst at highest point, triggered while skiing on convex rollover, 6-9 inch deep crown face, ran 3/4 of the way to the bottom, happened at 12:30pm (failed on suncrust).

– North face of Sunburst near the top of Elevator Shaft. The skier triggered it on his third jump turn from the top. It broke 2-3 feet deep and ran 1000 feet (probably buried surface hoar).

-South face of A1 in the Upper Girdwood Valley between 4000 and 5000 feet.

In summary appears that Northern aspects are holding layers of buried surface hoar and southern aspects have a crust. Either way, we have a dense heavy slab on top of these weak layers and avalanches have occurred as recently as yesterday afternoon.

Glide cracks are secondary concern today. These crevasse like features are those cracks that go all the way to the ground. Avoid traveling underneath these cracks.

WEATHER FORECAST (National Weather Service)

WESTERN PRINCE WILLIAM SOUND-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKDT TUE MAR 30 2010

…STRONG WIND WEDNESDAY MORNING THROUGH EARLY THURSDAY MORNING

THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…

.TODAY…CLOUDY WITH SCATTERED RAIN AND SNOW SHOWERS IN THE MORNING.

HIGHS IN THE LOWER TO MID 40S. LIGHT WINDS EXCEPT NORTH 10 TO 15 MPH

NEAR SEWARD.

.TONIGHT…SNOW SHOWERS DEVELOPING AFTER MIDNIGHT…MAINLY ALONG THE

COAST AND NORTH OF MOOSE PASS. SNOW ACCUMULATION UP TO 2 INCHES. LOWS

IN THE MID 20S TO MID 30S. LIGHT WINDS. THROUGH PORTAGE VALLEY AND

TURNAGAIN ARM…LIGHT WINDS BECOMING EAST 10 TO 20 MPH AFTER

MIDNIGHT.

.WEDNESDAY…SNOW SHOWERS…MIXED WITH RAIN AT SEA LEVEL. SNOW

ACCUMULATION UP TO 2 INCHES. HIGHS IN THE UPPER 30S. EAST WIND 10 TO

15 MPH. THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND

20 TO 35 MPH INCREASING TO 35 TO 50 MPH BY NOON.

TEMPERATURE / PRECIPITATION

SEWARD 44 32 44 / 40 60 80

GIRDWOOD 43 26 43 / 30 60 70

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

Sea-level: temps are forecasted between 26-43 and between 0.25-0.5” of water forecasted

3000′: temps are forecasted in the range of 23-32 degrees F with winds 5-10 mph

6000′: temps are forecasted in the range of 14-23 degrees F with winds 5-10 mph

WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS

3800′-Sunburst Wx Station

Temp (5am): 25 (3 degrees warmer than yesterday morning)

Winds: In last 24 hours winds have been light averaging 8-14mph with a moderate max gust of 27mph

HIGH TEMP 28 between 2-3pm

2600′-Seattle Ridge Wx Station

Temp (5am): 28 (3 degrees warmer than yesterday morning)

Winds: In last 24 hours winds have been light averaging 2-17 mph with a moderate max gust of 24mph

HIGH TEMP 40 at 2pm

1800′-Center Ridge Wx Station

Temp (5am): 31 (3 degrees warmer than yesterday morning)

Precip: 0.2 new water and 5” snow melting/settlement

HIGH TEMP 42 at 2pm

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Wednesday March 31st, 2010.

Tue, March 30th, 2010
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. 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.