Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Saturday, February 5th 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 MODERATE with pockets of CONSIDERABLE. Large natural avalanches happened during and after the recent storm. Persistent instabilities will make deep avalanches possible to trigger today.
The avalanches we saw yesterday were exceptional for a number of reasons and really caught our attention. We believe in hindsight that yesterday warranted at least pockets of CONSIDERABLE danger rating and that extends into today. In specific areas human triggered avalanches will be LIKELY today.
The warm sunny weather may cause some undeserved optimism about stability this weekend. Conditions are NOT NORMAL right now. In a normal Turnagain pass year we can safely travel in steeper backcountry terrain by waiting 48 hours after a storm. This is proving to be a wrong way to approach the snowpack this year. Persistent instabilities are causing deep avalanches during each storm and remaining reactive long after the storm ends.
A couple of human triggered avalanches were reported yesterday in the Turnagain Pass region. One was visible on the Seattle ridge webcam and occurred between 1:30 and 2:30 in the afternoon. This was possibly remotely triggered by a snowmachine and was estimated at 8 feet deep. AnCNFAIC Staff reported slide was also on Seattle ridge in Triangle Bowl, 4-5 feet deep.
Natural activity was sparse in Turnagain pass, but many large avalanches happened on the southern end around Johnson Pass, Lynx creek, and Silvertip creek as well as Summit. Some of these events showed extensive and complex propagation. The thinner snowpack in this region seems to be the most likely reason for the instabilities. Expect dangerous avalanche conditions in these zones with remote triggering, upslope propagation, and abnormal distribution. Upslope propagation means that you could trigger the slope above you from a lower angle area below.
This avalanche is still visible today from the Johnson Pass parking lot. It is located between Lynx creek and Silvertip creek. The width is estimated at 2000 feet or more, and the propagation extended across complex steep ridges that normally isolate avalanches to one side or the CNFAIC Staff. See photo gallery for more examples from yesterday.
CNFAIC Staff more remote regions including Winner Creek, Bench Creek, and 20-mile also showed some very large avalanches during and after the storm. The avalanches we’ve seen have not been widespread enough to clean out entire zones. This winter is producing an uneven patchwork of areas that have slid. The spotty distribution is one of the more disturbing characters of the recent avalanche cycles, which could mean areas that didn’t slide naturally are teetering on the balance and waiting for the right trigger. We got reports of hair-trigger instabilities in the Chugach Front Range and Eagle river also.
Likely trigger points through Turnagain pass are shallow areas near rock outcrops (which could be slightly buried and not visible). In the shallower snowpack on the southern end of Turnagain to Summit, Snug Harbor, and Carter Lake the likelihood of triggering an avalanche will be higher. Avoiding complex steep terrain is a prudent decision this weekend, backed up by quite a bit of evidence.
The primary instability right now is deep slab on top of persistent weak layers. We haven’t been able to do a crown profile since the most recent storm, but all the characteristics incriminate the old facets and buried surface hoar, which formed during our two extended high-pressure events in December and January. Snowpit tests could easily show false stable results and inspire unjustified confidence in the snowpack. The distribution of the weak layers and the depth of the slab on top is too variable to make confident and accurate evaluations.
Encyclopedia of avalanche terms.
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The recent storm finished 48 hours ago. Total storm accumulation ranged from only 10-15 inches snow in upper elevations in Summit to over 50 at upper elevations in Girdwood. Turnagain pass was middle range with about 29 inches up high. Rain to 2000 feet saturated the lower elevations and is evident with a crust below treeline. Wind during the storm trended from the East with gusts into the 60s at the ridge-tops. Wind distribution from the storm is evident through the region. Temperatures have dropped since the storm started diminishing and remain below freezing but moderate. A few inches of colder snow fell at the tail end of the storm. Today clear skies, moderate temperatures, and light wind are expected. A chance of snow showers starts tonight but little precip is expected at this time.
Wendy 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 SAT FEB 5 2011
.TODAY…MOSTLY SUNNY. HIGHS IN THE LOWER 20S TO LOWER 30S…
WARMEST ALONG THE COAST. NORTH TO EAST WIND TO 15 MPH.
.TONIGHT…PARTLY CLOUDY. A CHANCE OF SNOW SHOWERS ALONG THE
GULF COAST. LOWS 10 TO 20 ABOVE INLAND AND LOWER TO MID 20S
ALONG THE COAST. LIGHT WINDS EXCEPT NORTH WIND 10 TO 20 MPH
.SUNDAY…MOSTLY CLOUDY. A CHANCE OF SNOW SHOWERS IN THE MORNING
ALONG THE GULF COAST. HIGHS IN THE LOWER 20S TO LOWER 30S…
WARMEST ALONG THE COAST. LIGHT WINDS EXCEPT NORTH WIND
10 TO 15 MPH NEAR SEWARD.
.SUNDAY NIGHT…MOSTLY CLOUDY. LOWS 5 TO 15 ABOVE INLAND AND
MID 20S ALONG THE COAST. LIGHT WINDS.
.MONDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW SHOWERS. HIGHS
IN THE LOWER 20S TO LOWER 30S…WARMEST ALONG THE COAST.
.MONDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF SNOW. LOWS IN
.TUESDAY THROUGH WEDNESDAY…SNOW AND RAIN LIKELY. HIGHS IN
THE 30S. LOWS IN THE 20S.
.WEDNESDAY NIGHT…SNOW LIKELY. LOWS IN THE 20S.
.THURSDAY AND THURSDAY NIGHT…MOSTLY CLOUDY WITH A CHANCE OF
SNOW SHOWERS. HIGHS 25 TO 35. LOWS IN THE TEENS.
.FRIDAY…MOSTLY CLOUDY WITH A CHANCE OF SNOW SHOWERS. HIGHS IN
TEMPERATURE / PRECIPITATION
SEWARD 31 22 32 / 0 30 30
GIRDWOOD 25 16 25 / 0 0 0
WEATHER STATION SUMMARY for Turnagain Pass:
-3800′ Sunburst Wx Station-
Temperature 20, on a rising trend. Wind is picking up slightly, steady at 11-15mph currently.
-2600′ Seattle Ridge Wx Station-
Temperature 17 degrees. Recent wind gust to 27 in the last hour.
-1800′ Center Ridge Wx Station-
Temperature 18 degrees. One inch of snow settlement in the last 24 hours.