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| 3.9.2009 |
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This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Monday, March 9, 2009 at 7:30 am.
City of Ketchum (www.ketchumfire.org) & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
In the North Valley, Salmon Headwaters, and Sawtooth Mountains, areas of CONSIDERABLE danger exist on slopes steeper than about 35 degrees and on recently windloaded slopes. Recent storms and wind transported snow have continued to load facet layers from mid-February that are now buried 2-3 feet deep. Recent natural and triggered slides illustrate that on some slopes the snowpack remains unstable. Steep, rocky, and shallow areas are the most dangerous, and the best course of action is to avoid slopes steeper than about 35 degrees.
In the South and Central Valley, the avalanche danger is estimated to be MODERATE at mid to upper elevations. This area has not received as intense recent loading, but a facet layer buried 1-2 feet remains a concern. Use extra caution on recently windloaded slopes where wind slabs may be poorly bonded to weak faceted snow beneath.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
Storms since mid-February have buried weak facet layers throughout our area. The northern mountains have seen the most intense loading, and this layer is now buried 2-3 feet deep in this region. In the South and Central Valley, this layer can be found 1-2 feet down at mid to upper elevations – at lower elevations, rain has wiped out the layering.
The first significant loading event at the end of February caused a widespread avalanche cycle in our northern mountains. Because the weak layer had a chance to strengthen a bit, the second loading event that took place last week didn’t cause widespread natural avalanching, and has lead to a more stubborn and insidious instability resulting in several triggered slides.
Two human-triggered avalanches occurred near Galena Summit on Friday, new natural slides were observed on Saturday, and yesterday a snowmobiler in the Salmon Headwaters triggered an avalanche from the flats nearly 1000 feet away. This slide occurred on an upper elevation westerly aspect, broke 2-3 feet deep and 100 feet wide. Skiers on Durrance also remotely collapsed a slope that cracked but wasn’t quite steep enough to slide.
In areas where the snowpack is deeper, the Friday the 13th facet layer appears to be strengthening. Unfortunately, this has lead to a false sense of security since avalanches triggered in weaker, shallower areas can easily propagate into the stronger snow. These are dangerous conditions because you may not see signs of instability, and snowpits may give you the impression that conditions are stable. But if you happen to find the self-destruct button on a given slope, you could trigger an avalanche. Most dangerous are slopes that match the recent pattern – steeper than 35 degrees, rocky, with shallow areas harboring weaker snow. The best approach is to ski or ride conservatively, and personally I think it’s wise to avoid terrain steeper than about 35 degrees.
In Lake Creek yesterday, I found the Friday the 13th facet layer buried about 1.5 deep at mid to upper elevations. While it didn’t seem particularly sensitive, I was still able to get relatively clean and propagating shears during stability tests. With less loading over the past several weeks, the South and Central Valley is not as suspect as further north, but a buried weak layer does exist and steep slopes should be evaluated carefully.
Secondary Avalanche Concern:
Strong westerly winds continued to blow through the day yesterday, averaging 20-30 mph at upper elevations and gusting to much higher speeds. The winds were not confined to upper elevation ridgelines and blew clear down to the valley floor. Observations from yesterday indicate that snow was being transported at all elevations, loading leeward ridgelines and cross loading mid-slope ridges, ribs, and gullies. The redistribution of snow likely added stress to buried weak layers, and may have been responsible for yesterday’s remotely triggered slide.
Avoid slopes with signs of recent wind loading today, as fresh wind slabs may be poorly bonded to lower density snow beneath, or may be stressing more deeply buried weak layers.
Current Conditions:
Apparently the wind did not slow down long enough yesterday to check what the computer models were predicting. Defying forecasts, westerly winds continued to average 20-30 mph yesterday on Titus Ridge, with gusts into the 40s. In the higher peaks of the Soldier Mountains, winds averaged 25-45 mph gusting into the 60s. Snow transport was visible at all elevations during the day. The wind finally eased yesterday afternoon, and has tapered off to around 5 mph this morning although it remains breezy on the valley floor.
Mountain temperatures remained cool yesterday, hitting only the mid teens to around 20 degrees. Overnight, temps have continued to drop and this morning it’s in the single digits – positive or negative – at all elevations.
Mountain Weather Forecast:
A low pressure trough remains parked over the Pacific Northwest, and a minor wave of energy from this system will bring partly to mostly cloudy skies today along with a chance of snow. Temperatures will remain below seasonal values, with mountain highs only forecasted to reach the mid teens. The valley should reach the upper 20s. Ridgeline winds should (repeat, should) blow lightly at 5-15 mph from the west to northwest.
Photos:
1. Titus slide from Friday
2. Natural on Galena Peak
3. Gladiator accident overview
4. Crown of Gladiator Accident
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
-2 |
6 |
3 |
| 6am Temperature |
-1 |
6 |
3 |
| 24 hr Maximum |
8 |
15 |
33 |
| Winds |
| Current Winds |
6 WSW |
7 SW |
- |
| 24 hr Average |
16 WNW |
9 W |
- |
| Maximum Gust |
51 WNW |
29 WNW |
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| Snow - Storm Interval # 22 |
| Total Depth |
66" |
57" |
28" |
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