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| 3.23.2008 |
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This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Sunday, March 23, 2008 at 7:30 am.
The Twin Falls District Bureau of Land Management & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Special Announcement:
We plan to pull the Avalanche Rescue Training Park tomorrow or Tuesday. Today may be your last chance to get out for some beacon practice!
Bottom Line:
In our northern mountains, the avalanche danger is estimated to be MODERATE. The exception is on upper elevation, easterly aspects in the North Valley where the avalanche danger is estimated to be CONSIDERABLE. Facets sitting atop a crust buried 12-16 inches deep are showing clear signs of instability and are capable of producing human-triggered slides.
In the South and Central Valley, the avalanche danger is estimated to be LOW. Much less recent snow in this region means fewer stability concerns.
The danger of wet snow avalanches will increase today with daytime heating, especially in our northern mountains where more recent snow sits atop old melt-freeze crusts.
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Primary Avalanche Concern:
Over the past week or so, our northern mountains have picked up 1-2 feet of new snow, with the trend being more snowfall the farther north you go. The mountains closer to Ketchum have received significantly less. Since the new snow has fallen bit by bit, the snowpack has generally handled the load well. However there is one very important exception.
In the North Valley, upper elevation aspects ranging from northeast through east through southeast have a layer of facets above a crust buried 12-16 inches deep. On due north aspects, there is no crust present and there is no defined weak layer beneath the recent snow. On more southerly aspects, the sun has glued the newer snow onto the old melt-freeze crust. I was in Grand Prize Gulch north of Mushroom Ridge yesterday, and found this layer in a pit on an east-northeast aspect. It produced very easy and clean shears during stability tests (see video, http://www.youtube.com/watch?v=eX8J2O3UiYg ) and when I skied away from my pit I received a collapse with cracks propagating out 50 feet (see photo below). Janet found this layer around Galena Summit, and even triggered a slide on Friday on Titus Ridge. Observers in the Boulder Mountains yesterday reported poor stability on east aspects as well.
A few reports from the Sawtooths seem to indicate that this layer is not present there, but we don’t have any information from the Salmon Headwaters region. In the North Valley, this instability is limited to a very specific aspect range, but any steep, easterly facing slope should be treated with caution. If you feel even a thin crust 1-2 feet deep, you should carefully evaluate the snowpack before exposing yourself to a steep slope.
Secondary Avalanche Concern:
In our northern mountains, shady slopes continue to be very “sluffy” with loose snow avalanches easily released in steep terrain. These are generally small in size but something to be aware of, especially in committing terrain or above terrain traps. As the day heats up, loose snow avalanches of the wet variety will become likely on sunny aspects, especially further north where recent snow accumulations sit atop old melt-freeze crusts.
Current Conditions:
Sunny skies prevailed yesterday but temperatures didn’t get too out of hand. Highs in the mountains reached the mid 20s to low 30s, and ridgeline winds were light. A temperature inversion has set up overnight, with mountain temperatures currently hovering around 20 degrees and its in the teens down in the valley.
Mountain Weather Forecast:
The ridge of high pressure is passing overhead this morning and high clouds are expected to increase through the day. A minor system will enter the region tonight, but I expect less than an inch or two of snow by the morning. A cold front is forecasted to pass through tomorrow morning, bringing cooler temperatures and increasing wind. Today, you can expect mountain temperatures to reach the low 30s, with valley temps in the mid to upper 30s. Ridgeline winds will increase a bit, averaging 5-15 mph from the southwest.
Photos:
1. Collapsing and cracking in Grand Prize
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
18 |
22 |
14 |
| 6am Temperature |
20 |
22 |
14 |
| 24 hr Maximum |
26 |
24 |
33 |
| Winds |
| Current Winds |
8 S |
10 SW |
- |
| 24 hr Average |
7 W |
4 W |
- |
| Maximum Gust |
15 W |
12 SW |
- |
| Snow - Storm Interval # 17 |
| Total Depth |
72" |
74" |
37" |
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