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| 12.23.2008 |
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This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Tuesday, December 23, 2008 at 7:30 am.
The Wattis Dumke Foundation & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
Today the avalanche danger is estimated to be CONSIDERABLE. Where there is deep enough snow to ski or ride, the snowpack consists of sugary facets buried 1.5 to 2 feet deep. Sunday night’s storm added significant weight to a snowpack with very little strength, resulting in unstable conditions on many mid to upper elevation slopes. We observed significant signs of instability yesterday, including triggered avalanches and collapsing and cracking of the snowpack, and I would expect these to continue today.
Below: This season we have begun using danger roses to describe the avalanche danger for our North Valley and South/Central Valley regions. Once you get a hang of interpreting them, they provide a quick, easy way to see how the danger relates to aspect and elevation. For a quick lesson in their interpretation, mouse over the question mark icon.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
While yesterday’s storm wasn’t enough to fully tip the balance of our weak snowpack, it did make for some spicy conditions in the backcountry. By stomping around near the top of Timber Bowl on the backside of Titus Ridge, we released a small slab about 40 feet wide and breaking about 1.5 feet deep. On the other side of the ridge above Titus Lake, we triggered another, larger avalanche – this time about 100 feet wide and 1-2 feet deep. It initiated in steep rocks near the ridgeline, but propagated into lower slope angles. (See photos below.) A group near the Cross remotely triggered a small slide from a good distance away, received numerous collapses, and reported poor stability test scores.
The cause of the excitement is a scenario we are all too familiar with in this area – a slab sitting atop very weak faceted snow above the ground. The slab in this case is 1.5 to 2 feet of snow that’s fallen over the past two weeks, and the weak layer is loose, cohesionless facets beneath the slab. Yesterday’s conditions illustrated that snow doesn’t have to be very dense or cohesive to be a slab; in this case it was very fluffy powder. It just has to have more strength than the underlying weak layer, which I could practically blow out of my snowpit like a bunch of pixie dust. This unstable snowpack is most prevalent on mid to upper elevation, shady aspects that had pre-existing snow prior to the recent storms.
Yesterday we stuck to lower angle terrain, and I think it would be wise to do the same today. Watch for the classic signs of instability – which you will likely see – such as collapsing and cracking of the snowpack, and of course, natural or triggered avalanches.
Additional Discussion: Northwest winds increased a bit yesterday afternoon and overnight, especially in the high peaks of the Solider Mountains. With plenty of low density snow available for transport, watch for freshly formed wind pillows today on the lee side of ridges and gullies. Any wind loaded slope will be especially dangerous and should be avoided.
Current Conditions:
We picked up another 2 inches of new snow during the day yesterday, bringing storm totals up to about a foot in most locations. However, Soldier Mountain reported 18 inches from the storm, and it looks like the northern Sawtooths received more as well.
Northwest winds increased yesterday afternoon, and averaged 15-25 mph through the evening. The high peaks of the Soldier Mountains received even stronger winds. Ridgeline winds have moderated this morning and are currently averaging 5-10 mph. Mountain temperatures are currently in the single digits above or below zero at all elevations.
Mountain Weather Forecast:
The low pressure trough that brought yesterday’s storm has shifted to the east and will leave fairly benign, albeit chilly, weather in its wake. Skies should be partly to mostly cloudy today, and mountain temperatures should reach the single digits to low teens. A high in the low 20s is expected in the valley. Ridgeline winds are forecasted to blow 5-10 mph from the northwest.
We should see some clearing tonight and tomorrow before the next significant storm arrives on Christmas Eve. I hope you’ve all been good girls and boys!
Photos:
1. Crack on Titus Ridge
2. Triggered avalanche on Titus Ridge
3. Triggered avalanche in Timber Bowl
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
-2 |
6 |
1 |
| 6am Temperature |
-2 |
6 |
1 |
| 24 hr Maximum |
14 |
16 |
27 |
| Winds |
| Current Winds |
4 NW |
9 NW |
- |
| 24 hr Average |
8 W |
12 W |
- |
| Maximum Gust |
25 NW |
35 NW |
- |
| Snow - Storm # 5 |
| New Snow |
2" |
2" |
2" |
| Storm Total |
12" |
10" |
11" |
| Total Depth |
32" |
27" |
16" |
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