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| 12.24.2008 |
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This is Blase Reardon of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Wednesday, December 24, 2008 at 7:30 am.
Blaine County Search and Rescue & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
Today the avalanche danger is estimated to be CONSIDERABLE in the north valley. The snowpack is weak and can barely support recent snowfall. The load of a person may be just enough to trigger a slide on steeper slopes.
In the south and central valleys today, the avalanche danger is estimated to be LOW on most slopes but MODERATE on upper elevation slopes where a deeper snowpack exists.
The avalanche hazard will increase this afternoon and tonight as an approaching storm adds more load to the snowpack.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
North Valley: For the past two days, the signs of instability have been obvious and unambiguous. Backcountry parties near Galena Summit are reporting widespread collapsing and short shooting cracks, poor stability test scores, and several small, human-triggered avalanches. Several natural avalanches were reported yesterday in the north Boulder Mountains; these ran at the end of Monday's storm on upper elevation slopes and involved wind-deposited snow. More distant but perhaps more ominous are the reports of an extensive natural avalanche cycle west of our area that occurred during the last storm.
Stick your hand in the snow and the reason for the instability is clear. The one to two feet of recent snow near the surface isn’t very cohesive, but the faceted snow below it is so weak it can barely support even that small load. On slopes steeper than about 30 degrees, particularly on shadier northwest through northeast aspects, the weight of a person may be enough additional load to trigger an avalanche. Pockets of wind slab exist on the surface on open, upper elevation slopes, and these will be the most sensitive to human triggering.
This weak, unstable snow structure isn’t going away soon. In fact, it’s only going to get worse in the short term. The avalanche hazard will increase this afternoon and tonight as a winter storm passes over our area. Gusty southerly winds this afternoon may begin transporting the light density snow currently at the surface. We expect the winds to continue overnight as the storm drops up to a foot of new snow.
Secondary Avalanche Concern:
South and Central Valley: A secondary avalanche concern is small wind slabs in the south and central valleys. These developed when northerly winds moved around the recent light density snow. The wind has cross-loaded steeper gullies on easterly slopes and left drifts and small cornices near many ridgelines, even at lower elevations (see photo). On upper elevation slopes where a deeper snowpack exists, these wind features may be overlying older, weak snow, and isolated, unstable wind slabs may exist. Hikers in particular may encounter these conditions on trails near ridge crests.
Current Conditions:
This morning winds are light and temperatures in the valleys are a little colder than at ridgelines as Santa packs up the goods for tonight's deliveries. Skiing the past few days has required fat skis to stay on top. Off-trail conditions are still thin and variable for snowmobiles, especially in valley bottoms.
Mountain Weather Forecast:
A storm is currently approaching our area from the west. Temperatures, wind speeds and cloud cover will increase this afternoon and evening. Ridgeline temperatures will climb slowly into the teens during the day. Winds will back from southwest to south, and wind speeds will average from 10 to 20 mph, with some stronger gusts. Though snow will start this afternoon, the bulk of the storm will occur overnight. I expect from .5 to .75 inches of water and 8 to 12” of snow by tomorrow morning.
Photos:
1. South Valley Wind Effects
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
-2 |
3 |
4 |
| 6am Temperature |
0 |
5 |
-2 |
| 24 hr Maximum |
6 |
14 |
20 |
| Winds |
| Current Winds |
11 SW |
4 W |
- |
| 24 hr Average |
9 WNW |
7 NW |
- |
| Maximum Gust |
26 NW |
26 NW |
- |
| Snow - Storm Interval # 5 |
| New Snow |
0" |
0" |
0" |
| Storm Total |
0" |
0" |
0" |
| Total Depth |
30" |
26" |
12" |
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