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| 12.30.2008 |
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This is Blase Reardon of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Tuesday, December 30, 2008 at 7:30 am.
The Wattis Dumke Foundation & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Special Announcement:
The Beacon Training Park is up and operating in the field next to the Catholic Church on Sun Valley Road. Users need to bring a beacon and probe as well as snowshoes or skis to get around.
Bottom Line:
Today the avalanche danger is estimated to be HIGH in the north valley, Salmon Headwaters, and Sawtooth Mountains. Strong, gusty winds have added yet more load to slopes already unstable from recent snow on top of a weak base.
Today the avalanche danger is estimated to be CONSIDERABLE on upper elevation slopes in the south and central valley. Strong, gusty winds in the past 24 hours have created windslabs on slopes above 8000 feet. Below that elevation, the avalanche danger is estimated to be MODERATE.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
The primary avalanche concern in the north valley, Salmon headwaters, and Sawtooth mountains remains deep slab avalanches. The snowpack in these areas consists of a cohesive slab perched on a weak base – the ideal structure for deep slab avalanches. Settlement and yesterday’s warm temperatures have consolidated the storm snow from the past two weeks into a relatively homogenous slab two to four feet thick. On most aspects above about 8000 feet, this slab sits on a layer of weak, sugary snow; above about 9000 feet, there are one or two decomposing crusts near the top of the weak snow.
In the past two days there’s been plenty of evidence that this relatively uniform structure is unstable. Small to medium sized avalanches have occurred naturally, and on steep slopes that haven’t slid, it seems any additional load – a person’s weight, wind-transported snow, and the debris from a snowplow – is enough to trigger an avalanche (see photos). Skiers are reporting relatively widespread collapsing of the snowpack, with cracks propagating a hundred feet or more in some cases. Yesterday on Titus Ridge, I triggered two avalanches with one collapse (see photos), and the crowns from these and the natural avalanches were two to three feet deep.
The weather in the past 24-hours - 0.5” of precipitation, strong, gusty winds, rising temperatures - has only thickened the slab and increased the load on the weak layer at the base of the snowpack. The collapsing snowpack means it is possible to trigger a slide on a slope well above you. Today's break in the weather will not be enough to stabilize the snowpack. These are very dangerous avalanche conditions, and backcountry travel on or below steep slopes is not recommended.
Secondary Avalanche Concern:
A secondary avalanche concern is windslabs formed by the strong, gusty winds of the past 24 hours. On many slopes in the central and south valley, these slabs sit on top of lighter density, weaker storm snow. In the northern part of the forecast area and on shady slopes in the central valley with weak, faceted snow at the base, these wind slabs are contributing to the deep slab instability. The slabs will have a chalky, hollow feel and be most common near ridgelines. The high wind speeds measured in mountain basins and valleys indicate you may also find wind slabs on mid slope features as well. Though they may feel supportable, the wind slabs will be sensitive to a person’s weight today and human-triggered avalanches are possible.
Current Conditions:
If this were the evening news, there would be three headline stories – high winds, rising temperatures, and continued precipitation. Wind speeds at ridgeline weather stations increased yesterday afternoon and early evening to hourly averages of 25-45 mph, with the station near Soldier Mountain reporting a 24-hour average of 60 mph. Mountain basins and valleys haven’t been spared the wind; Titus Lake saw gusts over 30 mph, and Stanley reported gusts from 30-60 mph. The winds veered from southwest through west to northwest and have calmed a little; ridgeline wind speeds this morning have dropped to 15-20 mph except near Soldier Mountain.
The second headline story was rising temperatures. Yesterday’s highs were the warmest in the past two weeks, with valley temperatures well above freezing and ridgeline highs in the upper twenties. Clear skies overnight have allowed temperatures to drop back to the teens and single digits.
Lastly, most SNOTEL stations and weather stations reported over 0.5” of precipitation yesterday. That translates to roughly 4-6” of new snow, but because of settlement the automated stations aren’t measuring the new snow depths accurately. At Soldier Mountain and in the south and central valley, some of this precipitation fell as rain. The rain extended to around Chocolate Gulch or the SNRA.
The warming has improved backcountry travel conditions a little. Trailbreaking isn’t as epic as it has been, and snowmachiners aren’t sinking in as far or as often. Nonetheless, it’s still difficult to get around in the backcountry, especially on safer lower-angled slopes.
Mountain Weather Forecast:
Our area will see a brief break in the weather before several approaching storms bring more snow. Wind speeds will diminish during the day and back to the the southwest. Temperatures will be cooler than yesterday, with ridgeline highs in the upper teens and valley highs approaching freezing. There's enough moisture in the atmosphere for mostly cloudy skies and a possibly a few flurries. More snowfall is forecast starting later tonight, though the northwest flow doesn't favor high amounts. A second, warmer storm is forecast for Thursday night.
Photos:
1. Cracking, Titus Ridge 12-29-08
2. Remotely triggered avalanche, 12-29-08
3. crown of remotely-triggered slide, 12-29-08
4. Roadslide slide
5. 2 remotely-triggered slides on Titus Ridge.
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| Reported Conditions |
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| Temperatures |
| Note: Yesterday's highs were the warmest in the past two weeks. |
| Overnight Low |
2 |
10 |
17 |
| 6am Temperature |
2 |
11 |
17 |
| 24 hr Maximum |
24 |
27 |
38 |
| Winds |
| Note: Winds speeds were the highest in the past two weeks. |
| Current Winds |
20 NW |
16 NW |
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| 24 hr Average |
22 WNW |
17 SW |
- |
| Maximum Gust |
78 WNW |
47 WSW |
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| Snow - Storm Interval # 8 |
| Note: Settlement means new snow depth is underestimated. |
| New Snow |
5+" |
6" |
4" |
| Storm Total |
5+" |
6" |
5" |
| Total Depth |
49" |
45" |
29" |
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