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| 3.19.2009 |
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This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Thursday, March 19, 2009 at 7:30 am.
Idaho Department of Parks & Recreation in partnership with Idaho's Snowmobile License Plate Program & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
At mid to upper elevations in the South and Central Valley, the avalanche danger is estimated to be CONSIDERABLE. In our northern mountains, the avalanche danger is estimated to be MODERATE. The greatest concern are buried persistent weak layers from mid-February that are struggling to deal with the added stress of recent snowfall, wind, and warming temperatures. Use caution on steep slopes, especially those that may be weak, rocky, or shallow. Also avoid wind loaded areas, where wind slabs may remain sensitive or are overloading buried weak layers.
Wet Snow Stability: With a good freeze overnight, the possibility of wet snow slides will increase with mid-day warming temperatures and sunshine.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
Over the past few days, we’ve talked about several different types of avalanche problems. The snowfall from Sunday and Monday was inverted (meaning heavier snow over lighter snow), and combined with significant wind loading, produced unstable conditions and numerous natural and triggered avalanches. Warm temperatures and rain at lower elevations resulted in wet snow conditions. And all of this atop the old mid-February weak layers.
Instabilities within the recent storm snow are healing, and unless they are overloading older facet layers, wind slabs seem to be less sensitive. So at this point, it’s time to turn our focus back to the Friday the 13th facet layers. Like so many bad horror movies, this one seems to have too many sequels.
In the past weeks, we have been most concerned about our northern region because it had been receiving nearly twice the snowfall of the mountains closer to town. The mid-February weak layers are buried around 3 feet deep in the North Valley, and 4 or more feet deep in the Salmon Headwaters and Sawtooths. Especially with today’s warming temperatures, these areas remain a concern, but as the facets become more deeply buried, triggering becomes less likely. The trouble is if you do trigger a slide from a shallow or weak spot, the slide could be a monster.
This past storm has helped the South and Central Valley to catch up, and now I believe this area is our main concern. On mid to upper elevation shady aspects, the facet layer is now buried 2-3 feet down – deep enough to have a significant load above it, but shallow enough for easier triggering. Yesterday a cornice that broke from the ridge dividing Trail Creek and Lake Creek rolled down the slope and triggered a small avalanche on a wind loaded roll. This slide sympathetically triggered a larger slide that zippered a long distance down a sub-ridge into Trail Creek. Two other avalanches released at the same time, one over a half mile away. These avalanches likely ran on surface hoar or faceted snow that was buried in mid-February. A slab avalanche was reported in the Mine Bender chutes out the East Fork, and I saw a recent natural slide that stepped down into older weak layers on the dividing ridge between Murdock and Eagle Creeks.
The storm from earlier in the week reloaded a weak layer that has been active in the past, and warming temperatures in the forecast today will exacerbate the problem. While I think the possibility of triggering a slide is the greatest in the South and Central Valley, a conservative approach is warranted on any slope steeper than 35 degrees. Especially as the weak layer becomes more deeply buried, remember that likely trigger points include steep, rocky terrain or slopes with variability in depth.
Secondary Avalanche Concern:
We received a good freeze overnight, but as temperatures heat up today the wet slide danger will increase on sunny aspects. On lower to mid elevation slopes, this will involve the softening of the surface crust; once this becomes weak, punchy, and unsupportable it’s time to go somewhere else. At higher elevations, warm temps will turn the recent storm snow to Idaho mashed potatoes and increase the likelihood of wet snow avalanches.
Current Conditions:
Yesterday produced some very warm temperatures with many mountain stations reporting highs in the mid to upper 30s. Ridgeline winds remained light from a westerly direction. Overnight, temperatures have dipped into the mid to upper 20s at most elevations.
If you want my recommendation for good ski conditions, I recommend harvesting corn on the lower elevations slopes around town. Yesterday on Durrance, these were the best turns we found. Very shady, north-facing slopes at upper elevations may still have decent powder, but even some northeast aspects had heavy, dense snow yesterday. Snowmobilers may not have to be as picky to find enjoyable snow.
Mountain Weather Forecast:
A southwesterly flow is pushing warm temperatures into the region, and today will likely be a bit warmer than yesterday. We may see some increasing high clouds today, but they shouldn’t provide much shelter from the sunshine. Mountain temperatures are forecast to reach the mid 30s to near 40 degrees, and a high near 50 is expected in the valley. Ridgeline winds should remain fairly light from the southwest.
Photos:
1. Trail Creek Avalanche 1
2. Trail Creek Avalanche 2
3. Trail Creek Avalanche 3
4. Trail Creek Avalanche 4
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
23 |
29 |
25 |
| 6am Temperature |
25 |
29 |
25 |
| 24 hr Maximum |
30 |
37 |
45 |
| Winds |
| Current Winds |
11 WSW |
1 NW |
- |
| 24 hr Average |
11 W |
4 SW |
- |
| Maximum Gust |
20 W |
15 NW |
- |
| Snow - Storm Interval # 23 |
| Total Depth |
68" |
58" |
25" |
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