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| 2.27.2009 |
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This is Blase Reardon of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Friday, February 27, 2009 at 7:30 am.
Smiley Creek Lodge & Marley in the Mountains & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
Today the avalanche danger is estimated to be HIGH on upper elevation, wind-loaded slopes throughout our advisory area. On these slopes, several persistent weak layers are now buried two to three feet deep by recent snow, new snow, and wind-drifted snow. The rapid loading of these layers has created very dangerous avalanche conditions on steep, easterly-facing slopes. Travel on or below these slopes is not recommended.
A similar snow structure exists on slopes in our advisory area that have not been wind-loaded, but the avalanche danger on these slopes is CONSIDERABLE because the load is smaller and has not occurred as rapidly. This snow structure still makes for dangerous avalanche conditions, and safe travel on these slopes requires conservative decision-making and careful route-finding.
WEEKEND OUTLOOK- The avalanche hazard will slowly moderate during the high pressure forecast for today and Saturday, but another storm will arrive Sunday afternoon before conditions fully stabilize.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
Anyone who’s lived in a small town knows that dating there can be a cycle of excitement, disappointment, and diminished expectations. Pretty soon you or your friends are tempted to take some risks when something new comes along. That’s the situation we face with the snowpack right now; after a long wait we finally have the makings of good time - new snow and blue skies. But don't let your powder goggles blind you to the dangerous instabilities and weaknesses lurking on many slopes. Many of the weaknesses are baggage left from the prolonged mid-winter dry spell – facets, crusts, and facet-crust combinations that were buried February 13th. Recent snow and wind have created a substantial load on these weak layers - 18-30” of recent and new snow. That snow was slowly adding up until yesterday, when a burst of intense precipitation and very strong winds left 8-10” of new snow in most mountain locations as well as dense wind slabs on many easterly slopes.
Such rapid loading caused a natural avalanche cycle yesterday. Poor visibility limited observations, so the extent of this cycle is not yet clear, though I saw six natural avalanches in the Durrance area yesterday. These were on east-northeast facing slopes between 8300 and 9200 feet, ran on the Friday the 13th facets, and involved both recent snow and snow cross-loaded by the strong winds. Crowns averaged 20 inches deep, and one slide on The Horseshoe was about 500 feet wide. There are links to photos at the bottom of the page.
Yesterday’s natural avalanches are an obvious sign of instability, and as much as the powder goggles might have me wishing otherwise, that instability hasn’t gone away overnight. It will be most acute on southeast, east, and north facing slopes throughout our advisory area, particularly near ridgelines. However, cross-loading has likely created similar slabs around exposed mid-slope features, and terrain-driven channeling of winds may have created similar conditions on other aspects. Because the instabilities involve the near-surface layers of the snowpack, it’s relatively easy and quick to dig down to the mid-February snow surface and conduct quick tests. Be aware that today’s direct sun and slow heating can increase the stress on a slope with buried weak layers and thus may increase the chances of triggering slides on steep slopes as the day progresses. Cornices built rapidly yesterday, and some were breaking further back than expected.
Current Conditions:
Yesterdays saw full conditions in the mountains – fierce winds, cold temperatures, blowing snow, and high precipitation rates. Winds averaged 25-30 mph for several hours with gust s near 60 at both Titus and Baldy. Near Soldier Mountain, winds averaged 20 to 50 mph with gusts to 80. The strong winds even reached into the valley bottoms, with gusts from 25 to 40 mph recorded at many valley stations. The winds were southerly yesterday morning, but veered abruptly to the northwest at midday.
Mountain weather stations have recorded 4-10’’ of snow and 0.3 -.8” of SWE since yesterday morning, with a little more in reports from area yurts. The highest snowfall was in the western and northern part of our advisory area – the North Smokys, Salmon Headwaters, and Sawtooth Mountains. People are reporting very good powder skiing even on slopes 30 degrees or less.
Temperatures have dropped steadily behind the cold front that brought this snow and wind. Yesterday’s highs occurred mid-morning and ranged from the low twenties to near freezing. Temperatures this morning are near zero at most mountain stations. Winds have calmed somewhat, and veered further to the north.
Mountain Weather Forecast:
High pressure building over our area will bring clear skies, sun, and moderate winds for today and tomorrow. Winds today will blow from the north at 15-20 mph at upper elevations, then moderate and shift to the south by tomorrow morning. Temperatures today will be cool, with highs in the upper teens near ridgelines and in the upper twenties in the valleys. Lows tonight will drop into the single digits at most locations but rebound to near freezing tomorrow at upper elevations. Another storm is forecast for Sunday afternoon and evening.
Photos:
1. Slides in The Horseshoe on Durrance
2. Slide @ the bottom of Sam's
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| Reported Conditions |
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| Temperatures |
| Note: Temperatures peaked yesterday morning. |
| Overnight Low |
-5 |
1 |
2 |
| 6am Temperature |
-4 |
3 |
2 |
| 24 hr Maximum |
16 |
19 |
34 |
| Winds |
| Current Winds |
N16G24 |
NNW14 |
- |
| 24 hr Average |
WNW15 |
W18 |
- |
| Maximum Gust |
WNW60 |
S57 |
- |
| Snow - Storm # 19 |
| New Snow |
8" |
6" |
4" |
| Storm Total |
9" |
7" |
5" |
| Total Depth |
61" |
51" |
28" |
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