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| 3.12.2007 |
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This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Monday, March 12, 2007 at 7:30 am.
City of Ketchum (www.ketchumfire.org) & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Special Announcement:
We’d like to extend a special “Thank You” to all of our partial advisory sponsors who may not get mentioned every week. The Deer Creek Fund in the Idaho Community Foundation, the Wattis Dumke Foundation, Galena Lodge, Blaine County Rec, Sun Valley Tele Series, Idaho Power, and Smiley Creek Lodge.
Bottom Line:
The danger of triggering a dry slab avalanche is estimated to be MODERATE on upper elevation, northerly facing slopes steeper than 35 degrees. The avalanche danger on less steep slopes is estimated to be LOW.
Recent close calls demonstrate the problematic nature of steep, upper elevation, northerly facing slopes. Few signs of instability are present, but it remains possible to trigger an avalanche breaking 1-3 feet deep on buried facets. Variability resulting from the January dry spell means weaker areas do exist, warranting a conservative approach in this type of terrain.
With no freeze overnight, and warm temperatures in the forecast, the danger of triggering a wet slide will quickly rise to CONSIDERABLE today. Sunny slopes will have bottomless wet snow conditions. It will be a good day to avoid traveling on or beneath steep, southerly aspects.
Outlook for Tuesday:
The slab avalanche danger is expected to remain the same. Watch for the development of shallow wind slabs with increasing winds on Tuesday. Slightly cooler temperatures are forecast for tomorrow, but the wet snow avalanche danger will depend primarily on how well the snowpack freezes Monday night and the cloud cover on Tuesday.
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Primary Avalanche Concern:
Yesterday, the light overnight freeze broke down quickly resulting in increasing wet snow avalanche danger. A couple parties reported extremely weak, rotten snow beneath the surface crust, with one group stating that once the crust thawed they sank up to parts unmentionable. This will be especially true at mid to lower elevations.
Temperatures remained warm overnight which means that no surface crust will have formed to keep the wet slush in place on steep slopes. With partly cloudy skies and very warm temperatures forecasted today, the wet snow avalanche danger will begin increasing early this morning. Natural wet slides can be expected, and it will be wise to avoid traveling on or beneath steep southerly facing slopes.
Secondary Avalanche Concern:
Yesterday I investigated the avalanche in Apollo Creek that buried and injured a snowmobiler on Saturday (see photos below and video clip http://www.sawtoothavalanche.com/video/3.11.07.mpg ). The avalanche was about 250 feet wide, averaged 2 feet deep, and ran 400 feet vertically. The slope angle in the starting zone ranged from 37 to 40 degrees. The avalanche fractured in weak, faceted snow that formed during January.
Stability tests near the crown of the avalanche and elsewhere along the ridge required significant force to fail, but once they did they broke very cleanly. More telling was the snowpack structure, which consisted of a dense slab over loose, sugary facets. My take on this is that if you hit a weaker spot on the slope, the snowpack has the ability to propagate a fracture and release an avalanche. The prolonged dry spell back in January left us with a more variable snowpack than usual, which means weaker areas are more likely to exist.
Put simply, many northerly facing slopes steeper than about 35 degrees remain suspect. Conditions are especially deceptive because it’s been more than a week since significant snowfall, and you aren’t going to see the usual signs of instability such as recent avalanches or collapsing of the snowpack. Stability tests may even lead you to believe that the snowpack is fairly strong. As recent close calls have demonstrated, this is the classic false-stable situation, and the best solution is a conservative approach in steep, northerly facing terrain.
Current Conditions:
Yesterday, a light surface freeze combined with warm temperatures meant that corn conditions were fleeting and the snowpack on sunny slopes became wet and rotten by midday. Rollerballs and pinwheels were observed, even on north facing slopes.
Overnight temperatures at upper elevations only dropped to 35-40 degrees, so it goes without saying that the snow surface didn’t freeze. Sunny slopes should be avoided today, and while preserved powder still exists in favored locations, it’s getting harder to come by.
Mountain Weather Forecast:
The ridge of high pressure over the northwest is flattening and turning to a more westerly flow. A weak cold front heading south towards our region will bring increasing clouds overnight through tomorrow, along with a chance for light snowfall. Today is forecast to be partly cloudy, and mountain temperatures should reach the mid to upper 40s. Temperatures on the valley floor will likely exceed 50 degrees. Ridgeline winds are expected to blow 10-20 mph from the west to southwest.
Mostly cloudy skies will prevail overnight into Tuesday. Temperature forecasts for Tuesday are across the board but with the weak cold front they should be at least a little cooler than today. Ridgeline winds are forecast to increase to 15-25 mph from the southwest with stronger gusts likely.
Photos:
1. Apollo Avalanche Overview
2. Apollo Burial Location
3. Apollo Avalanche From Top
4. Aerial View of Apollo Avalanche
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
34 |
38 |
30 |
| 6am Temperature |
35 |
38 |
30 |
| 24 hr Maximum |
40 |
49 |
50 |
| Hours Below Freezing |
0 |
0 |
4 |
| Winds |
| Current Winds |
14 W |
6 W |
- |
| 24 hr Average |
12 W |
7 W |
- |
| Maximum Gust |
25 W |
29 NW |
- |
| Snow - Storm Interval # 18 |
| Total Depth |
57" |
46" |
23" |
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