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| 3.17.2009 |
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This is Blase Reardon of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Tuesday, March 17, 2009 at 7:30 am.
The Wattis Dumke Foundation & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Bottom Line:
In the northern part of our area, areas of HIGH danger exist on steep, upper elevation slopes with fresh deposits of wind-drifted snow. On other slopes in this part of our advisory area, the avalanche danger is generally CONSIDERABLE. Heavy snowfall and strong winds over the past two days have created unstable soft slabs one to two feet thick on slopes steeper than about 35 degrees; these slabs are easily triggered by the weight of a person, and we do not recommend playing or traveling on such slopes today.
In the central and south valleys, the avalanche danger on steep, upper elevation slopes today is estimated to be CONSIDERABLE. The storm has dropped less snow and brought more moderate winds, so although unstable slabs are generally shallower and less widespread, dangerous avalanche conditions still exist. The danger is greatest on east through north-facing slopes steeper than about 35 degrees, including the sides of gullies.
On low-mid elevation slopes throughout our advisory area, the danger of wet snow avalanches remains CONSIDERABLE due to above freezing temperatures and recent rain.
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Danger by Aspect and Elevation:
Primary Avalanche Concern:
Rapid loading from storm snow and wind transport created very sensitive slabs on steep slopes in the Galena Summit area yesterday. These slabs averaged about 15” deep, but ranged from 6 to 24”. They were easily triggered by a person’s weight or by small chunks of cornice; most were near wind-exposed ridgelines, but we also found several on cross-loaded, mid-slope terrain features and on a variety of aspects: southeast, east, northeast, northwest, and west.
The fractures were propagating disturbingly fast and far, even in areas with very shallow slabs; simply stepping on the top of a steep slope often caused cracks to shoot over 50 feet from our skis and around terrain features. We remotely triggered several slides on slopes adjacent to the one we were testing, including slopes a couple hundred feet away (links to photos below). Though most of the slides we triggered weren’t very wide – 25-50 feet – the slabs were deep and involved enough snow to bury a person. I’ve had reports of similar slides in out-of-bounds areas near Baldy.
Yesterday’s avlanches resulted from rapid loading of the weak layers at the old snow/ new snow interface - a layer of colder, light density snow or small facets. On shady slopes, both of these layers exist about 2-3 inches apart. On sunny slopes, the light density layer sits on top of melt-freeze crusts. These weak layers will become less sensitive as they adjust to the recent load, but that will take awhile because moderate to strong winds will continue to drift the recent snow and to load many slopes. As the storm and wind-drifted snow consolidate, slabs may become less sensitive to the weight of a person or snowmachine, but propagate further, resulting in larger, much more dangerous avalanches.
Today I expect that slabs will remain sensitive to human triggering on slopes steeper than about 35 degrees that have recent deposits of wind-drifted snow. These conditions will be most common near ridgelines and on open, upper elevation slopes that face southeast through northeast. The gusty winds have also been cross-loading mid-slope features such as the steep sides of gullies, making unstable slabs a real possibility in these locations as well. Many but not all of these slabs will give themselves away with shooting cracks. Be particularly careful where slopes roll from a lower angle to a steeper angle and on convex-shaped terrain features which often collect wind-drifted snow but cause higher stress on a slab.
Additional discussion: I’m concerned that the weight of the recent storm snow may reactivate the persistent weak layers buried in mid-February. These layers seemed to have stabilized last week, but the current storm has created a sudden, rapid load on those older layers. They are now buried three feet or more in most places – just about the depth at which a person’s weight has little effect. The problem is again the exceptions – the shallow places where the snow is weaker to start with and the load is proportionately greater. It’s a familiar refrain, but be very leery of exposed slopes with variable snow depths and lots of rocks where it may be possible to trigger a deep, very dangerous avalanche on this layer.
Secondary Avalanche Concern:
Wet snow avalanches remain a concern on steep, low-elevation slopes throughout our advisory area. These slopes have been saturated with rain the past few days and have had little if any refreeze. With daytime heating today, the snow on these slopes will remain wet and cohesionless, and natural and human-triggered wet snow avalanches are possible, particularly on slopes steeper than about 40 degrees below about 7000 feet. Avoid traveling on or beneath these slopes as temperatures rise today.
Current Conditions:
Rain, snow and wind dominated yesterday’s weather. Mountain stations have recorded another 0.5-1.4” of snow water equivalent in the past 24 hours, bringing storm totals to 1-3” of SWE and 8-18” of new snow. Snow totals depend heavily on elevation because of the warm temperatures, and the storm definitely favored the northern part of our advisory area, especially the Salmon Headwaters and the northern Sawtooth Mountains. The rain/ snow line yesterday was just below 7000 feet on Baldy and about Baker Creek in the north valley. Below that elevation, most of the precipitation fell as rain, or, when it did fall as snow, was quickly melted by subsequent rain. Valley precipitation totals are roughly 0.5-1”.
Strong winds were an unwelcome tag-along with the storm snow. At Titus Ridge, winds have averaged 25 mph from the west for the past 24 hours, with gusts to 60. Wind speeds were more moderate on Baldy; the 24-hour average was 16, with gusts from 25-50 mph. The winds veered from south during the day to west-southwest overnight. The wind speed sensor near Soldier Mountain has been down or rimed for most of the past 24 hours.
Temperatures hovered near freezing at most mountain stations yesterday, with temperatures near ridgelines in the mid 20s. Highs in the valleys were in the upper 30s. Partly cloudy skies allowed overnight lows to cool to the upper 20s at mountain sites and near freezing in the Wood River Valley and Stanley Basin.
The combination of wind and warm temperatures made for dense, sometimes upside down skiing and riding conditions yesterday. In wind-drifted areas, trailbreaking was difficult and the skiing was a variable. On sheltered slopes, it was easier to fond more consistent powder.
Mountain Weather Forecast:
Precipitation today will be limited to a few snow showers; these will end by nightfall. There will be a marked difference in temperatures with elevation. Ridgeline tempretures will only reach the mid 20s, but at lower elevations, temperatures will climb to near freezing, with valley temperatures in the low 40s. Winds will gradually diminish today, and continue to be westerly. Partly cloudy skies tonight will make for overnight lows in the mid-teens at upper elevations and mid 20s in the valleys, with winds turning slightly to the north.
Photos:
1. Titus_remote_trigger_3_16_09
2. Titus_Ridge_crack_&_remotely_triggered_slide
3. Cross-loaded slope
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| Reported Conditions |
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| Temperatures |
| Overnight Low |
19 |
23 |
32 |
| 6am Temperature |
19 |
23 |
32 |
| 24 hr Maximum |
26 |
30 |
40 |
| Winds |
| Current Winds |
W24G38 |
WSW11 |
- |
| 24 hr Average |
W25 |
SW16 |
- |
| Maximum Gust |
W60 |
S50 |
- |
| Snow - Storm # 23 |
| New Snow |
4" |
4" |
0" |
| Storm Total |
12" |
10" |
2.5" |
| Total Depth |
73" |
59" |
26" |
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