|
|
|
 |
| 3.18.2007 |
|
|
 |
This is Chris Lundy of the Sawtooth National Forest Avalanche Center with your Backcountry Avalanche Advisory and Weather Forecast for Sunday, March 18, 2007 at 7:30 am.
The Twin Falls District Bureau of Land Management & the Friends of the Sawtooth Avalanche Center bring you this advisory.
Special Announcement:
Due to the early arrival of spring and waning backcountry interest, we will begin providing general snow and weather information as conditions warrant.
Bottom Line:
The Wet Snow Avalanche Danger will start out LOW this morning and rise to CONSIDERABLE by this afternoon. A weak freeze overnight will break down during the day under partly cloudy skies and another day of warm temperatures. The possibility of triggering a wet slab exists on east and west aspects that are undergoing a transition to a spring snowpack.
The Dry Slab Avalanche Danger is estimated to be MODERATE on upper elevation, northerly aspects steeper than 35 degrees. The snowpack on these slopes remains dry with a layer of weak facets buried about 2 feet deep.
Outlook for Monday and Tuesday:
Continued warm weather is expected on Monday so the wet slide danger will again rise in the afternoon. On Tuesday, a storm system may bring a few inches of new snow accompanied by strong winds. Decreasing temperatures should freeze the snowpack, and the main avalanche concern will be shallow wind slabs that form along upper elevation ridgelines.
|
Primary Avalanche Concern:
Despite above freezing temperatures overnight, clear skies will have provided at least a weak re-freeze of the snow surface this morning. As the crust breaks down with daytime heating, the danger of triggering a wet snow avalanche will increase.
Temperatures in the South and Central Valley are staying warmer overall, which means less freeze overnight and more melting during the day. This combined with a weak, faceted snowpack means that with warming temperatures the snowpack on all aspects will quickly turn to a wet, soupy mess. If you get out in this region, even if on cross-country skis or snowshoes, avoid travel on or beneath steep slopes during the heat of the day.
In our northern mountains, the snowpack is receiving a better re-freeze overnight, but by afternoon it’s getting wet enough on sunny slopes to result in fairly large wet snow avalanches. Wet slab avalanches have been observed on east and west aspects as these make the transition to a spring snowpack.
Here are a few tips for springtime backcountry travel: Determine how well the snowpack has froze overnight by checking our weather station links in the morning. Look for near or below-freezing temperatures combined with clear, starry skies. Start early so you have plenty of time to be off of steep slopes before the snow becomes dangerously soft. At the first sign of your skis, boots, poles, snowmobile track, or other parts punching through the thawing surface crust, it’s time to be off of and out from beneath steep, sunny slopes.
Secondary Avalanche Concern:
In our northern mountains, the snowpack on upper elevation, northerly aspects remains dry and winter-like. The buried layer of weak January facets can be easily identified about 2 feet deep. This snowpack layering has the ability to propagate a fracture if you hit a weak spot, and it remains possible to trigger a dry slab avalanche on slopes steeper than 35 degrees, especially in rocky terrain or shallow areas. Given this buried weak layer’s track record and it’s extremely variable nature, I think it still has the ability to surprise us.
Current Conditions:
Despite above freezing temperatures Friday night, clear skies provided a decent freeze of the snowpack surface yesterday morning. In the North Valley, corn conditions started mid-morning on southeast aspects and lasted until early afternoon on southwest facing slopes. Northerly aspects are wet on the surface and offer poor skiing, but tolerable snowmobiling.
Temperatures yesterday hit the low 50s at most mountain locations, and it climbed to 67 degrees in Ketchum. Overnight temperatures have only dropped into the mid to upper 30s, but clear skies should have provided at least a weak surface freeze. Ridgeline winds are currently averaging 10-20 mph from the west.
Mountain Weather Forecast:
A weak shortwave passing to the north will bring a few clouds today, but temperatures are still expected to be very warm. Temperatures at 9-10,000 feet should reach the mid 40s to low 50s, and a high near 60 degrees is expected in the valley. Ridgeline winds will continue at 10-20 mph from the west to northwest, decreasing this afternoon.
High pressure will rebuild briefly on Monday before a Pacific low pressure trough enters the region Monday night. On Tuesday, 2-4 inches of new snow are possible accompanied by southwest winds. A cold front will be on the system’s heels bringing much colder temperatures Tuesday night through Wednesday.
Photos:
1. Wet Slab Avalanche
|
|
|
|
|
|
| Reported Conditions |
|
 |
|
 |
|
 |
| Temperatures |
| Overnight Low |
34 |
39 |
30 |
| 6am Temperature |
34 |
39 |
30 |
| 24 hr Maximum |
45 |
49 |
67 |
| Hours Below Freezing |
0 |
0 |
4 |
| Winds |
| Current Winds |
18 W |
10 NW |
- |
| 24 hr Average |
14 W |
9 W |
- |
| Maximum Gust |
29 W |
21 W |
- |
| Snow - Storm Interval # 18 |
| Total Depth |
49" |
44" |
13" |
|
|
){kind=link}