March 28 Avalanche Advisory

Weekend Avalanche Advisory
Posted March 28th at 0700

Greetings backcountry sliders and riders! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for March 28th – 30th, 2008. The avalanche danger rating expires at midnight March 28th. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas.

Weather and Snowpack Analysis

We are well beyond the first day of spring and nearly into April but you wouldn’t know it if you have been in the mountains this week. Temperatures were in the teens, the new snow was light and a delight to ski and the winds above 8000′ were blasting away at the exposed ridges and mountain tops just like we’ve seen all winter long. So is it Spring? You bet, but there is no sign of it in the mountains above 5000′. We did have a day or two early in the week when the sun brought temperatures above freezing and warmed high elevation slopes enough to leave a nasty breakable crust on all but the most shaded aspects.

Observers Thursday reported cold temps, a foot or more of new snow in the past few days and high winds at all locations in the Bitterroot and Rattlesnake mountains. The wind was the predominant weather feature as it was easily transporting the new snow onto leeward terrain. South to West aspects were getting scoured hard by the wind but we were finding pockets of sensitive snow wherever it could accumulate. Wind slabs were forming along the high ridges and open terrain and will be sensitive for a few days. East to North aspects have the best snow and where not influenced by wind loading, mostly stable conditions.

The Lolo Pass and Rattlesnake areas picked up the most snow and observers here and elsewhere were seeing low stability test scores associated with the new snow on a hard surface. Steep wind loaded terrain is the most sensitive. The variability in skiing and stability conditions was huge yesterday so it will pay to have a high level of awareness this weekend.

We did not receive a report from the Seeley Lake area this week. The North Fork Jocko SNOTEL reports 10” new snow in the past 48 hours so conditions in the Southern Missions and Southern Swans will be similar to those seen in the Rattlesnake.

Today’s Avalanche Danger

Above 5000′ on all wind loaded slopes steeper than 35 degrees, the avalanche danger is CONSIDERABLE. Natural avalanches are possible, human triggered avalanches are probable. Unstable slabs exist on steep wind loaded terrain. This condition will be found on the leeward side of ridges or open terrain where high winds are present. Any East to North facing aspect above the tree line will be heavily loaded just below the ridge top.

On all other slopes steeper than 35 degrees, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible. There may be unstable slabs on steep terrain. West to Southeast aspects where wind has filled in or cross loaded steep terrain are of particular concern. These slopes have a prominent sun crust that formed during the warm sunny weather last weekend and will fail easily with enough snow (10” or more) and a trigger.

Below 5000′ and on all other terrain less steep than 35 degrees, the avalanche danger is LOW. Natural avalanches are very unlikely, human triggered avalanches are unlikely. There may be isolated pockets of unstable snow at the lower elevations even during periods of LOW avalanche danger.

Weather Forecast and Avalanche Outlook

Another strong low pressure system will move into the area Friday with moderate to heavy snow accumulations Friday night and Saturday. 10”- 20” of snow with 13-18 mph South to West winds is possible above 6000′ by Sunday.

If these conditions materialize expect the avalanche danger to worsen especially on steep wind loaded terrain. The cooler temperatures are allowing weaknesses associated with buried sun crusts and small grained facets to persist for a much longer period than normal for this time of year.

The final formal advisory for the 2007-2008 season will be posted on April 4th, 2008.

If you are seeking further backcountry travel information please contact the local Forest Service Ranger District Office in the area where you wish to recreate or call us at 406-329-3752.