March 19, 2013 Avalanche Advisory
CONSIDERABLE avalanche danger exists on wind-loaded terrain steeper than 35 degrees above 5000 feet in the Bitterroot, southern Mission, and southern Swan Mountains. Natural avalanches are possible; human-triggered avalanches are likely in steep terrain. The avalanche danger is MODERATE on wind-loaded terrain steeper than 35 degrees in the Rattlesnake Mountains and on all other terrain in our advisory area.
Good Morning. This is Tim Laroche with the West Central Montana Avalanche Center’s backcountry avalanche advisory for Tuesday, March 19th, 2013.
Weather and Snowpack Analysis
Winter returned to west central Montana the past few days with a juicy storm that deposited over 20 inches of new snow in the Bitterroot, southern Mission, and southern Swan Mountains since Sunday. The Rattlesnake Mountains received about a foot of new snow. This latest storm dramatically improved skiing and riding conditions throughout our advisory area. The wind blew hard, gusting in the upper 30 mph range from the west and southwest until switching to the northwest late last night and calming to the mid teens. Currently, the bulk of the storm system has exited to the east, mountain temperatures are hovering in the low twenties, and winds are blowing from the west at 15-20 mph.
The primary avalanche concern is fresh wind slabs that have formed on upper elevation ridgelines and cross-loaded gullies. The new snow sits on a stout melt-freeze crust that developed last week during warm weather and has now frozen. During field assessments yesterday, we found the new snow bonded to this crust, but you will have to assess each slope to see if this is the case where you are recreating. On Sunday, skiers in the Bitterroot Mountains turned around when they experienced instabilities created by the storm snow that had not yet bonded to the underlying surface (public observation). The new snow should settle out fairly quickly, but until it does be aware of the potential to trigger fresh wind slabs that formed the past few days.
The secondary avalanche concern is for storm snow avalanches. There are variable weaknesses in the new snow due to differing temperatures and precipitation rates. These weaknesses will be sensitive to human triggers today on steep slopes. The storm snow is fairly cold, so expect loose snow avalanches in steep terrain where the wind has not yet formed a slab. These sluffs could catch and carry you down slope if you are not suspecting them.
Watch for the snow to change today on aspects that receive sun. Wet loose slides will occur as the sun comes around and warms the southerly facing aspects. Stay off and out from underneath any steep slopes that start to get damp. Visual indicators are small point release slides starting from rock outcrops and pinwheels that roll down steep slopes and gather snow as they descend.
Weather Forecast and Avalanche Outlook
Today will be a mostly dry day under a slight ridge of high pressure. Winds will be light out of the south with temperatures reaching the mid 30’s under partly cloudy skies. Beyond today, expect another round of precipitation and for unsettled weather to continue through the week.
I expect the avalanche danger to decrease as the new snow settles and until the next cold front arrives and delivers another round of precipitation.
I will issue the next advisory on Friday, March 22nd.
If you get out and have the time to send us some information about what you are seeing, please use our “public observations” link on our website or send us a quick note at i[email protected]. Thank you for your continued support!