April 6, 2012 Avalanche Advisory
LOW to MODERATE avalanche danger now prevails in the mountains of west central Montana. This is Steve Karkanen reporting from the West Central Montana Avalanche Center.
Weather and Snowpack Analysis
The mountain snowpack in western Montana has experienced several melt-freeze cycles the past 14 days with afternoon temperatures reaching into the 40‘s and 50’s then dropping back into the twenties and teens. This back and forth (temperature exchange) has strengthened the snowpack greatly but there are a few exceptions to keep in mind.
The weak faceted layers that formed in late January and February have not completely gone away and still fail cleanly during stability testing but only when shocked with an unusual amount of force. The important exception to this is when temperatures rise above freezing and stay there for several hours (especially overnight) or if rain water percolates down to one of these weak layers or an old crust.
We found a very strong, thick crust Thursday which struggled to warm up under partially sunny skies. Beneath this armor is a pencil hard snowpack that is still moist from the melting experienced during warmer weather the past few days. Above 8000 feet the northern aspects will hold drier snow but even after a big storm, the powder days will be short lived. Only an inch or two of snow has been recorded at advisory area SNOTEL sites this week.
The primary concern will be how well any new snow bonds to this hard surface with wind loaded terrain being the most sensitive after heavy snowfall. Until it snows heavily or warms up, there is very little chance of triggering an avalanche.
SNOTEL sites report a trace of new snow with NW winds at Point Six hitting 27 mph at 0600 this morning. Mountain temperatures are in the teens and low 20’s.
Weather Forecast and Avalanche Outlook
The Missoula office of the National Weather Service is forecasting a spring snow storm to impact areas south of Missoula Friday into Saturday with the Lost Trail Pass area having the best chance for significant accumulating snowfall. 9-15 inches of new snow is possible in the Lost Trail Pass area with less accumulation as the storm moves north.
High pressure builds back Saturday through Sunday with valley temperatures warming back into the 50’s and 60’s and mountain temperatures in the 40’s and 50’s. The high pressure ridge will remain for the early part of next week with temperatures warming 15 to 20 degrees above normal.
As temperatures remain close to seasonal norms, expect LOW avalanche danger in the mornings and on shaded slopes throughout the day. As the day warms up, the avalanche danger becomes MODERATE and the avalanche danger will quickly rise to CONSIDERABLE on aspects exposed to the sun or whenever temperatures remain above freezing overnight.
In locations that receive heavy new snow, pay close attention to how well the snow bonds to the now hard old snow surface. Expect this new snow to easily sluff on terrain steeper than 35 degrees immediately after a storm then again when it begins to be affected by warm temperatures or rain.
It’s a bad deal anytime it rains when you are playing on snow. Mostly because it isn’t any fun but more importantly rain quickly creates instability.
If this happens, expect the avalanche danger to rise to CONSIDERABLE or HIGH immediately.
Good clues of imminent avalanche danger in the spring are roller balls gaining size as they roll downhill, small point release avalanches entraining much larger amounts of snow or sinking easily past your boot tops or up to your knees in heavy wet snow. Move off of and out from under large open slopes and seek out the more shaded and cooler northerly aspects.
This is the last official advisory for the season but we will continue to post any observations sent in through the spring. So please do send us your reports. They are very helpful for the hard core skiers and riders chasing winter for as long as possible.