The Gallatin National Forest Avalanche Center issued its latest avalanche advisory on Thursday, April 5, with the following advisories issued by Doug Chabot:
Mountain Weather: Temperatures dropped into the high teens on Wednesday night with one to two inches of new snow falling early Thursday morning. Wednesday, under partly cloudy skies, temperatures reached the upper 40s as west winds blew 15-25 mph. A cold front moved in on Wednesday night as winds speeds reached 40-60 mph. Light snow continued on Thursday and picked up intensity through Friday. Winds were westerly at 15-25 mph with mountain temperatures dipping into the low teens overnight. The spring storm was expected to deliver four to six inches of new snow.
Snowpack and Avalanche Discussion:
The Bridger, Gallatin and Madison Ranges and the Lionhead area near West Yellowstone: Winter is back for her encore performance. The current below-freezing conditions have helped the snow stability; Wednesday’s wet surface is today’s thick crust. Mountain temperatures dropped to below freezing for six hours before the snow started to fall. This allowed the snow surface to freeze, which will kill the chances of wet avalanche activity. Thursday’s main avalanche concern was wind-loading. At the ridgetops the winds scoured any snowflake that was not glued down. Moonlight Basin had wind slabs two feet thick pull out on Tuesday. One to two inches of fresh snow this morning with a few more inches today will be enough ammunition to create wind drifts. With a thick ice crust on many aspects, a high-speed “slide for life” tumble has greater odds of happening than being buried by an avalanche.
Not all slopes have a crust. A few shaded slopes are holding onto powder snow too. At the base of the snowpack is a layer of depth hoar. This layer has proven itself to be unpredictable and untrustworthy no matter if the snowpack is moist or dry. Triggering a deep slab avalanche is still possible from thinner spots on a slope. Any avalanche that’s triggered at the ground will be large and destructive. I remain skeptical of this layer, even with a thick ice crust capping the snow. For Thursday the avalanche danger was rated moderate on all slopes since triggering wind-loads or something much deeper remains possible.
The mountains around Cooke City: Avalanche danger around Cooke City will be confined to wind-loaded terrain. These pillows will not be widespread or thick, but they will be reactive to ski cuts. Cooke City lacks the deep instability issues found in the rest of our region.
Consequently, for Thursday, the avalanche danger was rated moderate on wind-loaded slopes and low on all other avalanche terrain.