Good morning. This is Mark Staples with the Gallatin National Forest Avalanche Advisory issued on Thursday, December 20 at 7:30 a.m. Today’s advisory is sponsored the Montana FWP Recreation Trails Grant. This advisory does not apply to operating ski areas.
This morning temperatures were in the single digits F and ridgetop winds were blowing 15-20 mph from the W and SW with gusts of 30 mph. Major low pressure systems are impacting the west coast and the Midwest, but southwest Montana remains under a ridge of high pressure. Fortunately this ridge will flatten and move east this weekend. Today temperatures will climb into the teens F, and winds will increase and blow 25-40 mph from the SW.
Bridger Range Madison Range Gallatin Range
Lionhead area near West Yellowstone Cooke City
Yesterday’s winds transported some of the last remaining snow off ridgetops onto leeward slopes (photo). While this wind-blown snow will not alter the avalanche danger, it will remain the primary avalanche problem (example from Cooke City and the Bridger Range). Wind slabs will be tricky today because many will be stubborn and difficult to trigger. I have been fooled many times by wind slabs when some do not crack and move on small test slopes while others produce avalanches.
A secondary avalanche problem is a persistent weak layer of faceted snow within the bottom 1 ½ feet of the snowpack. This layer has both gained strength and adjusted to the weight of new snow from the last two weeks. We have not seen, nor heard reports of recent signs of instability on this layer other than results from ski area explosives. One exception is the west side of the Bridger Range where a weak snowpack and some collapsing was observed three days ago. Yesterday Karl and Doug investigated the snowpack on Saddle Peak where they found greatly improved stability test results in this faceted snow layer compared to what Doug and Eric found 10 days ago (previous snowpit vs current snowpit). However, wind slabs at the top of their snowpit easily fractured.
Wind slabs that can be triggered by a skier or rider combined with slopes harboring some weak snow near the ground mean it’s still possible to trigger an avalanche today. For this reason the avalanche danger is rated MODERATE.
I will issue the next advisory tomorrow morning at 7:30 a.m. If you have any snowpack or avalanche observations drop us a line at mtavalanche@gmail.com or call us at 587-6984.
Free 1-hour Avalanche Awareness lecture at Bridger Bowl at 1 p.m. on the 4th floor of the Saddle Peak Lodge on December 23. No registration necessary.