Northern San Juan
AvalancheColin Mitchell, Bill Nalli · Jan 19, 2020 · Northern San Juan
Weather
High thin clouds and mild temps made for a pleasant day in the mountains. Light north winds. Temps mid 20's near treeline.
Snowpack
Snow surface stayed dry on all northerly aspects but southeast through southwest to west got some form of sun affect by the end of the day. A new crust formed on these aspects, fragile on southeast and progressively harder around the compass to the west. Focus was on northeast aspects near and above treeline to determine the weak layer involved in a small avalanche cycle on east and northeast-facing slopes at the end of the storm on Friday January 17. From Red Mt. Pass to Cement Creek we saw at least six recent avalanches on northeast and east-northeast aspects. One of them was triggered by a rider in Porphyry Gulch likely on Saturday January 18. See pics below. The theme for these slides was the same. 30 to 50 feet wide and 1 to 2 feet deep, mostly travelling only a few hundred feet vertical, stopping at the first transition in slope angle. Size was D1.5 to D2, just large enough to bury a person. The largest one we saw was in Prospect Gulch on northeast facing Blackjack at 12,000 feet. The slope adjacent to it had not slid and the bottom portion of it is low angle so we targeted it for a snow profile. A small collapse on approach made us back away and find another test slope to dig with less consequences. There we found a very deep snowpack at 248 centimeters! Some north through east aspects that survived all of the last major wind events are very deep and strong. The snow near the surface however is becoming weaker. Small faceted grains formed over the last dry period and where they are more sheltered, are now buried under six to 16 inches of settled snow. These facets are likely the culprit in all the small avalanches from the last two days on northeast and east. Areas that received slightly more wind loading failed and others may just be waiting for the right trigger. It's unclear how long this juvenile persistent weak layer will last but I suspect at least another couple days if not more.
Avalanche
A string of small D1.5 to D2 avalanches on northeast through east-northeast occurred on Friday January 17 and one rider triggered on January 18.