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Avalanche Update 11/08



Friends,

This is Scott at the Colorado Avalanche Information Center at noon on Tuesday, November 08. The CAIC is gearing up for the 2005-06 season and we plan to resume normal operations with daily forecasts starting Friday morning, November 11. Until then, we will provide these updates as conditions warrant.



The central and northern mountains of Colorado saw a wide spread avalanche cycle from Wednesday November 2 thru Sunday November 6. People triggered, were caught, and one person was killed in this cycle. In all 15 slides were reported. Two in the central Mountains, outside Crested Butte near Yule Pass (on the 2nd), and one off Independence Mtn on the Aspen side of Independence Pass (on the 6th). These slides were on NE aspects, starting zones about 12,200 ft. All went long distance, from 1,000 vertical to 2,200 vertical at the Yule Pass area slides. All three were human triggered. The 2nd Yule Pass slide was a very close call. Early on the morning of the 2nd, two BC skiers triggered a large slide off Purple Ridge. As they were leaving the valley they noted a lone skier following their tracks to the ridge. Later that day or the next a woman called and left a message with the Crested Butte Avalanche Office stating her husband had been caught in a large slide, sustaining shoulder injuries and multiple cuts & bruises. The path he was caught in was adjacent to the one triggered earlier in the day. He ended up on a bench after a 1,000 vertical foot ride, the slide continued another 1,000 + vertical, off big cliffs to the valley floor.

The Independence Mountain slide was triggered by one skier in a party of six. He skied off the slab before it really began to move.

The patterns that stick out are similar starting zone elevations, similar slope angles & the same aspects.



In the northern mountains 13 soft and hard slabs ran from the 5th to the 6th. CDOT crews released 6 slides from the Seven Sisters on Loveland Pass on the morning of the 5th. These are all north aspect slopes, steep, starting near treeline. Also on the 5th, at Abasin, control crews ski cut 2 slides on a SE aspect above treeline. From the 4th to the 6th 3 other slides were human triggered on N-E aspects from Loveland Pass to Eisenhower Tunnel. These did not run long distances, except for one in Sheep Creek that ran about 1,000 vertical.

Finally, as we all know, a snowboarder triggered, was caught, & killed in a large hard slab off Mines Peak above Berthoud Pass. He was the second person to ride the path from a party of three. Reports indicate that there were several tracks along-side the path from the day before. For some reason the victim had left his beacon in the car, while the other two members of his party had their beacons on. The victim was found by a random probe from other skier/riders that were in the area at the time of the accident. A second, also large slide was trigged by one of the approaching rescue groups.

Similar features can be found from these northern mountain incidents to those from the central mountains. Most (except Abasin) were on N-NE aspects. All started above treeline in the 12,000 to 12,200 foot range. All were on fairly steep terrain, around 35 to 40 degrees. Almost all ran to the ground.



Weather and snowpack inputs tell of old snow across the north and central mountains that fell in October, had become faceted at & near the ground, with not much wind for redistribution. There was a report of an ice crust acting as the initial bed surface in the Yule Pass/Purple Ridge slides. On the 5th official reports had 6 to 7 inches of new snow around the Summit County area (Only two reports). Though one snowstake south of Breckenridge had 9 inches new. This snow had a low water content (Light and fluffy). It also came with a big wind event (winds from the SW). Reported wind gusts were into the 40s, & estimated wind gusts at ridgetops were 50s at the least. We have very little incoming data, so little weather data to relay from this last event. The strong SW winds, new low density snow, & early season enthusiasm certainly played a role in all the above incidents.



Where are we now?



Early season snows have left us with a typical Colorado weak foundation on the colder NW-N-NE aspects near and above treeline. Strong recent SW winds have built hard & some soft slab on these colder aspects. These hard slabs are capable of supporting a person where they are deep. But where the snow depth thins out a person will likely break through to the weak underlying layers & it then becomes a possibility that the weak layer will fail, & could propagate failure long distances, creating large avalanches, even though the overall snowpack is not deep. That is one characteristic of hard slabs. They can store a great deal of energy, for some time, allowing fractures to propagate long distances when fracture occurs.

Current warm temps will act to settle and strengthen the newer snow, but will have little to no impact on the older, weak snow layers below. Most snow that could be moved by wind has done so. So we suspect there will be little snow/wind transport going on until the next storm cycle. BC users will have to make on site determinations of this if out on a tour though. The cycle has run its course. But, given the nature of the weak foundation, we can not rule out further trigged slide activity in the near future. So continue to use wise BC protocols.

The big weather picture (from models) puts Colorado under a split flow. This means the storm track is well to our north, and to our south. In the short term, a little bit of new snow could fall over the next 36 to 48 hours, favoring the northern mountains. By Friday we will be issuing full reports, though our incoming data will still be very limited.



In the southern mountains, little snow is on the ground. But tracks have been noted off the higher peaks on northerly aspects. We suspect a similar weak base layer growing in the San Juans, only missing the slab ingredient from their neighbors to the north.



Toepfer