On Friday, April 17th two very large slab avalanches were triggered remotely on Tincan’s CFR ridgeline. One avalanche released on the North side of the ridge, reported to be ~D3 in size, and one released on the Southwest side of the ridge, D2.5 and possibly a 3 as well. Crown thicknesses were in the 2-5′ range.
A storm moved in literally minutes after the avalanches occurred and neither a crown profiles or investigation was able to be performed the following day. CNFAIC staff was able to speak with several people who were in the area and came across the slides only minutes after they released, but no one witnessed the avalanches in motion. We were not able to speak with the party that triggered the avalanches. This party of 4 snowboarders believed to be from Switzerland, we are assuming saw the avalanches.
The avalanches were triggered as the group of 4 snowboarders rode down the CFR ridge. It was reported that the North slide broke right at one the snowboarders feet but no one was caught. It is unclear what avalanche released first and how far up the ridge, into the Tincan Chutes, the North slide propagated. Reports were that debris could be seen coming out of the chutes. See the google image for a general idea of where the crowns were located.
After the slides were triggered, the group of 4 were slowly making their way down the ridge when a party of two skied down, saw the avalanche and immediately began questioning the snowboarders and looking for other parties in the area. At the same time another group of 4 was skiing down Common Bowl and noticed the slide. Another party of 6 also was on scene relatively soon. There was also another part of 2 that saw the aftermath. Many of these folks took photos and some of those are below while all of them are on separate observations. Since the Southwest avalanche was on the side with people skiing/boarding, that is the area they focused on.
Both the party of 4 and the party of 6 who came across the Southwest slide (CFR Bowl) performed beacon searches at different times; this was not the party of 4 that triggered the slide but another party of 4. There were some down tracks and at least one up track that was covered by debris. There was no signal found by either group and later that night it was determined there were no missing persons. KUDOS to these folks for performing a search!
The party of 2 on CRF ridge had skied the North side 1.5 hours earlier and their tracks and transition spot were taken out. They were aware of the groups in the area and felt confident that no other persons were on the North side. This turned out to be true.
AVALANCHE DETAILS (ESTIMATED):
Southwest slide CFR
Crown Depth: 3-5′
Vertical: 700′ (runout 1,500′ or more)
Debris field: roughly 1000’x100′
AVALANCHE DETAILS (ESTIMATED):
North slide, Tincan Chutes
Elevation: crown extended from 2,500-3,000′
Crown Depth: 2-4′
Width: ~1,500′ ? low visibility
Vertical: 800-1,000′ (runout 1,500′ or more)
Debris field: N/A
With little going on weather wise (snow and wind) there was distinct warming of the snow surface noted by each person we spoke with. The sun came out and people noted a green house effect occurring prior to the slide and before the storm hit. It is likely that this warming played a role in increasing the sensitivity of these slabs over the weak layer. Unfortunately, there are a lot of unanswered questions and it brings a new respect for unusual winters and the springtime transitions period from winter to summer. A bit more on weather and snowpack below.
Precipitation: The last precipitation event was 19" in 19 hours, ending at 6am on Tuesday April 14th.
Temperature: On April 17th, temperatures were steady in the low 20's F at the Sunburst weather station all day (3812') while the SNOTEL site (1880') jumped from 28F to 40F from 7am - 12pm and dropped slightly to 38 at 2pm (time of avalanches). Warming at the mid-elevations.
Wind: Ridgetop winds from Sunburst weather station averaged 13mph from the East on the 16th. On the 17th, the Easterly winds began to pick up and by 2pm they were averaging 23mph with a gust to 45. These winds were the leading edge of a storm heading in. There were reports of strong wind in certain areas of the Pass at this time but the wind was only beginning to impact the CRF area by 2pm.
~3,000' (Common Bowl area): Dry snow becoming damp by 2pm with sun and green house effect by thin cloud cover
~2,600' (Elevation of avalacnhe crowns): Damp snow becoming wet with a thin and soft crust on the surface
~2,000' : Snow became wet
We do not have snowpack data for this slope. But, we are suspect that due to close proximity (one ridge over) and similar elevation and aspects (only 500' vertical higher), that the snow structure is similar to the Eddies ridge area and the avalanches that occurred there on April 11th. That observation with a profile can be found at: https://www.cnfaic.org/observations/eddies-49/.
In short, there has been 4-7' of snowfall since the beginning of April that has settled to 2-5'. This snow fell on a variable surface that consisted of a stout melt-freeze layer at elevations below 2,500'. There are areas where a thin (1-3") layer of moist rounding facets sits sandwiched between the warm sticky slab and the hard crust. These facets were the culprit in the Eddies avalanches and may be the culprit in these slides.