While the persistent slab problem is becoming a bit less sensitive to triggering, last week's snow and wind has activated the deep persistent slab, the potential for these large destructive avalanches needs to be accounted for in your travel plans.
A bit of new snow and strong wind is expected to form fresh wind slabs which will be most prevalent at upper elevations. Fresh wind slabs in motion could step down to the February 19th surface hoar and release persistent slab avalanches.
30 to 50 cm of settled snow rests on a widespread layer of buried surface hoar. While this weak layer is thought to be present at all elevations, these slabs have been most sensitive to triggering at treeline which is a setup that can catch us off guard. Watch for deeper slabs at ridge crest too.
Consistent dribs and drabs of snow last week accompanied by strong southwest wind has aggravated the deep persistent slab problem. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine.
Out of the frying pan and into the fire, it looks like we’re in for another series of significant storms this week.
SUNDAY NIGHT: Freezing level around 1000 m, strong to extreme west/northwest wind, 1 to 5 cm of snow possible.
MONDAY: Overcast, freezing level rising to about 1000 m, strong west/northwest wind, 1 to 3 cm of snow possible during the day, 3 to 10 cm of snow possible Monday night.
TUESDAY: Broken cloud cover, freezing level around 800 m, moderate southwest wind, 1 to 5 cm of snow possible during the day, 1 to 5 cm of snow possible Tuesday night.
WEDNESDAY: Broken cloud cover, freezing level beginning at valley bottom rising to 600 m, moderate west/southwest wind, trace of snow possible.
It's been an active period for avalanche activity:
On Saturday a human triggered size 1 was reported on a northeast facing slope at 1500 m, failing on the Feb. 19th surface hoar. There was a great MIN report from Ashman too that shows some shallow slab avalanches.
On Friday a natural cornice failure produced a size 3.5 deep persistent slab on a northeast facing feature at 1800 indicating that the deep persistent slab problem continues to be active. Human triggered wind slab avalanches up to 30 cm in depth were running on the February 19th surface hoar too.
On Thursday natural avalanches to size 2 were observed running on the February 19th surface hoar.
Some very large and destructive avalanches have been sporadically running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. On February 17 and 18 there were three large (size 2.5-3) deep persistent slab avalanches in thin rocky terrain. Steady loading last week once again aggravated these deeper layers.
A few cm fell Sunday which may preserve yet another layer of surface hoar.
30 to 50+ cm of settled snow overlies the Feb 19 surface hoar in sheltered locations and a sun crust on solar aspects with the deeper slabs being found in the north of the region. The alpine has been heavily wind affected as evidenced by this MIN submission. The surface hoar may be most sensitive to human triggering at treeline which is a bit different setup than we're used to.
There are few different persistent weak layers (PWL's) we're watching in the mid and lower snowpack. A patchy layer of small surface hoar crystals and a sun crust both from early February are now 40 to 100 cm deep. A couple of weak layers that formed in January are buried in proximity to one another 80-140 cm below the surface. Below that, an early season crust lurks at the base of the snowpack. Some large avalanches have been triggered on these layers, usually during periods of rapid loading by new snow or wind and by large loads or from thin, rocky areas. We're now in another one of those periods that could wake up the deep slab.
