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Avalanche Forecast

Archived

Mar 1st, 2020–Mar 2nd, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

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. 

Confidence

Moderate - Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast. Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

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.

Avalanche Summary

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. 

Snowpack Summary

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.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Use careful route-finding and stick to moderate slope angles with low consequences.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.