Northwest Inland Avalanche Forecast

Mar 14th, 2020 4:00PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs, Persistent Slabs and Deep Persistent Slabs.

Strong northeast winds have formed wind slabs on atypical aspects. Watch for signs of warming and weakening snow from strong sun and rising temperatures. Buried weak layers and cornices could reach their tipping points.

Summary

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast. Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

Saturday night: Clear, decreasing northeast wind, alpine temperature -16 C.

Sunday: Clear, light northeast winds, alpine high temperature -5 C, freezing level 600 m.

Monday: Clear, light variable winds, alpine high temperature -1 C, freezing level rising to 2200 m in the southern half of the region.

Tuesday: Clear, light northeast winds, alpine high temperature 0 C, freezing level 1000 m. 

Avalanche Summary

Avalanches are expected to become more reactive with a rapid warm-up and intense solar radiation.

Over the past several days, large (size 2) human-triggered avalanches have been reported releasing in a drifted snow/weak interface combination and breaking 20-80 cm deep. These avalanches primarily occurred on leeward features above 1400 m. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below. 

Last week, there were reports of large (size 2-2.5) human-triggered avalanches failing on the March 1st surface hoar layer. Over the past two weeks, avalanches have been reported on an earlier surface hoar layer from February 19th as well as a deep persistent slab failing on basal facets. This pattern highlights how shallow avalanches in the surface snow have the potential to strain multiple weak layers in the snowpack and release larger avalanches.

Snowpack Summary

Temperatures are forecast to rise from -25 C to -5 C in the span of 24 hours and with intense solar radiation on Sunday. This significant warm-up is expected to rapidly destabilize surface snow and cornices as well as add significant strain to buried weak layers. 

Over the past few days, strong easterly winds drifted the 15-30 cm of recent snow into wind slabs in a reverse-loading pattern. Wind slabs will likely be more reactive where this snow rests on a weak layer of surface hoar and/or a crust on solar aspects. Cornices have also grown large with the recent weather, and a cornice failure could trigger a wind slab avalanche on the slope below.

Loading from new snow and wind has made several deeper weak layers problematic over the past week. A surface hoar layer from March 1st may be found 40-80 cm deep, and another combination of surface hoar and sun crust from February 19 may sit 60-90 cm deep. These layers seem to be most sensitive to human triggering at treeline elevations. 

A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack. Sunday's warm-up may have the potential to re-awaken these deeper layers.

Terrain and Travel

  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

Possible-Likely

Expected Size

1 - 2

Strong winds have drifted recent snow into wind slabs that are possible to trigger, especially in areas where surface hoar sits at the interface. A significant warming event on Sunday is expected to rapidly destabilize slabs, initiate wet loose avalanches, and bring cornices to their breaking point. This activity is likely to be most prominent on steep slopes that receive direct day-time solar radiation (slopes that face south through west). 

Aspects:

East, South East, South, South West, West, North West.

Elevations:

Alpine, Treeline.

Persistent Slabs

An icon showing Persistent Slabs

Likelihood

Possible

Expected Size

1.5 - 2.5

40 to 90 cm of settled snow rests above several layers of buried surface hoar that have produced avalanches over the past week. These slabs have been most sensitive to triggering in sheltered areas at treeline. Rapid warming is expected to increase the likelihood of triggering these layers.

Aspects:

All aspects.

Elevations:

Treeline, Below Treeline.

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Likelihood

Possible

Expected Size

2.5 - 3.5

Forecast warming may aggravate the region's 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. Cornice falls are a likely trigger for these slabs.

Aspects:

All aspects.

Elevations:

Alpine, Treeline.

Valid until: Mar 15th, 2020 5:00PM