Northwest Inland Avalanche Forecast

Issued: Mar 16th, 2020 4:00PM

Tue Mar 17th Current Conditions
Alpine Moderate Treeline Moderate Below Treeline Low
Wed Mar 18th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low
Thu Mar 19th 3 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low

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

Lingering wind slabs below ridgetops may remain reactive to human triggers. Strong solar radiation weakens cornices and often causes them to fall onto slopes and trigger large avalanches. Pay attention to what's above your head if the sun is out.

Summary

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Monday night: Clear, Moderate northeast winds, alpine low -10 C, freezing level valley bottom.

Tuesday: Sunny, light northeast winds, alpine high -2 C, freezing level 900 m.

Wednesday: Sunny, light west winds, alpine high -2 C, freezing level 800 m.

Thursday: Sunny, light southeast winds, alpine high temperature -1 C, freezing level 900 m.

Avalanche Summary

On Saturday, there were multiple reports of large (size 2-2.5) cornices failing naturally. Several of these cornice falls triggered slabs on slopes below, and one triggered a deep slab avalanche releasing on weak snow near the ground.  

 

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

Cornices have grown large with the recent weather, and a cornice failure has the potential to release very large avalanches on slopes below.

A few days ago, strong easterly winds drifted the 15-30 cm of recent snow into wind slabs in a reverse-loading pattern (check out this MIN from HBMR). 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.  

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. The warming weather 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.
  • Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

Possible

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. 

Aspects:

All aspects.

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. 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

Unlikely-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 17th, 2020 5:00PM