Avalanche Canada ldreier, Avalanche Canada

South Coast Inland Avalanche Forecast

Jan 9th, 2020 4:00PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

The avalanche danger increases during the day with continuing snowfall and moderate to strong southwest wind. Forecast snowfall amounts vary for the region with highest amounts in the Coquihalla area. 

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain. Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

THURSDAY NIGHT: Cloudy with flurries, up to 5 cm snow accumulation, moderate southwest wind, alpine temperature -10 C, freezing level below valley bottom.

FRIDAY: Cloudy with snowfall, accumulation 15 to 30 cm with highest snowfall amounts in the Coquihalla area, moderate to strong southwest wind, alpine temperature -8 C, freezing level at 500 m.

SATURDAY: Cloudy with flurries, up to 10 cm accumulation, light southwest wind, alpine temperature -10 C, freezing level at 500 m.

SUNDAY: Cloudy with snowfall, 5 to 15 cm accumulation, light to moderate southwest wind, alpine temperature -12 C, freezing level below valley bottom.

Avalanche Summary

On Wednesday, avalanches were triggered with explosives and ranged from size 1.5 to 2.5. One avalanche of size 3 released on a deeper weak layer in the snowpack and reached a depth of 130 cm. It was triggered by explosives. 

On Tuesday, several natural and skier triggered slab avalanches of size 1 were observed. One wet loose avalanche of size 2.5 was reported. 

On Monday, a natural avalanche cycle occurred during the peak of the storm with avalanches up to size 3.5. Most avalanches released in the storm snow. However, several avalanches released on a deeper weak layer from mid November, predominantly in the north of the region. These avalanches were mostly between size 2 and 2.5.

Snowpack Summary

The storm will bring up to 5 cm of snow overnight and 15 to 30 cm during the day on Friday with the highest snowfall amounts in the Coquihalla area. Combined with moderate to strong southwest wind the deepest and most sensitive slabs will form in lee terrain features. This new snow falls on a variable snow surface which ranges from strongly wind affected to soft deep snow. The new snow will fall with cold temperatures and it is in uncertain how fast it will bond with the old snow surface. The old snowpack has not quite recovered and gained strength from the previous snow storm which had delivered between 30 and 60 cm with higher amounts of snow in the south of the region and mostly rain below 1500 m.

Deeper in the snowpack:

  • The snow loaded a touchy weak layer of feathery surface hoar in sheltered areas above 1800 m. Areas such as Manning, Henning, Stoyoma, Duffey, and Hurley should be treated as suspect.
  • A persistent weak layer is present near the bottom of the snowpack in the northern half of the region (e.g., Duffey, Hurley). The weak layer of sugary faceted grains exists around a hard melt-freeze crust from mid-November. This is an indicative snowpack setup for large and destructive avalanches. The likelihood of human-triggered avalanches decreases as the layer gets deeper but the consequence of triggering it would be severe.
  • There are currently no concerns deeper in the snowpack near Coquihalla summit.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.

Problems

Storm Slabs

An icon showing Storm Slabs

Likelihood

Very Likely

Expected Size

1 - 2.5

With up to 5 cm snow overnight and 15 to 30 cm during the day the avalanche danger will increase throughout the day. Slabs will likely be more sensitive in lee terrain features in the alpine and around treeline, where the snow will be transported with moderate to strong southwest wind. 

Forecast snowfall amounts vary within the region with the Coquihalla area receiving the highest amounts. The new snow will fall with cold temperatures and it is uncertain how fast the new snow will bond with the old snow surface. Assess the bond of the recent snow before committing to avalanche terrain and travel conservatively.

Aspects:

All aspects.

Elevations:

All elevations.

Persistent Slabs

An icon showing Persistent Slabs

Likelihood

Possible-Likely

Expected Size

2 - 3.5

Two layers that cause concern:

  • In many parts of the region, with the exception potentially being near Coquihalla summit, recent storm snow may overly a touchy surface hoar layer found around treeline elevations in sheltered areas. This layer has produced large avalanches that have propagated far.
  • In the north half of the region, a weak layer is buried near the bottom of the snowpack. This layer has been responsible for recent large, destructive avalanches.

The likelihood of humans triggering these layers remains elevated due to the recent load applied to them. Storm slab avalanches could step down to these layers.

Aspects:

All aspects.

Elevations:

Alpine, Treeline.

Valid until: Jan 10th, 2020 5:00PM