Avalanche Forecast
Issued: Apr 16th, 2019 4:55PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Loose Wet.
cgarritty,
Avalanche Canada
Overnight winds are expected to keep slab problems firmly on the radar for Wednesday. The crust under our new snow hasn't been a great bonding surface and wind loading won't help matters.
Summary
Confidence
Moderate -
Weather Forecast
Tuesday night: Increasing cloud with a chance of isolated flurries. Winds increasing to moderate or strong from the southwest. Freezing level remaining near 1700 metres.
Wednesday: A mix of sun and cloud. Moderate to strong southwest winds. Alpine high temperatures around 0 with freezing levels peaking at 2200 metres and rising overnight.
Thursday: Cloudy with sunny periods. Moderate to strong southwest winds. Alpine high temperatures around +5 with freezing levels rising to 2800 metres, remaining elevated overnight.
Friday: Cloudy with showers bringing 2-7 mm of rain to mountaintop. Strong southwest winds. Alpine high temperatures around +6 with freezing levels dropping from 2800 to 2400 metres over the day.
Avalanche Summary
An observation from the Waterton Park area describes a very large (size 3) deep persistent slab having released with a cornice trigger at some time in the past week. This occurred on a wind loaded high (2500m) north aspect on Mt. Blackiston and initiated as a wind slab before 'stepping down' to weak faceted snow in the basal snowpack. This should be treated as a reminder of weak basal snowpack conditions persisting in high alpine terrain.
The following activity resulting from the recent storm occurred in the neighbouring Lizard Range:
On Sunday storm slabs were quite sensitive to human triggering to size 2 on north through east facing slopes in the alpine and down into treeline. A little further down into treeline the snowpack was well bonded to the old crust, check out this MIN report for more details.
A natural avalanche cycle occurred Saturday night, the highlights were storm slabs up to size 2 on northeast facing alpine terrain around 1700 m. A natural cornice failure was observed from a north facing ridgeline which subsequently triggered a size 2.5 storm slab involving the new snow. We received a great MIN report of a small storm slab from Saturday morning.
Snowpack Summary
Saturday night's storm delivered 10 to 20 cm of new snow, making for 20 to 40 cm of accumulation in the last week. Collectively, this snow sits above a widespread supportive crust. Steep, north facing, alpine terrain may still hold a cold, dry, snowpack where fresh storm slabs are expected to be widespread. Steep north facing terrain is also harboring a deeply buried layer of facets. Although unlikely, human triggering of persistent slabs on this layer may still be possible in rocky alpine terrain with a shallow or variable depth snowpack.
It looks like it is going to warm up this week and all the new snow will likely produce a widespread and powerful loose wet cycle. During the heat of the day, especially under direct sun, the snow surface will become moist or wet almost everywhere (except for high elevation north) and loose wet avalanches could run far.
Below treeline the snowpack has melted or is isothermal.
Problems
Storm Slabs
10 to 20 cm of storm snow rests on a widespread crust. The "crust free" exception may be high elevation north facing alpine terrain. Storm slabs may remain sensitive to human triggering, especially in wind loaded terrain.
- Avoid freshly wind loaded features near ridge crests, roll-overs and in steep terrain.
- Look for signs of instability like whumphing, shooting cracks, and recent avalanches.
- Forecast strong winds will promote the formation of new slabs on leeward (north to east) slopes.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Sunny breaks combined with recent storm snow may allow for continued loose wet avalanche activity. The likelihood of loose avalanches increases as temperatures warm throughout the day and/or if the sun comes out for an extended period of time.
- A moist/wet snow surface, pinwheeling and natural avalanches all indicate a weakening snowpack.
- Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.
- Areas sheltered from wind but exposed to sun will be the most prone to loose wet activity.
Aspects: East, South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 17th, 2019 2:00PM
