Avalanche Forecast

Issued: Apr 15th, 2018 4:42PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs and Loose Wet.

jmcbride, Avalanche Canada

Purcells.

Winter continues with more snow and wind in the forecast. This will continue to build touchy slabs at upper elevations. While loose wet avalanches may be a concern at lower elevations.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

TONIGHT: Mostly cloudy, flurries. Accumulation 3-6 cm. Ridge wind moderate to strong, southeast. Alpine temperature near -5. Freezing level 1100 m.MONDAY: Cloudy, flurries. Accumulation 5-10 cm. Ridge wind moderate, southeast. Alpine temperature near -5. Freezing level 1700 m.TUESDAY: Mostly cloudy, flurries. Accumulation 10-20. Ridge wind light to moderate, northwest. Alpine temperature near -5. Freezing level 1600 m.WEDNESDAY: Mix of sun and cloud. Ridge wind light, south. Alpine temperature near -5. Freezing level 1700 m.

Avalanche Summary

Saturday, explosive control work in northern parts of the region produced several storm slab releases size 2-2.5 on predominantly treeline to alpine, west aspects with wide propagations up to 200 m. And ski cuts produced size 1-1.5 results on cross-loaded features.Friday several skier-triggered, size 1.5 wind slab releases were reported on north aspects at 2300 m. As well as a few solar-triggered storm slab releases (size 1.5) on steep, unsupported. west-facing features at 2200 m.Wednesday there was a report of an icefall-triggered size 1.5 wind slab (50 cm deep and 50 m wide) on a north aspect at 2400 m. On Tuesday a skier triggered a size 1 wind slab 20 cm in depth on an east facing feature at 2400 m. And on Monday wind loading produced natural wind slab avalanches to size 2 on north facing terrain around 2400 m. Solar radiation also produced natural storm slab and loose wet avalanches to size 2.5 on various aspects between 1700 and 2600 m.

Snowpack Summary

About 10-20 cm on new snow overlies a crust on all aspects to at least 2000 m (and possibly higher on south-facing slopes), while 20-30 cm can be found at higher elevations (and up to 60 cm in some areas in the northwest of the region). There are now a few different crusts within the upper snowpack, with only the mostly recently buried crust posing a concern in wind-loaded areas.There is some lingering concern around the mid-March Persistent Weak Layer (PWL) in the alpine where it is found 50 to 90 cm below the surface. This buried crust/surface hoar interface was widely reactive but has produced only isolated avalanche activity in last week. It is likely trending towards dormancy, but we need to keep it in the back of our minds as it is capable of producing large avalanches should it be triggered (think step-down from a surface avalanche or cornice collapse)Deeper persistent weak layers from December and January are considered dormant at this time.

Problems

Storm Slabs

Snow and wind Sunday night and Monday will likely form touchy storm slabs in the alpine and at treeline.

Use caution on leeward and cross-loaded terrain near ridge crests.Choose well supported terrain without convexities.Be cautious of sluffing in steep terrain.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Loose Wet

Warming temperatures, rain or bit of sun may destabilize surface snow and initiate loose wet avalanche activity, especially on steep south facing features. The likelihood of cornice failures also increases with sun and warming.

Cornices become weak with daytime heating, give them a wide berth when traveling above or below.Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.

Aspects: North, North East, East, South East, South.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

1 - 2

Valid until: Apr 16th, 2018 2:00PM