Avalanche Forecast South Rockies

Tuesday 12th February 2019

Avalanche Danger Ratings Tue 12th Feb 5:20PM Danger Ratings Alpine: High Danger Ratings Treeline: Considerable Danger Ratings Below Treeline: Considerable Storm Slabs Storm Slabs Loose Dry Loose Dry Persistent Slabs Persistent Slabs

Avalanche Canada Forecaster: cgarritty

Date Issued:

Valid Until:

Our new snow won't form a strong bond with the surface over the short term. Expect widespread instability in the new snow, with danger increasing as it forms storm slabs under the effect of wind and with settlement.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

Tuesday night: Cloudy with continuing flurries bringing about 10 cm of new snow. Light to moderate southwest winds.Wednesday: A mix of sun and cloud with easing isolated flurries and a trace of new snow, with new snow totals around 30 cm. Light west winds. Alpine high temperatures around -12.Thursday: Sunny with increasing cloud and flurries beginning in the evening. Light southeast winds. Alpine high temperatures around -10.Friday: Cloudy with continuing scattered flurries and a trace to 5 cm of new snow, with new snow totals of 5-10 cm. Light west winds. Alpine high temperatures around -9.

Avalanche Summary

Preliminary reports from Tuesday showed numerous loose dry and storm slab avalanches releasing within the height of new snow, upwards of 20 cm deep. These occurred naturally as well as with ski cutting and skier traffic. Expect a continuation of this type of activity on Wednesday.A report from the Flathead on Monday described continued observations of large whumpfs at 1700-1800 metres. This can be attributed to collapsing of the large, weak, January 17 layer of surface hoar. Collapses of this type on a sufficiently large and steep slope can be expected to produce persistent slab releases.A social media post from the Marten area of Elk Valley on Thursday described touchy persistent slab conditions, with lots of smaller slabs releasing over the mid-January surface hoar layer. Check out the post here. Of note is the fact that this area was previously untracked, leaving the mid-January layer undisturbed.

Snowpack Summary

Up to 30 cm of new snow has buried a variable surface of heavily wind affected old storm snow at alpine and wind-exposed treeline elevations, while adding to 25-35 cm of lower density storm snow from last week in sheltered areas above 1700 metres. In these sheltered areas, this older storm snow may cover a layer of weak, feathery surface hoar crystals. Below 1700 metres, the new snow buried a thinner cover (5-10 cm) of the same storm snow that instead overlies a melt-freeze crust.The mid-January layer of surface hoar or a crust is now buried around 55 to 65 cm deep. The surface hoar is found on shaded and sheltered slopes and is most prominent between 1600 m and 1900 m but has been found up to 2200 m. The melt-freeze crust is found on south aspects at all elevations. This layer was the subject of a recent Special Public Avalanche Warning.The middle of the snowpack is generally consolidated. The bottom half of the snowpack is unconsolidated and composed of weak and sugary faceted grains. The basal snowpack is becoming even weaker under prolonged cold temperatures, especially in thin snowpack areas.

Storm Slabs Storm Slabs

Likelihood

Very Likely

Expected Size

1 - 2

Up to 30 cm of new snow fell over the region on Tuesday, with snowfall forecast to continue through Tuesday night. Reactive storm slabs are forming in areas affected by wind and as the new snow settles and consolidates.

Use conservative route selection, choose moderate angled and supported terrain with low consequence.Anticipate more rapid slab formation in overhead terrain.

Aspects: All aspects.

Elevations: All elevations.

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Loose Dry Loose Dry

Likelihood

Certain - Very Likely

Expected Size

1 - 1.5

Loose snow avalanches are an increased concern for Wednesday with upwards of 30 cm of new snow in the region. Terrain traps and careless group management can greatly increase the consequences of loose dry avalanches.

Be aware of party members below you that may be exposed to your sluffs.Be cautious of sluffing in steep terrain.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

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Persistent Slabs Persistent Slabs

Likelihood

Possible

Expected Size

1.5 - 2.5

New snow has increased the load on a weak layer of surface hoar already buried 55-65 cm deep. This layer has been reactive to human triggers, particularly in undisturbed areas. It is best preserved in shaded, sheltered areas between 1600-2200 m.

Watch for signs of instability such as whumpfing, or cracking, especially from 1600-2200 m.Be aware of the potential for large avalanches due to the presence of buried surface hoar.Increase caution in open features at lower elevations, such as cutblocks, gullies, and cutbanks.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

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