Wednesday March 21, 2018
Wind Slab, Storm Slab
Travel & Terrain Advice
Large accumulations of new snow are forecast to fall over this forecast period. The timing of the largest accumulation is unclear in the weather forecast models. Some have the large snowfall predicted for Thursday, others have moderate amounts Thursday with the bulk falling on Friday. Either way, these new snow falls will certainly result in a significant increase in avalanche hazard. Study the bond of the new snow to the crust that it will fall on. As accumulations increase remember that significant new snow and difficult trail breaking are red light signs of instability. Strong winds from the SE will also create wind slab issues on lee slopes. Avoid exposure to storm and wind loaded open slopes as well as convex and unsupported features. Careful snowpack evaluation, cautious route finding and conservative decision making will be essential to avoid avalanche danger at treeline and below, and travel in the alpine when the danger rating is high is not recommended. Don’t let the new snow stoke blind you to the dangers that will exist.
No new avalanches have been reported over the past forecast period.
Large snow accumulations with cold temps will make conditions ripe for transport by SE winds. With winds moderate gusting to strong SE, N-W aspects in the alpine and treeline will see even great accumulations/overloading and wind slab danger will certainly be a factor to heavily consider. These slabs will likely trigger naturally and are very likely to almost certain to trigger with human activity up to size 3.
Significant new snow will fall during the forecast period. The bond of this new snow to our crust will be a significant concern. This new snow will very likely create slab avalanches on all aspects and elevations with the bonding potential (to the old crust) being less the higher/colder we go. These avalanches will likely trigger naturally and are very likely to trigger with human activity and have the potential to be up to size 2-2.5.
Warm day time temps and cold nights have beefed up our crust that now exists on all aspects and elevations. This thick supportive crust will be the sliding layer of concern at the next set of storms approach.
|Surface||Thick melt freeze crust|
Warm daytime temps (up to +7 at treeline in Strathcona Park) and cold nights down to around -4, have increased the surface crust. A trace to no new snow has fallen and winds have remained light and variable (bulk from SW).
New snow! Light snow fall starting Wednesday will increase in intensity overnight with a good wallop by Thursday morning and more during the day Friday. Temps and freezing levels will slowly drop as the storm moves through (hopefully falling warm at first and bonding and then cold and light on top, the desired right side up cake!). The majority of the storm winds will be moderate from the SE, however Thursday night into Friday morning may see gust to strong increasing wind slab issues for Friday’s adventures.
Wednesday - 5-12 cm of new snow, Winds light SE rising to moderate overnight, Temps -2 to -4, Freezing level 850-1000 m.
Thursday - 6-19 cm of new snow, Winds moderate SE, Temps -2 to -7, Freezing level 1000-350 m.
Friday - 16-30 cm of new snow, Winds strong SE early morning dropping to moderate SE then light SW, Temps -5 to -7, freezing level 450 to 850 m
Posted on Wednesday March 21, 2018 by Bill Phipps