It's a good day to make conservative terrain choices and assess how the buried crust reacts to new snow fall. Storm slabs will be most reactive in wind affected terrain to the lee of exposed ridgecrests. Watch for signs of instability. Be cautious with any steep terrain.
A widespread melt freeze crust from the first week of December exists up to 2500m in places. New snow from Wednesday night will bury this layer in 30-55cm of recent snow. This layer has shown signs of reactivity with explosive control work, remote avalanches, and numerous snow-pit results showing easy "sudden" or "pop" results.
Recent and ongoing snow & strong to extreme winds (southwest and southeast) are forming fresh storm slabs. Watch for fat deposits in leeward terrain features such as ridge crests and roll-overs. Recent winds have been predominately from the Southern quadrant so expect wind loading to be the most prominant on northeast to northwesterly slopes. However, some areas have reported northeasterly winds resulting in the possibility of reverse loading.
Wave after wave of frontal systems bringing precipitation move through the Sea to Sky region through the weekend. Wednesday night will see more snow with the first wave. Then a brief ridge of high pressure provides cloudy but dry weather on Thursday ahead of a stronger wave on precipitation Friday and a much stronger warmer wave for Saturday. In general, highest snowfall amounts will be experienced near Brohm Ridge and taper for the mountains to the north.
Wednesday Night: Snow flurries, with 10-35cm accumulations overnight, moderate gusting strong south ridgetop wind, alpine low -4 C, freezing level 1100 m.
Thursday: Mainly cloudy with isolated flurries, trace amounts of new snow, wind easing to light southwest, alpine high -4 C, freezing level 1200 m.
Friday: Snow with 15-30 cm new snow WB 23cm/Brohm 38cm, wind increasing moderate to strong southwest, alpine high -3 C, alpine low -6C, freezing level 1200 m.
Saturday: Snow, heavy at times with WB 47cm/Brohm 55, moderate southwest ridgetop wind gusting extreme, alpine high +2C, alpine low -7 C, freezing level 2000 m.
Early reports from Wednesday indicate varied reactivity on a buried crust layer and include an explosive controlled large avalanches (size 2) on a ridge top feature near treeline. Elsewhere avalanches were small with average crown depths of 5-15cm deep.
On Tuesday there reports of numerous small avalanches (size 1-1.5) that were triggered natural, with ski-cutting and with explosives. More reactivity was noted at treeline below ridges. There were limited alpine observations due to stormy weather. However, natural and human triggered avalanches remain likely with the forecast new snow and strong to extreme southern winds.
This MIN from Blackcomb backcountry reports a large avalanche (size 2) skier accidental on a windslab on a NW slope in the alpine. This kind of avalanche activity illustrates how reactive wind slab avalanches can be during periods of strong wind and that they don't always fail on known weak layers (like crusts) but instead on density or crystal differences within recent snow. We're glad everyone was ok!
Where no slab properties are found, loose dry sluffing is being reported in steep terrain to size 1.
Cracking and other signs of instabilities were reported from Wednesday with a lack of propagation. We expect this to change as more snow is loaded onto the known weak layers. In other words, as more snow accumulates, avalanches will likely release on these weak layers where previously the cracking was merely illustrating a weak bond.
Have you been out and about in the mountains? If so please submit your observations to the Mountain Information Network (MIN). It doesn't have to be technical - photos are especially helpful! Thank you so much for all the great MINs submitted so far!
As new snow begins to accumulate, be on the look out for how new snow is bonding to storm interfaces and known weak layers (such as the crust and surface hoar).
Overnight Wednesday another 10-35cm of snow falls on top of recent 10-30cm of snow bringing total recent snowfall amount to 20-65cm. (Highest levels forecast for the Brohm Ridge region). However, this snow is heavily wind affected from strong to extreme southeast, northeast, south and southwest winds resulting in variable distribution of snow across the alpine and ridgecrests at treeline. In lee areas expect to find wind slabs and heavier snow fall accumulations. On windward slopes sastrugi, scoured areas and old surfaces may be encountered.
In wind sheltered areas, new snow has fallen on a variety of surfaces. Most notable is the melt-freeze crust from last week and in some places a buried surface hoar layer. The surface hoar is more likely preserved in sheltered locations at treeline.
This melt-freeze crust is widespread and now buried by 30-65 cm of snow at elevations above 1000 m. A MIN from Dec 15 near Metal Dome trees had test results on this crust buried 30cm. This crust was also reported in a "very easy" sudden character test pit (ECT) reported in this MIN at 1850m in the Flute/Russet/Fissile area on Dec 15. Another MIN did not find notable test pit reactivity on the crust near the Apostles.
Snowpack depths change rapidly with elevation, with 0-50 cm below treeline to 170 cm in the alpine. Below treeline conditions are slowly improving but remain variable as indicated in this MIN from Singing pass trail.
