Monday January 22, 2018
Wind Slab, Storm Slab
Travel & Terrain Advice
Don’t do it! Widespread evidence of natural avalanche activity is a great indicator of significant snow instability. We just witnessed a massive blizzard that dumped huge quantities of snow (90-120 cm fell overnight Saturday-Sunday alone with massive winds). This snow will take significant time to gain stability and strength. Slight reprieve from the torrent on Monday is no excuse to stick your neck out into the GNAR. Just because you can see it, does not mean you should ski (snowboard, sled) it! Stick to low angled treed slopes, heck that is where all the soft pow is anyways, the big open slopes and bowls are all wind pressed. Moderate snow fall will continue to load and destabilize slopes during this forecast period with the larges gains Tues into Wed morning. Strong winds from the south east to south west have and will continue to make north west to north east slopes in the alpine, treeline and open below treeline extremely dangerous.
Wide spread naturally triggered avalanche activity observed Sunday from multiple locations in the forecast area. Slab avalanche activity reported up to size 3 mainly on lee, north east to north west slopes in the alpine, treeline and open below treeline elevations. These avalanches were deep, big, wide and were running far into the runout zones. Would expect some even bigger activity in the extreme alpine up to size 4, but no reports to confirm that (might have to wait till spring to see the destructive evidence).
Extreme to strong south east to south west winds have and will continue to transport snow to lee (north east to north west) aspects in the alpine, treeline and open slopes below treeline. This transport has and will continue to form wind slabs that are almost certain to trigger naturally and with human loads. These avalanches have the potential to be widespread on lee aspects in avalanche terrain up to size 2 and could produce isolated slides up to size 3.5.
Massive amounts of new snow (and more on the way) have loaded all aspects and elevations. The new snow may create slabs that will likely trigger naturally and will certainly trigger with human loads. These slabs have potential to produce up to size 3 avalanches in isolated terrain features and up to size 2 in specific terrain.
Well if this is La Nina, then she is back with a vengeance! 90- 120 cm of new snow fell over night Saturday into Sunday. This snow has significant weakness and has already shown naturally triggered signs of deep failures/shears. These failures maybe due to the slight temperature spike that accompanied the mid point of the storm. Strong to extreme (up to 200 kph) winds accompanied this huge snowfall and have resulted in loaded lee slopes (north east to north west). Open zones in the alpine, treeline and below treeline have firm wind pressed slabs overlying very soft weak new snow (hard over soft = bad!). Wind sheltered zones in forest cover are still soft and wonderful below treeline.
|Surface||Wind pressed in open, soft in sheltered|
|Upper||Weakness due to density changes (driven by slight rise in temp and freezing level) in the new storm snow.|
90- 200 cm of new snow has fallen during the past forecast three days. This snow (majority falling Sat-Sun night) fell with south east to south west winds. The winds have been moderate to extreme (hurricane force) at times. Temps in the hills were mainly around -3 to -5 with a short lived spike from 0 to -2 during the onslaught of the storm.
Crazy but true… More snow will fall during the next three days with the biggest accumulations coming Tues into Wed morning. A slight reprieve Monday will see light-moderate snowfall with light to moderate winds, and then here we go again Tuesday…. but not another super blizzard at least! The Tues-Wed storm will see increasing winds and a slight temp and freezing level spike like the Sunday’s blizzard.
Monday 7-20 cm of new snow with light to moderate winds from the south to south west. Freezing levels 600-800 m
Tuesday 20-40 cm of snow with strong south east winds (some extreme winds for the north island). Freezing levels 700, climbing during the height of the storm to 1000-1200 m.
Wednesday another 15-40 cm of new snow falling at the tail end of the storm with winds strong, tapering to moderate south east. Freezing levels 1000 m dropping back to 600 m as the storm passes.
Posted on Monday January 22, 2018 by Bill Phipps