Monday February 22, 2021
Wind Slab, Storm Slab
Travel & Terrain Advice
Well we hope you got out to enjoy the powder Friday, Saturday or early Sunday (before the warm up). We have got a few reports of folks playing in some epic powder in sheltered lower angle treed terrain.
The big question is what elevation did the Sunday night rain event make it up to? Areas where the rain saturated the upper snowpack will quickly freeze up as temp begin to drop significantly Monday morning limiting concerns. High zones where the rain did not get to will certainly be primed to avalanche as we now have wet heavy snow loaded over older cold dry snow. Be very careful once you transition into the non rain affected elevations.
Winds will transport new snow that falls Monday onward. Study how much new snow you encounter to assess the avalanche danger (potential size of avalanches one may experience). Snow fall amounts are forecast to be moderate for the north island and mid west coast zone, while the rest of the island has very little predicted. The new snow will be easily moved by winds as it will bond poorly to our new crust formed after the rain event. Be on the look out for zones of wind slab, as they will bond poorly to the new rain crust.
We have no reports for the extent of the storm damage for late Sunday at the time of writing (Sunday night), but one can expect there to be widespread loose wet avalanche activity below treeline and up into the lower treeline zones. For the alpine we may even get reports of widespread slab activity…. We will try and update the bulletin as data flows in.
Mt Washington patrol did report widespread loose wet avalanche activity in the area up to size one during the day Sunday.
Saturday a few reports came in mentioning wind slabs up to size 1.5 being triggered by human travel on N aspects.
New snow this forecast period will fall on a crust forming after Sunday’s rain event. This new snow will most likely have a poor bond to the crust and will be easily transported by the forecast winds, resulting in wind slab formation.
Location: treeline and alpine elevations on NE to SE aspects.
Likelihood of triggering: likely with human triggers and possible with natural activity.
Wet snow falling Sunday night into early Monday morning at high elevations will cap colder dry snow below. This upside down snowpack creates a potential for avalanche activity early in this forecast period.
Location: High alpine elevations that escape the rain event. All aspects. Greater potential for the tall north island mountains.
Likelihood of triggering: very likely to human activity and likely with natural events occurring.
A rain saturated upper snowpack is spread island wide (only very high north locations may have escaped the rain, but they will have wet snow instead). Expect to find evidence of a widespread loose wet avalanche cycle, that will freeze into fields of frozen nasty debris. Higher elevation wet snow will sit on colder dry snow (upside down snowpack). The good news is this wet snow that will freeze up will make our persistent weak layer (now down 80 to 120 cm) next to impossible to trigger.
|Surface||Rain saturated or wet snow (freezing as temps drop Mon). Some wind affected wet snow at upper elevations.|
|Upper||Moist snow over some colder dry snow|
|Mid||Well settled snow with a few crusts above our old persistent weak layers|
Good quality snow with cold temps were found Friday and Saturday. Sunday temps began to climb and light to moderate amounts of rain started to fall as early as midday at low to mid elevations. Guess it was only a mater of time before we had one of our typical island high elevation rain events. It was amazing while the cold snow lasted, and I am sure it will be back again soon.
Rain and some wet snow falling Sunday night. Monday we return to the cold snap with light to moderate snow falls Monday into Tuesday and some strong winds. Things settle down Tuesday into Wednesday with the next snowstorm on the horizon for Wednesday night.
Monday: A trace to 7 cm of new snow fallowing the rain event. Winds moderate to strong SW. Temps for 1500 m -2 to -6. Freezing level 1000 dropping to 500 m.
Tuesday: a trace to 13 cm of new snow (the greater amounts for the north island). Winds moderate to strong SW to NW. Temps for 1500 m -10 to -6. Freezing level 500 o 800 m.
Wednesday: 0 to 3 cm of new snow. Winds variable direction light to moderate. Temps for 1500 m -5 to -6 climbing to -2 early in the evening. Freezing level 0 to 700 m. The next significant snow fall is forecast to begin Wednesday night.
Posted on Monday February 22, 2021 by Bill Phipps