Rain vs. Snow: the Battle Continues
With avalanche problems highly contingent on the outcome of this battle, Tuesday is a day to travel in initial assessment mode. Be ready for active wet loose conditions at lower elevations until temperatures drop and bring your guard up as you encounter dry snow accumulations. Treat danger as HIGH if you encounter 30 cm or more.
A transition from rain to snowfall as temperatures cool through Tuesday will leave us with a new snow instability of uncertain scale.
With 15 or more cm of new snow, expect storm slabs to be an issue on steep slopes on all aspects and large wind slabs in loaded areas. With lighter amounts a smaller wind slab problem specific to wind loaded leeward areas will be more likely.
Heavy rainfall will sustain serious wet lose avalanche potential overnight but this potential should diminish rapidly with cooling temperatures during the day Tuesday.
Numerous natural wet loose releases were observed on Monday at treeline and below as rain saturated our recent storm snow. Alpine observations are lacking, however with precipitation falling as snow, higher elevations likely experienced a natural avalanche cycle during the most recent storm pulse.
On Saturday explosive control on the east side of the island triggered a size 1.5 wind slab on a north aspect at treeline. This avalanche ran on the crust from late January.
20-50 mm of precipitation is expected to cover the region by end of day Tuesday, heavily favouring the south and west of the island. Below about 1400 m it is likely to fall as rain before finishing with 5-15 cm of new snow on Tuesday, increasing with elevation.
The new snow adds to (while rain erodes) as much as 70 cm already accumulated above 700 m with ongoing southerly wind impacting open terrain.
This snow has fallen above a melt-freeze crust. The crust is up to 10 cm thick at lower elevations and tapers down to 1 cm thick on shaded or high elevation terrain. Other than a few cm of softer snow immediately below this crust, the remainder of the snowpack is strong and well bonded.
Monday night
Continuing snowfall bringing another 20-40 mm of precipitation, mostly as rain. Strong to extreme south winds. Freezing levels reaching 1500 m.
Tuesday
Cloudy with easing snowfall bringing another 10 to 20 cm of precipitation, increasingly as snow. Heaviest precipitation on the south island. Moderate to strong southwest winds. Treeline high temperatures around 0, falling over the day as freezing levels drop to 700 m.
Wednesday
Clearing with scattered flurries bringing up to 5 cm of new snow. Light southwest winds shifting southeast and increasing in the evening. Treeline high temperatures around -2 with freezing levels to 800 m.
Thursday
Cloudy with continuing snowfall from overnight bringing 10-20 cm of new snow, including overnight amounts. Strong to extreme south winds. Treeline high temperatures around 0 with freezing levels rising to 1400 m.
More details can be found in the Mountain Weather Forecast.
