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Valid until Friday March 01 2013.
Confidence: Fair. Caution Thursday watch timing of rain once started it will increase hazard quickly. Precipitation amounts could vary over the island depending on the aim of the storm.
Past Weather: The Island Alps saw 5 to 20 cm of snow Monday & Tuesday. The snow came with some moderate winds initially and then light winds from north switching to south east. Freezing levels remained cool below 1000 m.
Main Concerns: (Avalanche problems)
Loose Wet - Recent snow plus new snow and then rain will combine to make the surface very unstable. This will effect all elevations and aspects. Steep terrain and tree bombs will be the first to let go naturally. Any cross slope cutting would produce loose wet slides on terrain steeper than 35 degrees. Size expected large enough to knock a person over or injure them to large enough to trigger slab.
Wind Slab - New snow and moderate wind from the South will add to the recent widespread wind slab on west through north to east aspects and in cross loaded features at tree line and in the alpine. These slabs could fail naturally once the rain starts and will remain easy to stubborn to trigger by humans through the forecast period. Avalanches could be large en to very large if they step down.
Wet Slab -Persistent Slab - A little more snow and then rain will add load to existing Storm Slabs on all aspects and at all elevations. These slabs may be triggered naturally by rain or other avalanches causing a potential 'step down' avalanche.. Human triggering may be difficult but would produce avalanches from Medium to more than large enough to bury, injure or kill a person. The persistent crust sliding layer is more than 50 to 100 cm down.
Cornice - Rain in large amounts may weaken cornices by Friday and Saturday. Cornices are found on West, North and East aspects at higher elevations. Natural triggering likely. Avalanche size from small to very large possible if the cornice triggers deeper layers.
Weather Outlook: (crappy til Saturday)
Thursday starts with snow 5 to 15cm before freezing levels spike up and rain starts 20 to 30 mm. Winds light gusting to strong from the south east. Freezing level rising from near tree line to over the mountain tops.
Friday - More rain 50 to 100 mm. Winds from light to gusting to strong and mostly from south east to south west. Freezing levels around 2000m.
Saturday - Rain 30 mm changing to snow in the AM. Winds light gusty to 40 southerly. Freezing level dropping from 2000 m to 1500 to 1000m by AM or mid day, cooler on North Island.
Snowpack/avalanche activity : A wide spread natural cycle occurred over night Sunday up to size 2 in the Mt. Cain area.|
Lighter snow caps off 40 to 80cm of last weekend's storm snow. Some shears exist in the old storm snow. A weak layer adjacent to a thin temperature crust on sunny aspects is now about 40 to 100cm deep in the snowpack. This layer delivered easy, planar results in tests on last Friday and Saturday and likely remains reactive though being so deep in the snowpack it will become increasingly difficult to trigger with human loads. Sunny aspects may have more of a crust problem deeper down. While a human load is not likely to trigger it, the load of even a small avalanche could.
Stay out of avalanche terrain in times of high hazard and until the rain stops. Stick to lower angled and/or forested slopes. Stay out from under large slopes. Be aware that wet cornices are suspect give them a wide berth. Even smaller avalanches can deliver a much heavier load to a slope than a skier; these avalanches could act as possible triggers of a deeper instability as mentioned in the snow pack discussion above. The coming forecast includes the possibility of large to very large avalanches which could run the full length of their path. Keep this in mind if choosing to cross paths even if you are very low in the run out zone. Wait until things refreeze later Saturday or Sunday before approaching avalanche terrain.
Prepared by: Niko Weis.
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