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Valid Until: Saturday 04 February, 2012.
Confidence: Fair. The coming weather pattern looks quite predictable but freezing levels are less certain. Watch temperatures on Friday as a rapid rise could cause instability in the snow. As the west coast received much more precipitation recently than the central, north and east island hazard there is higher than it is on the east and north island. Ratings above are median values. Check local conditions.
Main Concerns: (Avalanche problems)
Wind Slab - While the north, east and central island alps received only 10-20cm of new snow, the west coast got about 60cm. This snow came with winds strong enough to move the new snow to form slabs on west through north to north east aspects at tree line and in the alpine. These slabs could be triggered by the weight of a human especially in the alpine causing avalanches big enough to bury, injure or kill a person. Whereas mild temperatures will promote settlement in these layers at first on Thursday, if the rise in temperature is too rapid overnight to Friday, as now appears possible, this could cause instability in the snow.
Storm Slab - In areas of higher precipitation storm slab has developed with variations in temperature causing density changes within storm snow. This will remain triggerable by humans certainly in the alpine and possibly at tree line on Thursday. Whereas mild temperatures will promote settlement in these layers at first on Thursday, if the rise in temperature is too rapid overnight to Friday as now appears possible, this could cause instability on the snow.
Cornices - Cornices have continued to build and are a hazard in their own right but can also act as very heavy triggers for large avalanches in the alpine in particular but also possibly at tree line on west through north to east aspects.
The west coast mountains on the island saw about 60cm of new snow since Monday whereas the central, east and north island got only 10-20cm. The snow came with moderate winds from the east through south to south west and from the north west on Wednesday. Freezing levels stayed well below tree line except for a short rise to near tree line on Wednesday afternoon. The outlook is as follows:
Thursday: No new snow. Moderate south west winds rising possibly to strong in the alpine overnight Thursday/Friday. Freezing level around 700 to 1000m and rising rapidly possibly to as high as 2000m or more overnight Thursday/Friday.
Friday: A few cm of snow. Freezing levels dropping from their overnight spike back to 1000m or below. Winds light and variable but mostly from the west.
Saturday: No new snow. Light north west winds. Warm temperatures and strong solar radiation.
Wind slabs and storm slab mostly on the west island in areas of higher precipitation will continue to settle with warm temperatures on Thursday. If temperatures rise as much as forecast on Friday the snow could become less stable. The mid and lower snow pack are strong.
Small skier triggered avalanches less than size one have been observed in the recent snow at tree line.
Extra caution is advised as you move from below tree line to tree line and above into areas where wind has been able to move snow and where storm snow instabilities remain. These instabilities will become less touchy with continued settlement on Thursday but may destabalise with rapid warming overnight to Friday. Give cornices a wide berth both when traveling above and below them. As temperatures rise and the sun comes out, watch for signs of thaw instability in the snow such as moist surface snow, balling and pinwheeling.
Prepared by Jan Neuspiel