That Pesky Surface Hoar

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That Pesky Surface Hoar

Postby JBella » Wed Jan 20, 2016 8:32 am

http://www.rgj.com/story/life/outdoors/ ... /79038598/

While we've been seeing relatively "safe" conditions in northern New Mexico during January, we know there are widespread, buried persistent weak layers including old surface hoar that grew between the early season's storm cycles. This article brings up an important point, that surface hoar layers are not always a problem in the Sierra Nevada due to typical warming periods and wind. Often, similar environmental conditions exist in New Mexico, being as far south as we are temps can warm up significantly during Winter, affecting the bonds between layers and overall strength of the snowpack. This year that hasn't happened, precipitation has been above average and temps below average. January often brings a thaw but this year temps have been cool until two days ago when it warmed up a bit. The frequency of storms has been consistently reliable, and several surface hoar layers have been covered with fresh, dry snow as they were developing, we just haven't had the longer dry and warm periods typical to the Sangre de Cristos. This is an important factor to keep in mind, especially on northerly and easterly aspects that aren't affected by direct sunlight until later in the season. Another important point this article mentions is that it is difficult to know where surface hoar layers exist, the only ways to know are to be on the same slopes to check conditions daily, and to know how to recognize these layers while conducting snow pit analyses. We know these layers exist, and how they can affect the overall stability of the snowpack, the challenge is often to take the time to properly study and assess snowpack within localized regions. It's much easier to go on visible, more easily noticeable signs and warnings, which are not always present. This January has provided a rare opportunity to ride some lines that usually aren't safe enough until Spring, yet it is important to be aware of the lurking presence of potential deep slab instabilities. We know there has been a weak interface at the ground since early November, a couple weak layers showing up within the pre-Christmas layers, and some significant and modest accumulations since late December. The past two days, with warm daytime temps and cool nights the snowpack has continued transitioning to a mid/later-season pack, showing signs of drying up and loosing cohesion as lower layers continue the faceting process, in many areas, buried beneath heavier accumulations. This coupled with the fact that there has not been a significant natural avalanche cycle, beyond a few isolated storm slab releases, indicates that the relative safe period we've been experiencing may be subsiding. Look for weaknesses within the layered cake below the surface, seek out consistent and inconsistent results, and keep mental notes of what is observed from zone to zone while travelling in the backcountry. We know a bit about what has been happening, we know there are potential weak layers that have not released yet; the current snowpack is a classic intermountain amalgamation of varying densities and structures. As conditions have been safer than what is typical during January, I haven't seen anything to indicate the green light is on anywhere, it's been mostly in the yellow to lime-green state for the past couple weeks. Southerly aspects are holding much less snow than others but are pretty well filled in compared to most (and average) seasons and there are layers present. I would recommend to be extremely cautious, and follow standard protocol on all NW-SE aspects, especially on open, alpine terrain, like large aprons that are at the prime angle to release if a weak spot is triggered, and higher elevation bowls and chutes where hard slabs have developed during the windier storm events. Convex slopes are holding a deep pack with lots of tension above and onto rollovers, and concave slopes are holding layered weight that may not have enough compression strength holding them up. Think trigger points, along rock outcrops and trees, where slab boundaries blend with cohesionless tg snow. Study route options and consider consequences before entering avalanche terrain.
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