[CMCEmail] Avalanche Bulletin

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Avalanche Bulletin

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These bulletins are now issued by the Inyo National Forest
<http://www.fs.fed.us/r5/inyo/index.shtml>  in conjunction with the Eastern
Sierra Avalanche Center. The CSAC Avalanche Center (avalanche-center.org)
takes absolutely no responsibility whatsoever for the content or technical
accuracy of this information. 

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Current Keywords: CONSIDERABLE; MODERATE; avalanche
<http://www.avalanche-center.org/Education/danger/usdanger.php>  danger;
human <http://www.avalanche-center.org/Education/glossary/trigger.php>
triggered avalanches; natural
<http://www.avalanche-center.org/Education/glossary/natural-avalanche.php>
avalanche; rescue <http://www.avalanche-center.org/store/index.html>  gear;
settled <http://www.avalanche-center.org/Education/glossary/settlement.php>
; settlement
<http://www.avalanche-center.org/Education/glossary/settlement.php> ; slab
<http://www.avalanche-center.org/Education/glossary/slab.php> ; trough
<http://www.avalanche-center.org/Education/glossary/trough.php> ; wind
<http://www.avalanche-center.org/Education/glossary/wind-loading.php>
loaded; 

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Good morning, this is Sue Burak with the Inyo National Forest with an
avalanche advisory for Wednesday December 17, 2008. 


Mountain Weather


Cold weather continues all this week and into Christmas week. As one NWS
forecaster wrote last night, temperatures are cratering under clear skies.
Mountain locations will have morning lows of below zero and only warm up to
around 12-18 today. Expect cloudy skies and light snow showers today. By
tonight cold north winds will usher in the next system for Thursday night
and Friday. 

The low that produced the latest dumpage has moved west and opened the door
for more cold systems to drop south every couple of days. At this point in
time, models are forecasting 6-12 of snow for the Tahoe Basin but if the
moisture band loses strength, Mammoth Mountain and the Mammoth Basin might
be the only locations that receive a few inches of snow. Winds will pick up
over the ridgetops by Thursday afternoon but will nothing in comparison to
the 100+ mph winds on Friday evening and Saturday. 

The coldest weather of the season is due over the weekend. The pattern will
continue to be unsettled with the position of the longwave trough
<http://www.avalanche-center.org/Education/glossary/trough.php>  making it
possible for cold Canandian air to dominate our weather. There is a storm
forecasted for Sunday and Monday but we need to wait a few days to see if
our area will receive any precipitation. 


Snowpack and Avalanche Discussion


The lack of natural
<http://www.avalanche-center.org/Education/glossary/natural-avalanche.php>
avalanche activity and very limited avalanche activity on Mammoth Mountain
yesterday suggests the period of the greatest instability occurred during
the storm. The snow
<http://www.avalanche-center.org/Education/glossary/settlement.php>  settled
a few inches today which is a sign of increasing stability but trail
breaking is a wallowfest. 

I did several extended <http://www.avalanche-blog.com/item/15/>  column and
compression tests today. The extended column tests showed no results while
the compression test gave easy results on two interstorm layers. The old
snow and new snow interface did not fail. So which is the better test? 

The consensus among avalanche forecasters is that the extended column
<http://www.avalanche-blog.com/item/15/>  test gives more reliable results
than the compression test for a couple of reasons. The ECT tests a larger
volume of snow as the width of the isolated column is 90 cm compared to 30
cm for the compression test. The number of false positives or the number of
times the test result show the snow is unstable when it really isn't is very
small compared to the compression test. Just think of the number of times
your compression test gave scary results and you skied the slope anyway and
are here today to try a new and better test. I will post more information on
the ECT in the next couple of days. 


Bottom Line


The avalanche
<http://www.avalanche-center.org/Education/danger/usdanger.php>  danger
rating today is MODERATEon slopes steeper than 38 degrees. There are
isolated areas of CONSIDERABLEdanger on high elevation, north to east facing
wind <http://www.avalanche-center.org/Education/glossary/wind-loading.php>
loaded slopes. CONSIDERABLEmeans dangerous avalanche conditions- be
conservative in your selection of skiing and riding terrain and put one
person a slope at one time. The MODERATErating means human
<http://www.avalanche-center.org/Education/glossary/trigger.php>  triggered
avalanches are possible and dangerous avalanche conditions exist on some
terrain features such as steep rollovers and convex slopes where the snow is
under tension. Evaluate each slope carefully and use good travel habits.
Cold temperatures reduce the amount of snow
<http://www.avalanche-center.org/Education/glossary/settlement.php>
settlement so shaded north to east facing slopes will have the weakest snow.


Please note that the avalanche
<http://www.avalanche-center.org/Education/danger/usdanger.php>  danger
rating in this advisory expires in 24 hours. This advisory is our best
interpretation of snow pack conditions and NWS forecasts issued today.
Backcountry travelers should be aware that elevation and geographic
distinctions are approximate and that a transition zone exists between upper
and lower elevations. 

Avalanches do not happen by accident and most human involvement is a matter
of choice not chance. Most avalanche accidents are caused by slab
<http://www.avalanche-center.org/Education/glossary/slab.php>  avalanches
that are triggered by the victim of member of the victim's party. Even small
slides can be dangerous. Always practice route finding skills and carry
avalanche rescue <http://www.avalanche-center.org/store/index.html>  gear.
Remember that avalanche
<http://www.avalanche-center.org/Education/danger/usdanger.php>  danger
ratings are only general guidelines. Distinctions between geographic areas,
elevations, slope aspects and slope angles should be made. 

Another persepctive on <http://www.avalanche-blog.com/item/15/>  the
Extended Column Test

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