THREE SNOWMOBILERS CAUGHT, ONE KILLED
                             IN MONTANA AVALANCHE
                                March 9th, 1996
Three snowmobilers from South Dakota were caught in a large avalanche they
triggered while riding in the vicinity of Sheep Point, located approximately 12
miles northwest of West Yellowstone, Montana.  The avalanche released in a
northeast facing bowl at an elevation of 9500 feet.  The slide was 3 to 5 feet
deep, 1200 feet across, ran approximately 800 vertical feet, and fractured
around several rock ridges and cliff bands.  The slope angles in the bowl
ranged from 35 to 40+ degrees with steeper angles in the cliff bands.
Montana has seen variable weather conditions this winter, ranging from subzero
cold to mountain temperatures in the 40's with rarely much in between.  A clear
spell in late December resulted in the formation of a widespread weak layer.
Subsequent burial of this layer in January led to a number of avalanche
incidents.  The end of January brought bitter cold, with nighttime temperatures
down to 47 below in West Yellowstone.  After a little more snow, the week of
February 11th was warm and dry.  During the week of February 18th,
approximately 3 feet of new snow fell in the mountains around West Yellowstone,
with associated strong southwesterly winds.  On February 25th, a snowmobiler
was killed in an avalanche on Lionhead, which is approximately 5 miles of
advanced snowmobile riding southeast of Sheep Point.  Very little precipitation
fell in the area during the last week of February.   Extrapolating data from a
couple National Resource Conservation Service SNOTEL sites, it looks like about
2 inches of snow water equivalent fell at Lionhead during March 3-5, with less
than 0.2 of an inch falling from March 6-9.  Field tests conducted on Saturday,
on Lionhead, indicated fairly stable snowpack conditions.  However, due to
variable snowpack conditions that existed throughout this area, the avalanche
advisory for March 9th called the snowpack instability in the Lionhead area
moderate on slopes steeper than 40 degrees, with a strong warning to avoid
upper elevation slopes near the ridgetops facing from north to east to south
that had significant wind slabs deposited on them by westerly winds that had
blown earlier in the week.
On Saturday, a large group of snowmobilers were riding in the Sheep Point area,
when three riders triggered an avalanche at approximately 1130.  One rider was
able to ride off of the debris, one rider was knocked off of his sled, and one
rider was buried approximately 4 feet deep.  We were unable to interview the
survivors, so it is unclear exactly where the people were when they triggered
the slide, and what kind of self rescue efforts were undertaken.  The victim
was eventually located by an organized search party that had been dispatched
from West Yellowstone.  The body was located by a spot probe only a few minutes
after the search party arrived at approximately 1400 hrs.  The body was located
near some trees, approximately 20 feet upslope from his snowmachine which was
on the surface of the debris.
On Monday, March 11th, Karl Birkeland and I investigated the avalanche site.
In areas along the crown face, it appeared that the avalanche failed on two
layers of faceted grains buried deep in the snowpack.  Though these layers
didn't appear to be very weak, we did find areas with weak faceted and cup
shaped crystals where the snowpack was relatively thin.  It is possible that
the the riders "hit" an area on the slope that had a relatively thin snowpack
and weak faceted crystals.  The stress applied to this weak area may have
caused local failure, which then spread to the surrounding snowpack, resulting
in the large avalanche.
If people have questions or comments, they can contact myself or Karl Birkeland
at (406) 587-6984.
Ron Johnson
Avalanche Specialist, Gallatin NF Avalanche Center
March 13th, 1996