Wind slabs and wind loaded terrain continue to be a problem in the upper elevations. The mountains have picked up another 17-24 inches of snow this week with winds gusting into the 40+mph range. More snow and wind arrived yesterday and more is forecasted for today. The new snow combined with strong winds out of the south and southwest today will make areas that are already suspect even more sensitive. These sensitive wind slabs may be found on southeast, east, northeast, north, northwest,and west facing terrain. Wind slabs are widespread in the upper elevations and range in sensitivity from easily triggerable to resistant. They also range in size from a few inches to a few feet in some areas. New snow over the last few days has also done a great job camouflaging some of the wind slabs formed earlier this week, so you may not see any visual clues. These slabs range in density between soft to hard which means they may let you get well out onto them before they fail. We have been intentionally triggering relatively large avalanches with relatively small cornice bombs in these areas which means they are more likely to fail under a skier or snowmobiler. Also, some of these wind slabs may be resting on a layer of weak, faceted snow or surface hoar that was buried last week. Your best bet until our snowpack begins to 'heal' is to avoid skiing or riding on or below steep, wind loaded slopes.
While time heals everything, our current persistent weak layers are doing so slowly. Several different weak layers formed on or near the snow surface during the last period of high pressure and are lurking in our snowpack right now. These persistent weak layers have slowly been covered over the last 8-10 days and have failed in some areas, and are still waiting for a trigger in other places. These layers include buried surface hoar and near surface faceted snow on shadier aspects and a series of unsupportable and supportable crusts on the southerly (solar) aspects. If you take 5 minutes to dig into the snow right now, you can see exactly where these layers are, they are pretty obvious in the snowpack. Buried surface hoar and faceted snow have the ability to propagate over large areas when they fail and are responsible for most avalanche incidents and fatalities. These are photos showing the depth of the buried surface hoar as it failed in our propagation tests yesterday. Test results ranged slightly in the moderate levels in both shear and propagation testing on a 25 degree NNE slope. All tests failed with Sudden Collapse of the weak layer with very well preserved 8+ mm surface hoar visible throughout the layer. The slab overlying this layer has become very firm this week with the warming temperatures and would be a nasty grinder if you were caught in the debris. Because of the random distribution of this weak layer, it is very hard to predict exactly where you will trigger it.
We saw several "generations" of slides yesterday as we toured the steep drainages and mountain lake basins north of Goose Lake yesterday. Some of these were older natural release slides that happened early this week, some were more recent and had very little new snow covering up the crown and bed surfaces. There were also several new slides that were triggered over the last 2 days, one was a very large slide that was intentionally triggered by a professional ski guide ski cutting a steep NNW rollover. The layer responsible for the slide in that area appears to be the buried surface hoar that we found reactive in our pit tests yesterday and have been discussing through the last 2 weeks. We also saw a lower elevation slide that was triggered by a snowmobile playing on the roadcut on the Goose Lake Rd. near Clow Point. That smaller slide also failed on the layer of buried surface hoar. We noted that in the older slides we saw yesterday that the crowns were very wide or had propagated easily over a very long distance through complex terrain which is what happened on the ski cut triggered slide near Brundage Reservoir. This buried layer of surface hoar DEMANDS your respect right now, it is buried up to 2 feet deep in places below a dense slab of new and windblown snow. Where it did not release naturally, it is still waiting for a tigger.
TODAY THROUGH SUNDAY...AN UPPER LEVEL LOW IS JUST
COMING ONSHORE IN WRN OREGON AND NW CA THIS MORNING. IT WILL
WEAKEN AS IF MOVES ENE ACROSS THE REGION TODAY THROUGH TONIGHT.
THIS WILL HELP SPREAD RAIN/SNOW TO ALMOST ALL OF THE AREA TODAY
WITH DECREASING AMOUNTS TONIGHT AND SUNDAY. A WEAK SECONDARY
SYSTEM WILL MOVE IN IMMEDIATELY BEHIND THIS FIRST SYSTEM...
BRINGING SNOW TO HARNEY COUNTY LATE SUNDAY AFTERNOON. AMOUNTS FOR
BOTH SYSTEMS WILL BE RELATIVELY LIGHT...AND NO ADVISORIES ARE
ANTICIPATED. TEMPS TODAY WILL BE MILD AS THE REGION BEGINS THE DAY
WITH SW FLOW ALOFT IN ADVANCE OF THE INCOMING SYSTEM. TEMPS WILL
COOL SEVERAL DEGREES TOMORROW COMPARED TO TODAY...BUT WILL REMAIN
A FEW DEGREES ABOVE NORMAL. SNOW LEVELS WILL FALL FROM AROUND 5000
FEET TODAY TO 3000 FEET TOMORROW AFTERNOON. WINDS WILL BE OUT OF
THE EAST THROUGH SOUTH TODAY...AVERAGING 10-20 MPH WITH GUSTS TO
25 FROM MOUNTAIN HOME EAST INTO THE WESTERN MAGIC VALLEY...AND 5
TO 15 MPH ELSEWHERE. A COLD FRONT WILL MOVE THROUGH TONIGHT...
BRINGING NW WINDS TO MOST OF THE REGION FOR SUNDAY. WINDSPEEDS
WILL BE SIMILAR TO TODAY...WITH THE STRONGEST WINDS AGAIN FROM
MOUNTAIN HOME EAST INTO THE WESTERN MAGIC VALLEY.
This avalanche advisory is provided through a partnership between the Payette National Forest and the Payette Avalanche Center with help from the Friends of the Payette Avalanche Center. This advisory covers the West Central Mountains in the Northern half of Valley County and the Eastern portion of Adams County. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general snow and avalanche conditions throughout the advisory area, local variations should be anticpated. This advisory expires at midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.