Wallowa Avalanche Center

Conditions Summary for January 29, 2010

Disclaimer: Wallowa Avalanche Center assumes no liability for the accuracy of the information contained in this summary. Information provided does not constitute or imply any type of forecast, advisory, or recommendation. Users are warned to use this information at their own risk.

The Bottom Line Synopsis:

We received 8 inches of new snow last Saturday and another 4 to 6 inches on Tuesday. With winds this last week quite manageable compared to the week before and with the new snow addition, conditions are very pleasant for recreation. The upper level snowpack this week has had the usual loose snow sluffs, point releases and a bit of fairly benign wind slab weaknesses in the low to mid elevation non-wind exposed terrain. There is a problematic weak layer to watch for about 35 to 45 cm (15 inches) down from the surface. This is a thin, almost undetectable weak layer (buried surface hoar?) and so visually in a pit you cannot see it. Proper tests are necessary to bring it out. On ridgelines even with some trees the wind drifts are sometimes so tall it’s real work to climb over them. The lower snow pack weakness definitely showed it’s hand after observations found some older natural avalanches.

Photo 1: NNE aspect Dollar Ridge natural avalanche release. Photos taken 1-24-10. HS-N-R4-D2.5 width = about 1000 feet (wider than photo) and dropped approx. 500 vertical feet

Photo 2: Another view of the natural in Aneroid basin

Recent Activity:

Perhaps during the snow events and very strong winds of January 16-20 two separate natural avalanches released on the lower snowpack weakness. These were fairly large avalanches easily capable of seriously injuring and burying a person. One was in the Aneroid Basin and the other in the Wing Ridge area. These avalanche paths have since reloaded with wind transported snow and could slide again. See photos and details below. There have been no reports of human triggered avalanches, however these natural avalanches which are even better indicators of instability show that the lower weakness is still not to be fooled with!!

Photo 3: Starting zone of avalanche off Middle Ridge, N aspect @ 8300 feet. Crown and flank thicknesses ranged from 1 to 4 feet. Note the starting zone was a very thin snowpack area around a rock band  and the slide ran on the weaker sugary (faceted) granular snow near the ground. Photo taken 1-27-10.

Concern #1: Deep slab instabilities. I have moved this to the #1 spot for good reason this week. Weaknesses still exist in the lower one third of the snowpack. Again, as stressed for many weeks now, there are places where the snowpack is thin. On steep terrain these thin spots are a likely place where an avalanche will be triggered and when it goes, it will surely be big. These thinner snowpack areas are where the weight of a human can reach down, cause a failure of this weakness and possibly cause an avalanche. The trigger point could be anywhere on the steep slope and the fracture (crack) will propagate irregularly; above you, to the side of you, across from you, etc. and even into a much thicker snowpack area.

Concern #2: Some sensitivity was evident this last week in the upper one third to one half of the snowpack. There exists a weakness of some sort that seems to vary with elevation and aspect. In some places it is nestled in with a bunch of wind slab layers. In other places it seems to be a part of or above/below the old (3 weeks ago now) rain crust. It is now difficult to see this weak layer so careful  tests need to be employed to bring it out. Here, treat this as Don Sharaf would say, “Guilty until proven innocent”. If you don’t find it one place look elsewhere; sometimes it’s only 20 feet away.

Concern summary:
In the lower one third of the snowpack lurks a continuing weakness. This weakness has not strengthened in the last week. The upper one third to one half of the snowpack has a thin weak layer requiring careful detective work. Be cognizant of these concerns and evaluate them carefully when traveling within any avalanche terrain. Or simply stay out of avalanche terrain where much enjoyment can be had on low angle terrain.

Photo 4: Debris field of Middle Ridge avalanche. This slide ran full track and stopped well onto the flat valley below. HS-N-R3-D2.5, width = 75 feet at top and dropped approx. 450 vertical feet. Photo taken 1-27-10.

Brief update from Southern Wallowas:

Biggest news from the southern portal is a thin rain crust developed on January 24. Since then (Tuesday 1/26) 6 inches of new snow fell on top of this crust. There exists a similar weakness in the upper foot and a half of the snowpack. There have been reports of a reactive graupel layer but in isolated pockets. Generally until the recent crust deposit the snowpack has been relatively stable in comparison to the north.
 

Average snowpack depths at 8000 feet:

Northern and Eastern Wallowas: 12” new, 50” total depth

Southern Wallowas: 6” new, 102” total depth

Photo 5: NNW aspect at similar elevation as Middle Ridge  avalanche which is  just over the knob. Pit work here because safer spot. Note creep crack by shovel handle. 35 degree slope.  Photo taken 1-27-10.

The Juicy, Techie Stuff:

Of note: Where there exists a deep snowpack (1.5 meters or more) the deep faceted/depth hoar layer is still getting slightly stronger with smaller facets, rounding and sintering. The thinner snowpack areas (less than 1.5 meters) are not improving. Overburden is seemingly playing a large part in helping the thicker snowpack areas. The more digging around this season the more it is found that the lower weakness varies whether above the concrete old snow or in contact with the ground. Specific height of this weakness is less dependent on aspect and elevation than previously thought.

Photo 6: Creep crack detail. Crack is 2 inches wide at ruler. Crack extended from surface all the way to the sugary weak layer but stopped there. Why?

Stability tests this week have been too erratic to provide general indicators. Some reports from our observers are showing very sensitive weaknesses in the top half of the snowpack while others show only moderate weakness concerns. Spatial variability is here to stay. All agree that there is a persistent problem down under (no, not Australia!) near the ground. Surface conditions as of Wednesday 1-26-10 show only spotty surface hoar, isolated diurnal recrystalization and plenty of relatively abrasive wind slab surfaces in the previously wind effected areas. A thin sun crust developed at the end of the day Wednesday on southerly aspects.

Figure 1: Generalized snowpack diagram

QUIZ!!!

Photo below taken 1-24-10 ENE aspect Lookout Mtn. Aneroid Basin. On a scale from 1 to 5 how would you rate the level of wind effected terrain in this photo?

General Announcements

Don’t forget our Wallowa Avalanche Center biggest fun event of the year on February 11 (Thursday) 6pm at the OK Theater in Enterprise!!  Cool movies!! The event will feature pizza from Embers in Joseph and beer from Terminal Gravity, as well as a raffle and a drawing for items that include a backcountry safety package consisting of a backpack, shovel, probe and electronic beacon. Proceeds from the event will purchase additional educational materials to be offered free to the public; instrumentation for better wind data from the top of Mt. Howard; additional Web site enhancements; and support for WAC’s Observers Network.

Tickets are available for only $5.00, while children under 6 are free. Purchase tickets at the Bookloft, Central Copy and Shipping, and The Sports Corral. More information is available by calling Penny at 541-263-1454 or emailing info@wallowaavalanchecenter.org

Wallowa Mountain Weather Briefing:

We are working directly with the National Weather Service in Pendleton through special briefings to provide you with applicable local mountain weather information. You should always obtain a weather forecast from National Weather Service before undertaking any winter outdoor activities.

Simplest way to describe the upcoming weekend weather is off and on precipitation but nothing heavy. A weakening system is splitting, with nearly all of the energy either moving to the north or south of us. Today (Friday) expect increasing clouds with no precipitation. Overnight tonight and into Saturday the snow/rain line will be about 5000 feet and very light with SW winds. Saturday night may see a slightly heavier light snow event but all will taper off by Sunday afternoon with drying moving in. Monday brings in an upper level high pressure and NW winds. Temperatures will be seasonable to a little above normal. 8000 foot high’s each day about 30 and overnight temps in the low 20’s.

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE PENDLETON OR
302 AM PST FRI JAN 29 2010

CORRECTED PUNCTUATION ERROR IN SHORT TERM DISCUSSION.

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...SATELLITE IMAGERY SHOW
AN UPPER LEVEL TROUGH ALONG THE PACIFIC NORTHWEST COAST WITH A WEAK
RIDGE CENTERED OVER THE IDAHO PANHANDLE. A SHORTWAVE IS MOVING
THROUGH THE BASE OF THE TROUGH AND WILL RIDE OVER THE RIDGE LATER
TODAY AND TONIGHT. RADAR IS ALREADY SHOWING SOME RETURNS FROM THE
SHORTWAVE BUT THUS FAR PRECIPITATION REMAINS IN NORTHERN CALIFORNIA
AND FAR SOUTHERN OREGON. EXPECT THAT MAINLY RAIN WILL MOVE INTO
CENTRAL OREGON LATER THIS MORNING AND PROCEED NORTH ALONG THE
CASCADES. FURTHER EAST UNDER THE RIDGE...STABLE CONDITIONS AND LIGHT
WINDS WILL ALLOW SOME PATCHY FOG TO DEVELOP IN THE COLUMBIA BASIN
AND BLUE MOUNTAIN FOOTHILLS AND PERHAPS THE YAKIMA VALLEY THIS
MORNING. WHILE IT MAY LOWER VISIBILITY TO A QUARTER MILE AT WALLA
WALLA AND PENDLETON...EXPECT THAT IT WILL BURN OFF BY LATE MORNING.
THE SOUTHERLY FLOW AROUND THE TROUGH WILL BRING SOME WARMER AIR INTO
THE REGION AND TEMPERATURES IN THE EASTERN HALF OF THE AREA SHOULD
BE UP A COUPLE OF DEGREES FROM YESTERDAY. THE WEST WILL COOL A
COUPLE OF DEGREES UNDER THE HEAVY CLOUD COVER AND RAIN/SNOW. SNOW
LEVELS WILL BE FAIRLY HIGH...AROUND 4500 FEET SO SNOW WILL BE
CONFINED TO THE MOUNTAINS. IN THE AFTERNOON...THE SHORTWAVE WILL
PROGRESS TO THE EAST AND BRING RAIN AS FAR EAST AS THE TRI CITIES
AND PENDLETON BY LATE AFTERNOON AND FINISH CROSSING THE AREA
OVERNIGHT. SNOW LEVELS WILL DROP TO AROUND 3000 TO 3500 FEET
OVERNIGHT SO BEND MAY PICK UP A HALF INCH OR SO OF SNOW AFTER
MIDNIGHT. THIS IS NOT A WELL ORGANIZED SYSTEM SO EVEN THE WETTEST
LOCATIONS WILL SEE AROUND A TENTH OF AN INCH OF LIQUID PRECIPITATION
TODAY AND AGAIN TONIGHT.

ON SATURDAY...THE TROUGH WILL PUSH OVER THE AREA AND SHOVE THE RIDGE
TO THE EAST OF US. MOST OF THE ENERGY WILL HAVE SPLIT OFF AND GONE
SOUTH INTO CALIFORNIA LEAVING EASTERN OREGON AND SOUTHEAST
WASHINGTON WITH LIGHT SHOWERY PRECIPITATION. TEMPERATURES WILL BE IN
THE MID TO UPPER 40S WITH UPPER 30S TO LOWER 40S IN THE MOUNTAINS SO
AGAIN MOST OF THE PRECIPITATION WILL BE RAIN. SNOW LEVELS WILL BE
AROUND 4500 FEET. SATURDAY NIGHT WILL SEE A LOW FORM OVER IDAHO AND
WRAP AROUND CIRCULATION AND MOISTURE WILL GIVE THE EASTERN MOUNTAINS
A GOOD SHOT OF SNOW...PERHAPS 2 TO 4 INCHES IN THE NORTHERN BLUE
MOUNTAINS AND WALLOWAS WHILE THE REST OF THE AREA SEES A TENTH OF AN
INCH OR LESS OF RAIN. PERRY

.LONG TERM...SUNDAY THROUGH FRIDAY...A RATHER PROGRESSIVE WEATHER
PATTERN IS EXPECTED TO LAST THROUGH THE EXTENDED PERIOD. ON SUNDAY
THE LOW WILL MOVE INTO MONTANA...ALLOWING THE LOWER COLUMBIA BASIN
AND THE DESCHUTES PLATEAU TO DRY OUT. YET ALONG THE CASCADES AND IN
THE MOUNTAINOUS AREAS OF NORTHEAST OREGON AND SOUTHEAST WASHINGTON
SHOWERS WILL CONTINUE DUE TO WESTERLY UPSLOPE FLOW. THE SHOWERS WILL
DIMINISH ON SUNDAY NIGHT AS A TRANSITORY RIDGE BUILDS INTO THE
AREA...BUT SNOW WILL STILL BE POSSIBLE IN THE HIGHER TERRAIN.
ANOTHER LOW WILL APPROACH THE PACIFIC NORTHWEST ON MONDAY...BUT
THERE IS TOO MUCH MODEL SPREAD TO DETERMINE HOW MUCH OF AN IMPACT
THIS FEATURE WILL HAVE ON THE LOCAL AREA. THE ECMWF CONTINUES TO
HOLD A RIDGE OVER OREGON AND WASHINGTON THROUGH FRIDAY WHILE THE GFS
FLATTENS THE RIDGE...BRINGING SIGNIFICANT MOISTURE INTO THE REGION.
GIVEN SUCH UNCERTAINTY HAVE KEPT SLIGHT CHANCE POPS IN THE LOWER
ELEVATIONS AND CHANCE POPS IN THE MOUNTAINS MONDAY THROUGH FRIDAY.
CONVERSELY...THERE IS FAIRLY HIGH CONFIDENCE THAT SOUTHERLY FLOW
WILL BRING SLIGHTLY ABOVE NORMAL TEMPERATURES TO THE REGION
THROUGHOUT THE PERIOD.  82