Wallowa Avalanche Center

Conditions Summary for January 17, 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:

Bottom Line Southern Wallowas: (Courtesy our senior advisor Don Sharaf)
Warm temperatures and little new snowfall in the past week have given the snowpack time to stabilize. The only layers of concern currently is the recent windslabs deposited on north and east aspects from strong winds on Sunday night (1/10 evening- 1/11 am). These slabs spontaneously released in high angle leeward terrain during deposition and might be triggered by travelers in alpine terrain. Additional, possibly sensitive wind slabs from the Tuesday/Wednesday event bears careful evaluation.

Recent Activity: On January 6 and 7 warm temperatures and clear skies produced a widespread avalanche cycle on south aspects between 5500 ft and 7500 ft . Some of these wet loose slides scoured a foot (30cm) deep into the snowpack creating avalanches big enough to bury several people. (WL-N-D2-I).
Strong SW winds overnight on January 10 created relatively shallow winds slabs that naturally released on high angle (>45 degrees) easterly aspects between 6000’ and 7000’. (SS-N-D1.5-I)

No collapsing, or shooting cracks, were observed on any aspects in the past four days (1/8 to 1/11, 2010).

Concern #1: Shallow wind slabs (estimated one foot / 30 cm deep at most). Warm (above freezing) temperatures should cause these rapidly deposited slabs to gain strength fairly rapidly. Lingering sensitivity of these slabs may linger in deeper deposits near ridge-crests in the true alpine zone (above 8000’) or in very steep (>45 degree) terrain.

Concern #2: Although buried surface hoar (feather-like frost crystals that form on the surface and then can be buried by subsequent snowfall) lingers in the snowpack (between 16” and 24” €�/ 45-60 cm deep) it has appeared to gain strength and has low propagation potential. Several pits showed planar, but sluggish shears (Q2) on that buried surface hoar layer, but showed no propagation with the Extended Column Test.
Buried Surface Hoar is generally considered “Guilty until proven innocent.”

In a large number of test pits dug on all aspects in the Norway Creek, Pine Creek, Granite Creek, and Blue Creek drainages (more than a dozen pits) the surface hoar ended up free of charges, only one pit showed that it should remain on probation.

Photo 1 : Salt Creek headwall taken 1-15-10 (NNE aspect 8200 feet). Ken noted these ridgetop drifts occurred during a short duration gusty NW wind of 1-14-10.
 

Bottom Line Northern Wallowas: (Courtesy Keith, Ken and John Groom)
Several outings this past week noted continued strengthening of the lower (nearest ground level) weaknesses in the snow pack due to the continued warm weather. Recent moist new snowfall occurring Tuesday and into Wednesday morning yielded 2” up to 8” from about 5500 ft. to 8000 ft. respectively. There have been no reports of natural avalanches with this new snow and field work of ours has not produced any human triggered activity of note. After hard work we were able to get a very small slide on a test slope (N aspect, 7900 feet, 36 degrees) to release at the new snow / old snow interface. Hard work means two cornice cuts/drops and two ski cuts, releasing on the second. See photos 3 and 4. In places it stepped down to just below the new/old snow interface to underneath a 2’€” older wind slab. Occasional spotty collapsing and whumpfing was noted after this new snow event but generally on flatter terrain.

Photo 2: Pyramid Mtn. (Wing Ridge, taken 1-15-10). Note crossloaded pillows on ENE aspect downslope of the leftmost ridgeline at 8400 feet. These have built further with the SSW gales of 1-16-10.
 

Concern #1: Wind Slabs and drifts (see photo 1). With strong S -> SW winds early in the week, during the storm of last week and again on Friday (1-15-10) there are some fairly deep wind slabs on north through easterly aspects. These are areas where we saw movement of the new snow when given a pretty strong nudge. Cornices have grown more with this new snow and we are now giving them a wider berth when traveling on ridges. Steep chutes (35 degrees +) and couloirs seem to have captured a lot of this new snow. Tremendous cross-loading is now evident; see photo 2.

Concern #2: Buried surface hoar and facet layers. On one test slope near a creek bottom easy hand shears produced a noticeable weakness at a probable surface hoar deposit under this new snow. With the warmth we’ve had, this appears to be likely only on the most northerly shaded open terrain. Lastly, the more deeply buried loose granular snow, although strengthening now, could still be problematic where a thin snow cover exists. Just this past week on Wing Ridge, extended column test results show the propensity for propagation on this weak sugary layer nearest the ground.
This new snow was welcome and has helped to cover the rain crust yuckiness in many places. This is providing a much nicer skiing experience where our observers were skiing terrain and traveling about this week in terrain up to 35 degrees with only heightened avalanche condition awareness and with consideration given to the concerns above.

Photo 3: Result of a cornice drop in McCully Basin 1-13-10. UTM 0488866E 5006786N NAD27 @ 7900 feet.

The Juicy, Techie Stuff:

The general snowpack structure diagram shows the typical weaknesses found by all observers in the Northern and Eastern Wallowa Mtn. Range. Of note: The persistent weakness in any of the sugary layers located in the lower two thirds of the snowpack have begun to show increasing strength from the warmer (near freezing) weather which generally allows a snowpack to build bonds between snow grains. This improves cohesion within the layer and also helps to glue one layer to another adjacent layer.

Figure 1: Generalized profile, northern areas.

There still remains some risk to triggering an avalanche at a trigger point involving a thinner snowpack spot where the weight of a person or snowmachine can reach down to this layer. If this were to occur, tests are still showing a strong likelihood that this failure and subsequent fracture will propagate down to this weakness and across the slope, possibly resulting in a very large and destructive avalanche.

Photo 4: John Groom of the Payette Avalanche Center performing a second ski cut (same location as photo 3 on 1-13-10). Finally some results after 2 cornice drops and this second ski cut.

Ken Bronec’s January 15 report: I skied up to Fat Ridge (Wing Ridge area) and followed it up to 8000’ at the transition between the timbered and alpine areas.  Winds were out of the south at about 15 -  20 mph with higher gusts.  Gusts were mainly S. at 30- 35 mph. Lots of snow transport going on. In open areas the snow surface was sometimes completely obscured by the ongoing saltation.  The lower elevations still had 4“ - 6” of fluffy powder from the Tues/Wed storm.   By the time I got to 8000’ a soft wind slab 2” - 4” thick had formed on all aspects I traversed (SE, E, NE, and N). In treed areas the snow was good skiing, but a bit stiff. The ridge line on Fat was quite firm in places, having been either cleared down to the rain crust or having had a solid wind crust formed.  Previously erratic winds created quite a bit of variability in the snow surface.

On Whitebark Ridge we put in a pit at 7700’  on a 28 degree SE slope.  Snow depth was 75 cm and the snowpack was as follows:  15 cm of light powder; over a moderate density slab (4F) 35 cm deep; over 13 cm of faceted grains; over 12 cm of knife crust covering the ground.   In the upper 1/3 of the slab (25 cm below the snow surface) was a 1 cm thick layer of facets that appeared to be starting to bond.  We got easy to moderate shovel shear at the 1 cm layer of facets.   We also got a moderate (CT14) compression test to fail at that same thin weak layer.   That failure did not easily slide off, but sheared clean when it did.  An extended column test failed (ECT 24) at the faceted layer near the ground.  The low angle slopes there were quite good skiing, but large open areas with much south exposure were less than a foot deep.

In summary, the weak layer is still present in the upper 1/3 of the snowpack in the two spots (SE @ 7700’ and E @ 7800’) I observed.  It appears to be starting to bond, but is still a concern.  The thick layer of facets near the ground will continue to make the consequences high if something does break out on steeper open slopes. The high potential consequences, combined with the significant cross loading in the alpine bowls, signal that skiing in the steeper alpine terrain can be very risky right now.  Some great skiing could be had in the lower angled treed slopes but remember snowpack depth is not what is normal by mid-January.

Figure 2: Snow pit analysis from Chief Joseph, NW aspect.

Total snow pack depths on average:
Northern and Eastern Wallowas @ 8000 ft.: 40 inches
Southern Wallowas @ 8000 ft.: 80 inches

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.

Much instability will be with us over the remainder of the holiday weekend and into early next week. Temperatures at 8000 feet will be a bit over the freezing mark during the day and in the mid 20s overnight. Each day will bring, at most, a couple inches of accumulation as each little low pressure trough slips by us or splits a bit and goes around us. Continued winds from the S -> SW will be cranking right along in the teens (mph) with gusts well into the twenties throughout the period. Models from the NWS show no large storm systems on the horizon.

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
230 AM PST SUN JAN 17 2010

.SHORT TERM...TODAY THROUGH MONDAY NIGHT...THE MILD AND WET WEATHER
PATTERN IS EXPECTED TO CONTINUE THROUGH THE SHORT TERM FORECAST
PERIOD INTO THE EXTENDED PERIOD. A SYSTEM CURRENTLY MOVING ACROSS
THE PACIFIC NORTHWEST IS BRINGING RAIN ACROSS MOST OF THE FORECAST
AREA AT THIS TIME. THIS IS EXPECTED TO CONTINUE THROUGH MOST OF THE
DAY AND THEN TAPER OFF TONIGHT. SNOW LEVELS ARE QUITE HIGH SO SNOW
IS ONLY EXPECTED IN THE HIGHER MOUNTAIN. THIS SYSTEM WILL BE QUICKLY
FOLLOWED BY ANOTHER WEATHER SYSTEM LATER TONIGHT AND ON MONDAY. THIS
WILL BE A STRONGER SYSTEM AND WILL HAVE MORE MOISTURE WITH IT AS
WELL AS A STRONG SOUTHERLY FLOW AHEAD OF IT. THIS WILL CAUSE BREEZY
TO WINDY CONDITIONS TO DEVELOP IN MANY AREAS...ESPECIALLY ON THE LEE
OF THE BLUE MOUNTAINS...I.E. THE NORTHWEST  AND WESTERN SLOPES. ALSO
THE GRANDE RONDE VALLEY MAY EXPERIENCE WINDY CONDITIONS AS USUAL
WITH THESE WEATHER SYSTEMS MOVING ACROSS THE WESTERN CWA. DUE TO
SNOW LEVELS BEING SO HIGH DURING THE NEXT FEW DAYS WITH THE
EXPECTED PRECIPITATION HYDROLOGY ISSUES ARE POSSIBLE WITH RIVER
RISES. MOIST RIVERS IN THE HSA WILL REMAIN BELOW BANKFULL. THE
RIVERS OF MOIST CONCERN ARE THE NACHES AT CLIFFDELL AND THE UPPER
YAKIMA RIVER BASIN WHERE PRECIPITATION AND SNOW MELT WILL BE THE
GREATEST. UN-WINTER LIKE TEMPERATURES WILL PERSIST WITH ABOVE NORMAL
READING THROUGH AT LEAST THE SHORT TERM PERIOD INTO THE EXTENDED. 88

.LONG TERM...TUESDAY THROUGH SATURDAY. LOW PRESSURE OVER THE EPAC
WILL CONTINUE TO SPIN DISTURBANCES AND MOISTURE INTO THE TWO STATE
AREA THROUGH WEDNESDAY KEEPING CLOUDS AND SHOWERS IN THE FORECAST.
AS THIS LOW PRESSURE RETROGRADES WEST...ANOTHER TAKES OVER THURSDAY
AND DRIFTS INTO THE PACNW FRIDAY AND SATURDAY. THIS WILL KEEP THE
UNSETTLED WEATHER FOR THE REST OF THE WEEK AND INTO THE BEGINNING OF
THE WEEKEND. TEMPERATURES SHOULD BE NEAR NORMAL WITH HIGH SNOW
LEVELS. HOWEVER THE MODELS ARE HINTING AT A WEDGE OF COLD AIR ALONG
THE EAST SLOPES OF THE WA CASCADES FOR FRI NITE AND SAT WHICH MAY
ALLOW FOR SOME FROZEN PCPN IN THIS AREA.