Wallowa Avalanche Center

Conditions Summary for December 18, 2009

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:

One week of really cold clear weather, then last Friday night the clouds moved in and stayed with us all week with barely a break in the cover until Thursday where the sun finally came out. Off and on snow was with us continually; sometimes light snow impulses and sometimes heavier. Some observer reports along with remote station data indicates that the storm system began as a lighter density snow over last weekend and gradually has become wetter and denser. This generally creates an upside down effect where soft slab avalanche activity releases and slides on the lighter density snow. Although none of WAC’s Observers were able to get close enough to determine this for sure, it would seem likely. It is still unknown for certain whether any of the reported slides have been running on the weak layer of loose sugary snow on top of the melt freeze crust surface or some other weak layer.

Poor visibility with low clouds and snow, the hazard, and various other obligations this week has precluded photos and detailed graphical entries in the summary.

This week long storm system has favored the southern part of the Wallowa range where up to 2.5 times as much total precipitation fell compared to the northern and eastern Wallowas. Down near Cornucopia nearly 4 inches of water fell this week which the SNOTEL claims is about 30+ inches new snow. On the west side (Taylor Green) near Catherine Creek State Park, total precipitation (although lesser) is in relative agreement. North and east received about 1.8 inches of water and around a foot or a bit more of new snow. We see then that this is a very moist and dense new snow layer and has heavily loaded the older snow pack below it. So we clearly shouldn’t be too surprised that we have lots of avalanche activity; see the avalanche reports below. Also take a look at our Forum; Salt Creek Summit, Avalanche on Wing Ridge, for an entry by a backcountry skier there on Wednesday and what he came across.

Figure 1: Snow Depth data from remote sensing for NE Oregon.

The snow has ended only momentarily but will start up again this weekend just like last. The weak layers causing the avalanches this week and the persistent weak loose sugary layer on top of the old melt freeze surface will continue to cause trouble. More snow this weekend adding yet more weight will likely further  increase the chances of avalanches, both natural ones and human triggered. Those areas that have avalanched will reload with the new snow and could be a potential avalanche hazard all over again.

Figure 2: 24 hour change in SWE (Snow Water Equivalent) A slight bias toward the south end of the Wallowas is apparent from this last storm.

Concern 1: Where you travel or ski/snowmobile/snowshoe this next week. CHOOSE CAREFULLY! Any terrain above you or that you wish to travel upon that is 25 degrees or greater slope angle is avalanche terrain. And we see that avalanches are happening and are likely to happen this next week due to continued snow pack instabilities. All aspects and mid to upper elevations should be treated equally right now.

Concern 2: Continued snowfall causing both new snow instabilities and creating yet more weight on the already heavily loaded weaknesses in the existing snow pack. Constantly be on the lookout for recent avalanches, cracks, whumpf noises as you travel, drifts, pillows, etc.

Average snow pack depths are in the 22 to 30 inch range @ 7400 ft. in the NE Wallowa’s
Average snow pack depths are in the 50 inch range in the southern Wallowa’s at mid elevations. No reports of depths at the higher elevations.

Figure 3: Aneroid Basin Snow Level Forecast starting early Friday (12/18) morning.

The Juicy, Techie Stuff:

Avalanche reports from WAC qualified Observers:
(Note: In many cases it was not safe for the Observer to get close enough with the continued avalanche hazard to get all the details)
 

Charla reports:
In the Wing Ridge area (Thursday 12-17): Natural R3 D2.5 Easterly component Salt Creek headwall.
D2 north aspect off S. Ridge. Frequent whumpfing and cracking on N and S aspects.
22 inch average depth snow pack @ 7400 ft. 3 inches new on Thursday morning.
 

Joel reports:
In the McCully Basin area: 2009.12.14 11:30 7400 ft. East North East 32 degrees slope angle, The Pebble Beach Zone, Snow Depth 120cm ECT non-conclusive (could isolate column and then it progressively "mushed" through tapping, hitting, pounding) From the top down, 10cm new snow 70cm sugary facets 30 cm melt freeze chunky monkey 10cm depth hoar
 

Jeff and Mike reports:
In the northerly aspects accessed from the Wallowa Lake Trailhead (Wednesday 12-16):
1000 Whumpfing and shooting cracks in snow drifts in deeper snow, which continued all day
1100 test pit--Ct13Q3 down 30cm (deeper layers did not fail after 30 whacks)
1130 test pit--STE down80cm
1200 witnessed SS-N-D1.5-R3 (ran from high in couloir to bottom of slope, flanks showed sliding of top 30cm and stepping down below 45cm in main channel of slide)
1230 smaller SS-N-D1.5-R2 in lookers right side of previous slides path, Increasing winds and snowing
1400 SS-N-D1.5-R3 Initiated above line of visibility and ran over previous slides
All observations were on North facing terrain of 30 degrees or greater, test pits were between 7500' and 8000', and slides initiated above 8500'.
 

Jerry reports:
I went xc skiing at SCS yesterday (12-16) and every other step on that flat trail whumpfed. 3 to 20 inches of new snow with top 6 being moist and wind worked. Skis just sunk to the ground as the under layer is still sugar-ish.
 

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.

Weather:
Expect things to settle down for just a little while at the beginning of the weekend. Not much sun will grace the mountains, but temperatures will be seasonable to a little above average until early next week.

Today (Friday) light snow in the high country with snow levels above the 5500 ft level. Very little accumulation likely. Saturday will be mostly cloudy with south winds changing to westerly. No precipitation until Sunday afternoon when a low pressure trough leaves the Puget Sound area and moves over us. Late Sunday afternoon, throughout the night and into Monday morning, expect snow above 6000 ft.. A total of 0.5" water out of this system is likely and dependent on density could give us 6 inches at the upper elevations. Temperatures throughout the weekend will be hovering just below the freezing mark with more of a cool down early next week.

Figure 4: QPF (NWS Precipitation forecast) for the Northwest Friday-Monday.

General Announcements:

General announcements: Place January 12^th (Tuesday) at 6 pm on your calendar. A visiting Avalanche Forecaster from the Payette Avalanche Center will present an Avalanche Awareness Safety Seminar. Cool photos, short wild video clips and a talk  will highlight the dangers out there and then he’ll discuss how to become Avy Savvy to avoid these dangers and be able to more safely enjoy our mountains. He’ll even bring some props that show with a special sand-like material why one snow layer slides on another. Joseph Community Center, AND IT’S FREE!!!!

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
420 AM PST FRI DEC 18 2009

UPDATED AVIATION DISCUSSION

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...A WEAK MID/UPPER LEVEL
SHORT IS CURRENTLY MOVING THROUGH A MID/UPPER LEVEL HIGH PRESSURE
RIDGE PRODUCING LIGHT RAIN IN NORTHEAST OREGON AND SOUTHEAST
WASHINGTON. THIS WAVE WILL EXIT INTO IDAHO SOON AFTER DAYBREAK THIS
MORNING. THEN A SERIES OF THREE WEAK DISTURBANCES WILL MOVE THROUGH
THE RIDGE AND ACROSS THE PACIFIC NORTHWEST BEGINNING THIS AFTERNOON
WITH THE FIRST WAVE ARRIVING ON THE COAST AND THEN MOVING ACROSS THE
FORECAST TONIGHT. THESE WEAK SYSTEMS WILL KEEP THE WEATHER UNSETTLED
THROUGH SATURDAY WITH SNOW LEVELS REMAINING ABOVE 7000 FEET THROUGH
THE PERIOD.  POLAN

.LONG TERM...SUNDAY THROUGH NEXT FRIDAY...A PROGRESSIVE FAST MOVING
PATTERN WITH FREQUENT WEATHER SYSTEMS WILL CONTINUE ACROSS THE
REGION DURING THE EXTENDED PERIOD. THERE IS A TREND TOWARD COLDER
AIR MOVING BACK INTO THE AREA BY THE LATTER HALF OF THE EXTENDED
PERIOD BUT BY THEN THE VARIOUS MODELS BEGIN TO DIVERGE FROM EACH
OTHER CONSIDERABLY SO WILL ONLY COOL IT DOWN SLIGHTLY TOWARD THE
MIDDLE OF NEXT WEEK. IN THE MEAN TIME CAN EXPECT SHOWERY AND OVERALL
WET CONDITIONS TO PERSIST WITH BRIEF PERIODS OF DRY (I.E. NO
PRECIP...BUT STILL HIGH HUMIDITIES) BETWEEN EACH WEATHER SYSTEM.
SNOW LEVELS WILL BEGIN QUITE HIGH SO THEREFORE THE THREAT OF HEAVY
SNOW IS NOT ANTICIPATED...AT LEAST NOT THROUGH THE WEEKEND. AS THE
COOLER AIR MOVES IN (IF IT MOVES IN) AND SNOW LEVELS DROP LATER
DURING NEXT WEEK IT MAY BECOME A CONCERN AGAIN...OTHERWISE EXPECT
TEMPERATURES WILL REMAIN NEAR NORMAL WITH NO SIGNIFICANT COOLING OR
WARMING TAKING PLACE. 88