Wallowa Avalanche Center

Conditions Summary for December 11, 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:

Remember this rule: Extended cold to very cold clear weather nearly always equals a weakening snow pack; especially when the snow pack is thin. Houston: We have a situation here! More on that later. It was a welcome surprise last Saturday morning to enjoy nearly 8 in. of new light density snow at the upper elevations which has now settled to 4 to 5 inches. During last weekend temperatures just continued to plummet and have stayed down there all week. It’s been sunny without any precipitation all week and well below zero at night and barely above zero for our highs around the 8000 ft. level.

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Photo 1: Leftmost part of photo is N aspect, headwall of 1st Basin in distance is ENE aspect. Note the heavy scouring on N aspect from upslope winds of early this month. photo taken Tues 12-8

After a little poking around last weekend and more detailed work on Wing Ridge on Tuesday the concern just above the old melt freeze crust conglomerate of early November has become a manifestation. This layer measuring approx. 2 inches thick and located just above the melt freeze (MF) crust surface has weakened considerably this past week and is highly likely to plague us for some time to come as a persistent weak layer. Ugh. This weak layer so described can be found in many places at mid and upper elevations, generally on NW -> N -> Easterly aspects but I have seen it poke it’s nasty face out in localized areas on other aspects since I have been looking at this for a month now. There are still areas where the wind has stripped all snow down to this MF and this weak layer doesn’t exist, but this is much rarer than the other way around. However, where the MF surface is exposed (today for instance) there will likely be a bonding issue to the new snow since it is rather slick.

Photo 2: Same spot as photo 1 but looking to opposite side of Middle Ridge toward NE aspect.

I’ve noticed reduced sensitivity within the interface layering of the multiple wind slab/crust layers developed over the last several weeks. The main focus now is when enough additional weight gets applied to this MF weak layer through one really big storm or multiple smaller storms we will invariably see an avalanche cycle. Sometimes what happens in this situational setup is that we get a small storm or two but they don’t provide enough weight, then a storm after those finally tips the balance and the snow pack teeters on the edge of stability until that unsuspecting trigger comes along.

Concern 1. The weak layer just above the concrete MF surface. Continually evaluate whether this loose granular layer is under you when traveling or skiing. It is mostly on NW -> N -> Easterly aspects from about 7000 ft upward in elevation.

Concern 2. New snow. Some is expected, so beware that this will hide the places where the MF surface was once exposed. And we now begin the loading of the weak layer. But, as always, new snow often comes with wind so watch for new wind drifts, soft slabs, etc. and remember that these pockets and areas of additional wind transported snow contribute quickly to weight. And of course new snow instabilities (sluffs).

Concern 3. Coverage as usual. Still marginal on S -> W aspects, barely covering rocks and stumps; better coverage on Northerly aspects.

Average snow depths exactly the same as last weeks report. Have heightened awareness to these concerns when you venture through or up onto avalanche terrain.
 

Photo 3: Test pit  areas upper Middle Ridge NE -> E aspects 8200'

The Juicy, Techie Stuff:

The important thing is that the weak layer above the MF crust surface has weakened. The poly-crystals offering up some strength from last week have all but disappeared in these pits in this Wing Ridge Basin and likely elsewhere as well. This problematic layer is largely made up of 3mm striated, advanced facets. The size has increased in the last week from 2 to about 3 mm as of Tuesday. If you have a copy of Colbeck’s c1990 International Classification paper, the photo of Class 5 cp is identical to what is found right now in this layer.

Figure 1: Generalized Snow profile. Middle Ridge at or near UTM 0491757E 5004783N, ENE aspect, 8200'

I performed a series of shovel shears and CT’s but got quite inconsistent numbers and Q’s (2’s & 3’s) so I didn’t list them. When they are that erratic I would generally try other places but time ran short, I had another project to attend to. Last week I felt more comfortable with the scores and so placed them on snow profile. Surface hoar still being deposited in localized areas this week, mostly on the Northerly aspects. With this intense cold I have spotted some of it even in some slightly southerly components particularly east.

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.

Today (Friday) expect sunny skies in the morning with increasing clouds in the  afternoon. Snow will move into the upper elevations on a northerly jet stream track by Saturday morning with light snowfall rates through Saturday night. Winds change to a more westerly direction during the day Saturday when there is a possibility that the bulk of the storm track precipitation will be north of the Columbia Basin into the Tri-cities area. Snow should end sometime Saturday night with expected accumulations less than 6 inches. Sunday will bring only a short duration clearing before another possible wave of precipitation early next week. Confidence of exact storm track will be higher and validated by Friday afternoon. Temperatures will moderate some with the westerly winds and move toward more seasonable highs and lows by mid-weekend.

General Announcements:

To our Observers - Remember our observers meeting Monday night Dec. 14 at 6pm.   Come prepared to share some ideas for this season so we can be really efficient at getting those - all important Obs into the Center.

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
239 AM PST FRI DEC 11 2009

.SHORT TERM...TODAY THROUGH SATURDAY...TODAY WILL BE DRY FOR MOST OF
THE FORECAST AREA AS WE WILL STILL BE BETWEEN THE NORTHERN JET
STREAM AND AND ASSOCIATED MID/UPPER LEVEL LOW OVER WESTERN CANADA
AND THE SOUTHERN JET STREAM MAKING LANDFALL IN SOUTHERN CALIFORNIA.
LOOK FOR INCREASING CLOUDS THIS AFTERNOON IN CENTRAL OREGON AS
MOISTURE ADVANCES INTO OREGON FROM THE SOUTHERN JET STREAM. WATER
VAPOR IMAGERY LOOP SHOWS A DISTURBANCE RIDING ALONG THE SOUTHERN JET
STREAM NEAR 46N/155W...WHICH WILL MOVE EASTWARD TODAY AND APPROACH
THE NORTHERN CALIFORNIA COAST TONIGHT AND SPREAD MOISTURE ACROSS THE
OREGON PORTION OF THE FORECAST AREA INTO FAR SOUTHERN AND SOUTHEAST
WASHINGTON OVERNIGHT. WITH A VERY COLD AIR MASS IN PLACE...WARM AIR
OVERRUNNING TONIGHT WILL PRODUCE SNOW EAST OF THE OREGON CASCADES
WITH SNOW MOVING INTO THE EASTERN COLUMBIA GORGE TOWARD MIDNIGHT
WITH SNOW CONTINUING IN THE GORGE THOUGH SATURDAY. A WARM LAYER
ALOFT WILL PRODUCE A WINTERY MIX OF SNOW AND SLEET IN CENTRAL AND
NORTH CENTRAL OREGON WITH A CHANCE OF FREEZING RAIN AT LOWER
ELEVATIONS TONIGHT. A DEFORMATION ZONE SETS UP OVER EXTREME NORTHERN
OREGON ON SATURDAY AS THE NORTHERN JET STREAM DIPS SOUTHWARD INTO
THE PACIFIC NORTHWEST THUS ENHANCING DYNAMIC LIFT IN THE FOOTHILLS
OF THE BLUE MOUNTAINS FOR A DECENT SNOW EVENT THERE ON SATURDAY.
POLAN

.LONG TERM...SATURDAY NIGHT THROUGH THURSDAY.  THE ACTIVE WEATHER
WILL CONTINUE THROUGH THE LONG TERM PERIOD.  THE VARIOUS WINTER
STORM WATCHES AND ADVISORIES ARE IN EFFECT THROUGH SUNDAY MORNING.
ON SATURDAY NIGHT AND SUNDAY...A STATIONARY BAND WILL BRING STEADY
SNOW TO AREAS SOUTH AND EAST OF THE TRI-CITIES, WASHINGTON.  A WATCH
WILL BE ISSUED FOR THE BLUE MTN FOOTHILLS WHERE CONFIDENCE IS FAIRLY
HIGH THAT THIS AREA WILL SEE HEAVY SNOW.  ACCUMULATIONS IN THE LOWER
COLUMBIA BASIN NEAR THE WA/OR BORDER AS WELL AS THE NORTHERN BLUE
MTNS MAY REACH ADVISORY CRITERIA BUT WILL LET THE DAY SHIFT TAKE A
CLOSER LOOK TO DETERMINE IF WARNING AMOUNTS WILL BE MET.  THE NAM
MAY BE TOO BULLISH WITH PRECIPITATION AMOUNTS...SO WILL SEE WHAT 12Z
RUNS HAVE IN STORE.  THE WINTER HIGHLIGHTS MAY NEED TO BE EXTENDED
THROUGH SUNDAY NIGHT AND MONDAY AS WELL FOR AREAS ALONG THE WA/OR
BORDER...AS THE STATIONARY FRONT MAY LINGER EVEN LONGER DESPITE THE
UPPER LEVEL TROUGH EXITING THE REGION.  A WEAK TRANSITORY RIDGE WILL
MOVE EAST OF THE CASCADES MONDAY FOR A BRIEF BREAK IN PRECIPITATION
BEFORE THE NEXT SYSTEM ARRIVES MONDAY NIGHT AND TUESDAY.  THIS
SYSTEM HAS A DECENT MOISTURE TAP OFF THE PACIFIC...SO PRECIPITATION
AMOUNTS COULD BE SIGNIFICANT AND MORE WINTER HIGHLIGHTS MAY BE
NEEDED.  THE FORECAST AREA WILL REMAIN UNDER A MOIST W-SW FLOW ALOFT
THROUGH THE END OF THE WEEK FOR CONTINUED PERIODS OF SNOW.
TEMPERATURES WILL GRADUALLY WARM...SO THE BASIN AND SURROUNDING
VALLEYS MAY BEGIN TO SEE PRECIPITATION CHANGING FROM SNOW OR WINTER
MIX EARLY IN THE WEEK TO ALL RAIN BY WEDNESDAY.  JOHNSON