Wallowa Avalanche Center

Conditions Summary for March 5, 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.

Observations this week are from Mark at Big Sheep, and Trevor (Central Oregon Avalanche Association) who was also skiing in Big Sheep, Matt from the southern Wallowas, Keith from Aneroid and a few others.

During Keith’s mission to the Wasatch of Utah he worked alongside some Top Notch Avalanche Pros and has a brief report.

The Bottom Line Synopsis:

This week has generally been partly to mostly sunny and dry. While I did record a flurry while on Obs this week it only added up to an inch or so at 8000’. Good snow for turning is getting harder, but not impossible, to come by. We need some snow, ‘ya think? Some has been found hiding in N through NE facing terrain at the mid elevations where the wind can’t get in. These are just the areas where the upper snowpack appears to be the most sensitive to avalanche when considering making turns on steeper terrain. Right now there are a lot of places with a variety of crusts. The surface crusts or buried crusts are not adhering well to the snow beneath or the snow beneath is not adhering well to the crust below or above. And then there’s the buried surface hoar issue which isn’t adhering to any of the rules. This buried surface hoar demands detective work and attention.

Photo 1: Aneroid Basins’ Dollar Ridge and some evidence of continued avalanching. photo taken 3-2-10.

We continue to see varying weakness in the upper snowpack layers, in approximately the upper 18”. Some crusts are hard to get to move, others crack and move easier. The buried surface hoar which is quite scary when you dig down and look carefully at it causes an easy shear weakness in one place and a somewhat less sensitive weakness somewhere else. In some other places you can’t even find it. How do you know if it’s under where you want to recreate? My dear Watson – use your powers of observation I say!!  It’s not that far down and the snow is quite easy to move aside with your hand. So try that in many, many places and often, and LOOK, and do hand shears. I was amazed during my wandering around out there this week that I couldn’t get a single whumpf on this surface hoar layer. But another observer in Big Sheep did get some whumpfing on NW -> N -> E aspects on convexities higher up nearest ridgetops. Just because you don’t hear the red flag clue noise doesn’t mean it’s stable. After thinking on it some I might have the answer – feel free to offer yours on the forum ‘Avy Answers’ and let’s get a discussion going. More later, in the juicy techie section.

Photo 2: Aneroid Mtn. from Aneroid Basin. photo taken 3-2-10.

Recent activity: In the northern Wallowas there was some minor activity. There was a report on Friday 2-26 of a woman getting swept off her feet by a small point release on “The Burn”, a slope about 35 degrees in the Big Sheep area of Wing Ridge. She was not carried or injured.

A member of the same party saw a slab avalanche in the Wing Ridge area to viewers left of the top of Middle Ridge, in that little bowl that drops into the N end of the Salt Creek Bowl.  He said it ran down to the bench below. It was about 50' wide, with a crown just a little ways below the cornice, and ran about 100' - until the slope got below about 30 degrees. This was not a brand new avalanche. Isolated whumpfing was experienced but generally higher up.

Photo 3: Aneroid Basin near cabins, W aspect, @7800’. photo taken 3-2-10.

Southern Wallowa update:

From this region we have reports that are approximately up to 2 weeks old but certainly have relevancy. Generally reports are from the Blue Creek and Red Mtn. area. A skier triggered a soft slab (SS-R2-D1) on a NW aspect on 2/23. Check out last weeks last minute update showing a photo of the natural occurring in the Pine Creek drainage possibly a result of the 2/12 storm cycle. Widespread whumpfing was noted on 2/24 due to a new 20cm snowfall on top of the previous days near surface facets. Winds picked up after the 2/24 storm and wind loaded slopes began appearing from winds out of the NE -> E. Generally just before the 2/24 storm the new snow fell on stout southerly sun crusts or a variety of weak surfaces on the more northerly aspects.

Just like the northern Wallowas, the southern tier is being plagued  with that nasty buried surface hoar (SH) layer. This SH is causing much sensitivity in the upper 1 ½ feet of the snowpack also. Most stability tests are producing clean shears in the low-moderate range and failing almost exclusively on the SH.

Photo 4: Aneroid Basin pit on NE aspect Lookout Mtn. @8050’. Approx. location: 11T0484430E 5004400N NAD27.

Concern #1: Many weaknesses of varying sensitivity exist within the upper  1½ feet of the snowpack. In particular there are isolated pockets and areas of a very persistent buried surface hoar (SH). This surface hoar is still standing up and about a full centimeter thick. It appears to be mostly on N -> E aspects on leeward or wind protected areas at elevations from about 7000’ to just below ridges and summits. Other crusts, whether they be sun or wind related are also not bonded well to the snow either above or below them. These other crusts are not quite as sensitive or reactive as the SH but may become more so when (if) we get another good round of snow or other major weather change (warming, rain, wind, etc.) The surface hoar layer has me concerned because it can surprise anyone. In one place you can’t find it, in the next place it’s the best textbook example possible. Pay attention.

Photo 5: Sample of the surface hoar crystals. Don’t even need a magnifier to see the size and thinness of these.

Photo 6: Surface Hoar (SH) from Aneroid Basin pit on 3-2-10. It is still standing up, ready for collapse, and very sensitive. This is persistence at its finest. This SH was deposited during the cold of February 6-8. Note that creep is beginning within the SH layer evidenced by the tipping of the crystals.

Concern #2: Deep slab instabilities. For a long time now WAC has been keeping a level of awareness for this weakness in the summary. There has been no activity on this layer within the last 2 months. It should always be kept in the back of your mind but we will drop it until something of significance changes and causes us to bring it back into the spotlight.

Concern #3: Wet snow movement. This last week some evidence has surfaced. A bit of wet sluff action has occurred but very minor. Roller balls have bounced down sun heated slopes. Generally nothing hardly warranting a mention.

Concern Summary:

Surface hoar awareness. Be sure it’s not under where you want to be, when on steep slopes. Varying crusts require some evaluation with regard to bonding. Wet activity may or may not be a problem. Evaluate these concerns carefully when traveling in, around or below slopes exceeding 30 degrees.

Average snowpack depths at 8000 feet this week:

Northern and Eastern Wallowas: An inch or two new, 53 inch total depth
Southern Wallowas: Around the high 90 inch total depths

Photo 7: Aneroid Basins’ southern flank of Lookout Mtn. photo taken 3-2-10.

The Juicy Techie Stuff:

Very consistent results from stability tests from all observers this week provide some confidence that indeed the sensitivities exist primarily in the uppermost portion of the snowpack. Scores mostly in the low moderate range (11 to 15) failing primarily on the SH layer. The surface hoar layer is something to behold. Some reports are showing some failures with less stress applied on one of the nearer to the surface crust type interfaces. A quick test was performed to see if depth would be a factor for the depth hoar weakness  – see photo. All observers for a month or so now are honing in on a score of 25 for a failure on this depth hoar layer.

Photo 8: Comparing CT results for short vs. tall columns for a special depth hoar test. Here the numbers were basically the same. CTmid-20’sQ3.

Why, perhaps, would whumpfing not be noticed on elevations or areas where wind can’t easily effect the surface? What we know this week: On upper elevation ridgelines whumpfing is noticed (collapse of some weak layer). Lower, non-wind effected terrain whumpfing is not noticed. Maybe it’s because the lower terrain surface snow hasn’t firmed up enough yet (stiffer slab-like properties) to cause shear stress propagation. Maybe I can’t provide enough weight to cause collapse failure. Thoughts?

Figure 1: Aneroid Basin Pit Data

Whumpfing analysis answer courtesy Keith's friend and fellow UNH engineering alum -- contracted a while back by CRREL in Hanover, NH (a snow science research project that Keith originally began but turned over to the much more qualified Mr. Kestler)
 

From Ken Kestler:
 

Why, perhaps, would whumpfing not be noticed on elevations or areas 
where wind can’t easily affect the surface?
 
What we know from observation:
Upper elevation ridgelines => whumpfing is noticed.
Lower, non-wind affected terrain => whumpfing is not noticed.
 
What we know about whumpf creation:
At higher elevations, wind driven snow can create a firm surface layer 
over potentially weaker layers. This slab-like veneer exhibits 
relatively high shear strength along planes perpendicular to the snow's 
surface. Any surface loading will be horizontally carried away from its 
source via a wide pressure bulb. As the surface layer deflects under 
load, the lower weaker layer will fail over a wide area defined by the 
extent of the large surface basin undergoing the deflection. Sort of 
like a large piston suddenly lowering into the collapsing weaker layer. 
The end result -- that familiar rushing sound of air & snow, wwhhuummppff!
 
Proposed reason for low level whumpflessness:
Lower terrain surface snow hasn’t firmed up enough yet to create the 
slab-like veneer. Any loading will have a narrow pressure bulb. No 
slab, no transverse load carrying, no piston effect, no whumpf.
 

Figure 2: Idealized pit diagram

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.

Doldrums. Boring. Bleak. I don’t know how else to describe it. No snow in the forecast for the next 4 to 5 days. High pressure builds over the area through the weekend. A storm approaches the pacific northwest overnight but confidence is high that it shears and moves south into parts of California.

Warmer trending weather through the weekend but a colder frontal passage should occur late Sunday and with it a slight chance of precipitation. Don’t hold your breath. Cooler/colder conditions starting on Monday. Generally partly to mostly sunny skies for the next few days with gentle winds and temperatures slightly above normal.

General Announcements:

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Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
253 AM PST FRI MAR 5 2010

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...WATER VAPOR AND
INFRARED SATELLITE IMAGERY LOOPS SHOW A MID/UPPER LEVEL TROUGH NEAR
132W AND A MID/UPPER LEVEL RIDGE OF HIGH PRESSURE OVER THE FORECAST
AREA EARLY THIS MORNING. UP TO THIS POINT THE TROUGH HAS BEEN
DECELERATING IN ITS EASTWARD MOVEMENT AS A RESULT OF THE BASE OF THE
TROUGH DIGGING SOUTHWARD. THE TROUGH WILL CONTINUE DIGGING SOUTHWARD
TODAY...THUS HALTING ANY FURTHER EASTWARD PROGRESSION OF THE TROUGH
WITH RESPECT TO THE FORECAST AREA. BY THIS AFTERNOON THE TROUGH WILL
HAVE DEVELOPED INTO A MID/UPPER LEVEL CLOSED LOW OFF THE CENTRAL
CALIFORNIA COAST. AS SUCH THE COLD FRONT ON THE EAST SIDE OF THE
TROUGH WILL BE UNDERGOING FRONTOLYSIS TODAY JUST OFF THE WA/OR COAST
WITH THE AIR MASS CURRENTLY IN PLACE OVER THE PACIFIC NORTHWEST
PERSISTING THROUGH TONIGHT. GIVEN THAT THE LOW BYPASSES THE FORECAST
AREA ENTIRELY TODAY WE CAN EXPECT DRY CONDITIONS. HOWEVER...A
5000-1000 FT CLOUD LAYER AND A LAYER OF HIGH CLOUDS WILL PERSIST
TODAY ACROSS THE INTERIOR PACIFIC NORTHWEST. A WEAK MID/UPPER LEVEL
RIDGE OF HIGH PRESSURE BUILDS OVER THE REGION TONIGHT AND SATURDAY
WITH ATTENDANT SUBSIDENCE PRODUCING MILD WEATHER AND DRY CONDITIONS
THROUGH SATURDAY NIGHT. HIGH TEMPERATURES ON SATURDAY WILL BE AROUND
4 DEGREES WARMER THAN TODAY WITH HIGHS SATURDAY IN THE LOWER 50S TO
LOWER 60S...EXCEPT IN THE LOWER 40S TO LOWER 50S IN THE MOUNTAINS.
POLAN

.LONG TERM...SUNDAY THROUGH FRIDAY...HIGH PRESSURE WILL BEGIN
BREAKING DOWN ON SUNDAY ALLOWING A COLD FRONT TO SWEEP INTO THE
REGION SUNDAY NIGHT. SNOW LEVELS WILL DROP BEHIND THE FRONT BUT
PRECIPITATION WILL TURN TO SHOWERS AS THE UPPER LEVEL TROUGH PASSES.
THIS COULD LEAD TO A DUSTING OF SNOW IN THE LOWER ELEVATIONS WITH
HEAVIER AMOUNTS IN THE MOUNTAINS THROUGH MONDAY NIGHT. ANOTHER
SYSTEM WILL FOLLOW RAPIDLY LATE TUESDAY BUT THIS SYSTEM WILL NOT
HAVE MUCH COLD AIR SO IT LOOKS LIKE MAINLY A RAIN EVENT WITH SNOW IN
THE MOUNTAINS. SLIGHT RIDGING WILL BUILD OVER THE REGION THURSDAY
AND FRIDAY UNDER A MOIST SOUTHWEST FLOW. THIS COULD LEAD TO SOME
LIGHT PRECIPITATION MAINLY NEAR THE WASHINGTON CASCADES.

&&