Wallowa Avalanche Center

Conditions Summary for December 4, 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:

If you are looking for some turning and riding you actually will find it. Secret is to be OK with elevations between about 7500 ft and 8500 ft, NNW -> NE, and in some trees or low angle more open slopes. The upper elevation ridges/summits and steeper terrain are simply hammered by wind, have some stability concerns and/or either no snow or have wind crusts/slabs or rock hard MF crusts. There are still obstacles and booby traps everywhere. It was good to see over the Thanksgiving weekend some folks getting out on their snowmobiles and snowshoes.

Except for a piddling 4 inch storm up high last Friday there has been nothing and probably won’t be anything more of significance until sometime next week at the earliest. I found this 4 inch storm to be difficult to climb up on during my outing last Sunday (11-29) and when testing it a thin 1/2 inch wind crust underneath the new snow went with it on NW aspects in McCully on steep terrain. See photo 1. Apparently a fellow skiing off Ruby had the same experience trying to put in a skin track. (see Forum page Wallowa Lake).

.

Photo 1: McCully, NW aspect, 8350 ft, 31 deg slope, pencil just below a 1/2 inch wind crust

Wednesday (12-2) and also on 11-29 I tried to get numerous old wind drifts and pillows to release. I was only successful in cracking them and had to nudge them to get them to head downward (they were stubborn and solid). When the winds shifted from S during last Fridays storm and began coming from the west then toward NW a 2 inch thick wind crust has developed on easterly component aspects but also on other aspects where cross loading plays a part. These newer wind crusts are cracking easily and moving due to a very poor bond to the soft snow underneath. See photo 2.

Photo 2: McCully, NNE aspect, 8200 ft, 31 deg slope, note 2 inch cracked wind crust

Test pit work on 2 outings revealed many steps in weak layers all the way down to the top of the MF crust. From the surface down the sensitivities in weakness progressively diminish but there are weaknesses all the way down to the top of the melt-freeze crust. See photo 3. I am still not trusting this MF crust surface. Where there is less snow or a thinner slab on top of it, this sets up a likely trigger point, so keep checking in your travels.
 

Concern 1: Releasing a thin wind crust and having it trip you and you go bouncing down with it.

Concern 2: Multiple and varying interlayer weaknesses down to the top of the MF crust. There have been numerous wind events and between some layers there are weak bonds.

Evaluate these snow pack concerns with variability in mind (up/down and across the slope) when entering avalanche terrain

Photo 3: Same spot as photo 2 @0488000 5005930, saw stuck in MF crust and at ground some  movement toward depth hoar.

The Juicy, Techie Stuff:

This week I dug 3 detailed pits and will marry them into one since they were similar enough and it won’t jeopardize accuracy. As noted above the wc is very likely failing on diurnal re-crystallization and surface hoar that I have seen off and on for the past 2 weeks. These same surface conditions have been continually getting buried by wind transported snow or several very light accumulations and are likely the cause of numerous weak interfaces all the way down to the MF crust.

Figure 1: Generalized Snow profile. UTM 048800E 5005930N. Note the steep temperature gradient.

 For the past several weeks I’m continually running into a somewhat old but soft snow surface condition that I’ve not really seen in the Wasatch.I had to look it up and narrowed it down to a mix mostly of classification I2, rimed particles, (see McClung and Schaerer 3^rd Ed., p. 317) and some decomposed SH and small facets. I’ve not seen this I2 stuff before but it’s all over up there on top of the softer snow. This I2 stuff may be playing a factor in these multiple weak interfaces. Do the Wallowas get some kind of short duration midnight “Northern Lights” (ha-ha) rime event? Does I2 decompose? Any thoughts from out there? Where I have found heavy near surface facets (advanced SH) there have been many crystals mimicking a cup shape with striations and measuring 3mm. I’m still watching the faceting story directly on top of the MF crust. It’s varying from 1F+ to as soft as 4F- and 1.5-2mm facets.
 

Snowpack depths on northerly aspects:
20” avg. @7500 ft

27” avg. @8200 ft

Depths vary like crazy within even 50ft.
 

Photo 4: Looking W toward Bare from photo 3 location.

Photo 5: Knowledge Quiz

A “tougher” which way was the wind blowing to make these ripples?
a. from the left
b. from the right
Answer to last wind question of November 6 summary is (b.) From behind the trees.

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.

The big story is cold. Exceptionally so. Today (Friday) the NWS briefing of yesterday indicates a closed low positioning itself to the NW of us causing continually lowering temperatures. As this arctic front moves in, only very small accumulations of snow even in the higher terrain is likely. Yet another push of cold air which will further drop temperatures on Sunday causing another squeeze of precip., but again, nothing of significance. Perhaps as much as 2 inches €� through the weekend period ending Sunday night for the upper elevations. 8000 ft temps likely to be in the low teens daytime and near or below zero each night through this period. Winds NW changing to NE on Sunday. Latest models show an improvement in possible snowfall amounts starting mid to late next week.
 

General Announcements:

To our Observers: There was an error in the meeting date originally given as Dec. 12. It is Monday, December 14.

Be careful out there and keep looking at our Home Page Blog for updates during the week.

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
230 AM PST FRI DEC 4 2009

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...THE FIRST OF A COUPLE
OF SIGNIFICANT COLD FRONTS IS CURRENTLY MOVING NORTH TO SOUTH ACROSS
THE FORECAST AREA AND HAS JUST MOVED SOUTH OF THE OREGON/WASHINGTON
BORDER WITHIN THE LAST HOUR OR SO. SOME MID AND HIGH CLOUDS AHEAD OF
THE FRONT HAS BEEN PUSHED TO THE SOUTH OF THE COLUMBIA RIVER AND
CONTINUES TO BE PUSHED TO THE SOUTH-SOUTHEAST. THIS COLD FRONT
CONTAINS VERY LITTLE MOISTURE AND THE MOST THAT CAN BE EXPECTED FROM
IT IS A FEW LIGHT SNOW SHOWERS OR FLURRIES OVER THE NORTHERN BLUE
MOUNTAINS AND WASHINGTON CASCADE EAST SLOPES THIS MORNING. A COLD
NORTHERLY FLOW WILL SETTLE IN OVER THE REGION LATER TODAY AND
TONIGHT WITH CONTINUED COLD ADVECTION INTO THE REGION FROM THE
NORTH. TEMPERATURES WILL HAVE DIFFICULTY GETTING OUT OF THE 30S
TODAY AND SATURDAY...EVEN IN THE LOWEST ELEVATIONS. A SECONDARY COLD
FRONT ORIGINATING FROM ARCTIC REGIONS WILL DROP DOWN INTO THE
PACIFIC NORTHWEST BY LATE SATURDAY INTO SATURDAY NIGHT. IN ADDITION
A CLOSED UPPER LOW PRESSURE SYSTEM FROM ABOUT 850-700MB AND HIGHER
WILL ALSO DROP DOWN OVER THE CWA. CIRCULATION AROUND THIS LOW WILL
HELP TO TRANSPORT SOME PACIFIC MOISTURE INTO THE REGION OVER THE
DOME OF ARCTIC AIR. AS SUCH CAN EXPECT A BETTER CHANCE FOR SNOW
SHOWERS SATURDAY AND SATURDAY NIGHT. HAVE TRENDED THE POPS UPWARDS
FOR EACH PERIOD AS A RESULT...THOUGH STILL NOT CONVINCED THERE WILL
BE ENOUGH MOISTURE FOR WINTER STORM OR EVEN ADVISORY CRITERIA TO BE
MET...BUT PERHAPS A JUST A FEW INCHES IN THE MOUNTAINS AND A LIGHT
COATING TO PERHAPS AN INCH IN THE LOWER VALLEYS AND COLUMBIA BASIN.
THE MAIN FEATURE WILL BE THE VERY COLD AIR...COLDEST OF THE SEASON
SO FAR...WHICH WILL SETTLE INTO THE REGION BY LATE SATURDAY NIGHT
AND PERSIST INTO THE EXTENDED PERIOD.  88

.LONG TERM...SUNDAY THROUGH THURSDAY...AN UPPER LEVEL LOW WILL MOVE
FROM BRITISH COLUMBIA TO WESTERN OREGON SUNDAY. THIS WILL BRING A
WIDESPREAD CHANCE OF SNOW TO EASTERN WASHINGTON AND OREGON...WITH
SHOWERS LINGERING IN OREGON MONDAY. COLD NORTHERLY FLOW WILL REMAIN
OVER THE AREA TUESDAY. UPPER LEVEL FLOW WILL BECOME MORE NORTHWEST
WEDNESDAY AND THURSDAY WITH COLD AIR TRAPPED IN THE COLUMBIA BASIN.
A SERIES OF DISTURBANCES IN THE NORTHWEST FLOW WILL BRING VARYING
CHANCES OF WINTER PRECIPITATION WEDNESDAY AND THURSDAY.  COONFIELD