Wallowa Avalanche Center

Conditions Summary for November 13, 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:

Off and on instability with snow showers and light accumulations have been the rule this week, except Wednesday/Thursday where up to 10 inches fell. The temperatures up in high country have been quite mild actually. Highs during the daytime hours have been at or slightly above freezing, but the lows have only been in the mid 20's on average. Winds have been from the SSW all week and quite strong and gusty, particularly Sun., Mon. and Wed.

New snow for this week has totaled nearly a foot. It's hard to get an accurate number above 8000' since the wind has moved it around so much. Both of my observation outings (Sun. and Wed.) were met with wind scouring, drifts and you name it. Everywhere above tree-line has places with nearly no snow and others with drifts and pockets 2-4' deep. See picture of the scouring. Wind board surfaces above tree-line are the norm, but in sheltered places or below treeline there is 4-5" of uniform medium density powder as of late Wednesday over the solid melt/freeze crust(s) I've been talking about for the last two weeks.

Photo 1: Upslope Hidden Peak, NE aspect, showing much scouring

Wednesday I noticed that most of the snow has been deposited in small gullys and depressions on slopes facing the north through NE. However the larger the NE slope is, the less snow there is due to the cross-winds ability to reach down and scour. See the two pics of the NE aspect of Hidden Peak taken Wednesday at about 8700'. Speaking of the place where I stood for these pics (UTM 0487348E 5008070N); Last December 29 (2008) a party of two were ascending this ridge when they triggered a massive avalanche that pulled out nearly the entire NE aspect to ground (entire view of both pics). Fortunately they were not caught and went to a safer spot elsewhere.

Photo 2: Looking downslope, NE aspect, Hidden Peak from 8750' ridgeline

Things to watch out for in the next few days:
Concern 1: New snow instabilities as the new snow always varies slightly throughout a storm.
Concern 2: Cracking and movement of wind slabs above 8000' ,above tree line and on ridges and summits that will continue to build with the additional snow expected and continued strong winds from the SSW today (Friday) and into Saturday. There are still many places where these wind slabs (pillowing and drifts) are resting on top of the old melt/freeze (MF) crusts which are slick. The bonding of any wind slab is still not good to these crusts. Wednesday I was able to crack and get some movement downhill of these slabs (which were fairly fresh) on slopes as gentle as 25 degrees.
Concern 3: There are areas (various aspects) at and below 8000' which are developing a loose granular 2" or so layer of snow below the new snow but above the MF crusts. See picture.
Evaluate these concerns carefully, before entering into avalanche terrain.

The Juicy, Techie Stuff:

That 3mm rain crust is alluding me in the McCully Basin. I know it’s there somewhere, but where? The 1-2” wet snow event of 10/27-29 which was a stout MF crust is now weakening from about the ENE aspect -> SW. The bonding is being eroded and is causing the snow and/or wc on top of it to move downslope with only moderate slope angles and nudging. Remember, the snow pack is very thin from about the E -> S -> W aspects. NW to approx. NE still maintains the slick surface, really hard (P) MF crust. Temperature gradients just above this MF crust have been substantial on only the cold nights from instrumentation. Other warm nights and particularly this new snow on top have slowed further facet development. Continue to pay attention to the weakening of this crust in your travels. Depth hoar is not here yet but will continue its advance in the thinner snow pack aspects only if we see colder clearer weather which is uncertain in the long range forecast for next week. The snowpilot diagram below represents the best possible average for that aspect and upper elevation.
 

Photo 3: Many places 4" snow depth, others at 3' or so. This @8400'

Photo 4: This middle loose granular layer could be problematic as mentioned

Figure 1: Snowpilot Diagram

Total snow pack depths from observations in this basin:
12 inches avg. @7500ft, 3 inches new on Wed. 11-11
15 incches @8450ft, 5 inches new on Wed. 11-11

Local Weather Forecast:
 

Forecast for Friday through Friday night is snow with moderate to strong SSW winds. 3-6 inches storm total through this period for the upper elevations. Saturday a cold front moves in with the winds changing to NW, mostly cloudy with highs in the low 20’s. Saturday night - cold. Sunday cloudy again but slightly warmer. Little accumulation after Saturday morning expected.

General Announcements:

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
300 AM PST FRI NOV 13 2009

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...THE NEXT SYSTEM
CONTINUES TO DRIVE QUICKLY TOWARDS THE PACIFIC NORTHWEST. THE SYSTEM
IS ARRIVING A LITTLE SLOWER THAN WAS EXPECTED THIS TIME YESTERDAY...
BUT IT IS STILL A FAST MOVER. DARKENING ON WATER VAPOR OFF THE
WASHINGTON COAST INDICATES A THAT A GOOD UPPER LEVEL JET IS NOSING
INTO THE SYSTEM. THUS IT SHOULD BE VIGOROUS AND A GOOD PRECIP
PRODUCER. YET THE SYSTEM SPEED WILL LIMIT IT/S TIME OVER ANY GIVEN
LOCATION...REDUCING TOTAL PRECIP ACCUMULATION. SNOW LEVELS ARE AT OR
NEAR THE SURFACE THIS MORNING. THUS THE AREAS TO BE AFFECTED THIS
MORNING...OUR WESTERN MOST ZONES...WILL SEE SNOW (MIXING WITH RAIN
AT ONLY THE LOWEST ELEVATIONS...SUCH AS THE COLUMBIA RIVER GORGE).
THERE IS THE SOME WEAK WARM AIR ADVECTION AHEAD OF THE SYSTEM AND
THIS COULD POSSIBLY PRODUCE SOME SPOTTY LIGHT SLEET OR FREEZING RAIN
IN THE LOWER COLUMBIA BASIN...GORGE AND THE VALLEYS NEAR THE
WASHINGTON CASCADE EAST SLOPES. THUS WILL MENTION A SLIGHT CHANCE OF
SLEET OR FREEZING RAIN IN THESE AREAS...THOUGH EXPECT MAINLY SNOW
THIS MORNING. THERE SHOULD BE ENOUGH INSOLATION TODAY TO RAISE THE
SNOW LEVELS TO BETWEEN 2000 AND 3500 FEET. THEREFORE EXPECT SNOW TO
CHANGE TO RAIN AT THE LOWEST ELEVATIONS THIS AFTERNOON. OVER CENTRAL
OREGON...EVEN WITH SNOW LEVELS RISING TO NEAR 3500 FEET...SHOULD
STILL SEE 2 TO 4 INCHES OF SNOW ABOVE 3500 FEET TODAY. THUS WILL
CONTINUE THE CURRENT ADVISORIES FOR THE CENTRAL OREGON AND JOHN DAY-
OCHOCO HIGHLANDS ZONES. ELSEWHERE SNOW AMOUNTS SHOULD STAY BELOW
ADVISORY LEVELS. THE SYSTEM WILL EXIT THE REGION AFTER MIDNIGHT
TONIGHT AND WILL THEREFORE SEE A GRADUAL DECREASE IN SHOWERS
OVERNIGHT. CONDITIONS WILL MOSTLY DRY SATURDAY WITH ONLY A FEW STRAY
SHOWERS LINGERING OVER THE EASTERN MOUNTAINS. A DEEPENING TROUGH
OFFSHORE WILL INCREASE WARM AIR ADVECTION CLOUDINESS OVER THE REGION
SATURDAY NIGHT. ALSO MAY SEE SOME LIGHT SNOW RETURN TO THE
WASHINGTON CASCADE EAST SLOPES. 90

.LONG TERM...SUNDAY THROUGH THURSDAY NIGHT...MODELS HAVE DEVELOPED
SOME CONSISTENCY IN HAVING A DEVELOPING TROUGH IN THE EASTERN
PACIFIC SUNDAY THROUGH ABOUT WEDNESDAY WITH A RIDGE TO OUR EAST. THE
TROUGH BRINGS A FRONT TO THE COAST SUNDAY AND GRADUALLY DIGS IT
SOUTH ALONG THE WEST COAST BUT NEVER QUITE PUSHES IT EAST INTO OUR
AREA. THIS WOULD GIVE A CHANCE OF SHOWERS IN THE CASCADES AND THEN
RAPIDLY DECREASE THE POSSIBILITY OF SHOWERS AWAY FROM THE CASCADES.
HAVE CUT BACK ON POPS MONDAY THROUGH WEDNESDAY. HAVE BROUGHT THE
TROUGH THROUGH THE AREA WEDNESDAY NIGHT THROUGH THURSDAY NIGHT WITH
A CHANCE OF RAIN AND SNOW ABOVE 2500 TO 3000 FEET. TEMPERATURES
REMAIN NEAR SEASONAL AVERAGES...PERHAPS A COUPLE OF DEGREES BELOW
NORMAL SUNDAY THEN A COUPLE OF DEGREES ABOVE NORMAL TUESDAY THROUGH
THURSDAY DUE TO SOUTHERLY WINDS OVER THE AREA. PERRY

&&