Wallowa Avalanche Center

Conditions Summary for April 2, 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.

The Bottom Line Synopsis:

It has been a stormy week with typical spring time conditions of sun, graupel, sleet, rain, high winds and over a foot of new snow in a 24 hour period.  Before the weekend several inches of graupel and snow fell with temperatures in the mid 30’s and moderate N winds. Saturday was a glorious day to be out and enjoy the clear skies and relatively calm winds - which was the calm before the storm. On Sunday strong gusty winds 20 G 40 mph mainly out of the South began ahead of the storm which started late afternoon. Those winds continued for 24 hours creating a lot of wind deposition. The temperatures were cold initially then warmed up and precipitation fell as denser snow and sleet up to 7500 ft creating a crust at mid elevations  on 3/28-9 before changing to snow again and dropping 13” on the Mt. Howard Snotel and nearly 2 ft. in the Wing Ridge area through 3/30. on Tuesday stayed cool the mid to upper 20s with light winds out of the South and ridge top winds 15 – 20 mph with snow showers. Wednesday was warmer, with temps reaching into the mid-30s and light and variable winds mostly out of the South and heavy snow at times dropping several more inches of snow. The sun broke out briefly Wednesday and was starting to affect the snow by early afternoon, making it noticeably heavier and a little sticky in places. 

Latest Conditions

It is difficult to tell that nearly 2 ft. of snow fell due to the wetter snow, sleet and graupel forming a supportable crust with 6” of lighter snow on top. This crust is evidenced at lower and mid elevations while climbing by the sound of “squeegee” under foot and all though upper elevations did not see the rain, there is still a supportable crust from a denser layer of graupel and wetter snow so ski penetration is still only 6”. Ken Bronec observed the following on Tuesday and Wednesday:

The upper few inches are a fluffy stellar/graupel mix. The few inches below it are slightly denser, and made up of alternating layers of stellars and graupel.  These alternating layers bonded surprisingly well to one another.  However, one of the layers of concern identified with hand shears and a compression test is located where the bottom of this graupel layer cake interfaces with the previous snow surface.  The previous snow surface is topped with a 1-inch thick ice layer at lower elevations, generally below about 7500’.  Hand shears varied from easy to hard depending on whether or not the ice layer was present. Where it was present bonding of the new snow to the crusted old snow surface was poor. At higher elevations, where the crust was not palpable, bonding between these new snow layers and to the old snow was much better, but still a snowpack weakness (CTH (29) in one test). Ski penetration was only -6” inches on all slopes I observed, with skiers riding on top of the old surface below the stellar/graupel layers. 

Skiing both days was excellent on north to east facing slopes. A number of folks were out on Wednesday enjoying the new bounty. The stability suggested by hand shears and CT tests was confirmed with lots of ski testing.  By the end of the day Wednesday many of the bigger alpine lines from South Ridge to Middle Ridge had been joyfully skied.

Photo 1: Phat Ridge 2010-03-30. Layering of the graupel and stellers. On the right side of the photo is where the CTH (29) broke right at the interface of the graupel/stellars with the older snow.

Concern #1: New snow bonding with variable snow pack

In another test, the top 6” of new snow failed easily and cleanly on top of the crust on several N aspect tests at 8000. Below that was a fairly cohesive slab with a hard failure and moderate energy (CT23Q2) at 14” on a 45’ slope at 8000’. This assessment was done after the storm while at the same aspect and elevation on a 35’ slope in another location during the storm. Just one day earlier similar results were found in the top of the snow pack but not below. The slab failed easily and cleanly at 6” with moderate compression and energy at 15” (CT12 Q2) just as the first test but it failed again at 23” (CT14  Q2 ) and again at 30” with (CT16  Q2 ). Is this variability with the three above tests due to the observers or the observations happening during versus after the storm, or is the snow pack variable enough to cause these differences between N and NE aspects?

Concern #2: Wind Deposition

With strong southerly winds during the heaviest snowfall a lot of snow was deposited onto the NE aspects primarily and the cornices grew rather quickly. Be leery of getting too near or below these cornices and   cognoscente of the result of the wind slab and weight of the newly transported snow.

Concern #3: Deeper weak layers

Tests on S aspects showed good stability with the 6” of new snow shearing easily with moderate energy on top of a solid pencil/knife slab. However, the S exposures tend to have thinner coverage due to being blown off in a lower snow year so you will probably need to venture onto the N & E aspects. Those aspects still have a weakness about 3 ft. below the surface from the buried surface hoar which is now very hard to see. Be respectful of open terrain because we are still in a winter snow pack even though it is April and we just had a big storm.  If triggered from on or near a ridge where coverage is thinner, rock outcrops heat up in the spring sun or where wind loading is present there is still good potential for an avalanche. Though the snowpack is showing fairly good stability, there are still persistent weak layers several feet down and at ground level which need to be respected.

Figure 1: Generalized profile

Recent Avalanche Activity

No new avalanche activity has been observed since the storm. On Saturday before the storm when it was warm and sunny several CF’s were observed on NNE aspects of the Imnaha Divide and ridge above Bonny Lakes in the Big Sheep area.

Concern Summary

Remember that in the spring time when the sun comes out that rapid temperature changes cause rapid changes to the snow pack. There were 3 recent deaths of snow mobilers triggering avalanches in ID and the ID/MT border. Both had crowns of 2 – 4 ft and were 2-400 yds wide which most likely broke on the buried surface hoar layer that we have here and is still evident throughout much of the intermountain west. One avalanche occurred on Saturday after direct sun had been on the slope all day and the other occurred after the storm on Tuesday.

Use good judgment when assessing the snow, terrain and weather factors and remember that our snow pack is quite variable so what you see may not always be what you get next door or in an hour.

Lastly, with the large snow fall and heavy winds pay attention to wind loading, wind slab and loaded cornices especially with another large storm of equal magnitude and wind on the way today (Friday)!

Average Snow Pack depths at 8000’:

Northern and Eastern Wallowas: 12 – 24” new,  104” (8.5 ft). This is hard to believe but that is what several of us found.

Southern Wallowas: No recent data

Observations:

We received a limited number of observations this week. All are from the Wing Ridge / Big Sheep Area.

3/25: NW aspect Little Sheep Divide, 7500 ft  CT16 Q2 @ 16”

3/27: NW aspect South Fork of Big Sheep Divide, 8200 ft  CT6 Q1 at 14”  RB3 Q1 at 14— (Did not ski that slope)

3/28: Same slope as above RB2 Q2 at 18” (Still didn’t ski that slope)

Figure 2: Pit profile from Phat Ridge (Wing Ridge Area) 2010-03-30.

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 jet stream that has alluded us for most of the season is lining up for a direct hit on NE Oregon. National Weather Service models project a significant inflow of moisture from the Pacific with a strong westerly flow that should produce a good spring dumping. Winds at 500 mb (18,000 ft) are expected to double in speed to over 60 MPH by later today. Current projections show the greatest potential for snowfall amounts in the northern Blue Mountains. It’s still a little early (Thursday NWS briefing) to pin down what the Wallowa Mountains should expect, however at least a foot can be expected at the higher elevations. The bulk of the storm is on track to reach the Wallowa mountains late in the day on Friday, with partial clearing and unsettled weather expected through the weekend.

Look for continued strong winds through this storm cycle as well.

General Announcements:

April 9 is closing day for the weekly summaries. On that day we will have a host of generalities to watch for between that day and when people have had enough of hiking up with all their gear for 3 turns or some such thing. I grew up as a teen skiing on Tuckermans’ Ravine on Mt. Washington back east where yes they do hike all their gear up for about 3 turns; generally until about July 4.

We are now an official IRS 501(c)(3) non-profit organization. All proceeds you donate to us is fully deductible to the extent allowed by IRS law on your taxes. Please donate so we can begin setting aside important dollar amounts for the multitude of exciting projects we would like to plan for next year. Email us and tell us your thoughts: info@wallowaavalanchecenter.org

Current Regional Weather Synopsis:

.
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
230 AM PDT FRI APR 2 2010

.SHORT TERM...TODAY THROUGH SATURDAY...A WARM FRONT IS MOVING TOWARD
THE COAST...BRINGING RAIN AND SNOW TO THE CASCADES. THE RAIN AND
SNOW WILL SPREAD EAST OVER THE AREA TODAY. THIS SYSTEM HAS LOTS OF
MOISTURE. THE HARD PART IS HOW MUCH SNOW WILL MELT WITH THE WARMER
AIR ALOFT BEFORE THE COLD FRONT MOVES THROUGH THIS AFTERNOON OR
EVENING AND CHANGES THE PRECIPITATION BACK TO SNOW. THE
PRECIPITATION WILL BEGIN AS SNOW IN THE MOUNTAINS...BUT STRONG WARM
ADVECTION WILL OCCUR AHEAD OF THE WARM FRONT. THE WARM SOUTHERLY
FLOW WILL LIMIT SNOW ACCUMULATION TODAY EVEN THOUGH THE SNOW LEVEL
ALONG THE EAST SLOPES OF THE CASCADES WILL BE 2000-3000 FEET. IN THE
BLUE MOUNTAINS BELOW 4000 FEET THE PRECIPITATION WILL CHANGE TO RAIN
THE AFTERNOON...THEN BACK TO SNOW AFTER THE WARM FRONT PASSES LATE
IN THE AFTERNOON. THIS WILL BE A MOIST SYSTEM WITH SNOW PERSISTING
IN THE MOUNTAINS THROUGH SATURDAY MORNING. VARIOUS WINTER SNOW
WARNINGS AND ADVISORIES HAVE BEEN ISSUED.

HIGHER WINDS ALOFT WILL MIX TO THE SURFACE AT TIMES ON
RIDGETOPS...THE DESCHUTES PLATEAU...AND THE WALLOWA VALLEY.
FUNNELING WILL ALSO INCREASE THE WINDS IN THE GRANDE RONDE VALLEY
AND THE NORTH-SOUTH PART OF THE JOHN DAY VALLEY. THE COLD FRONT WILL
MOVE ACROSS EASTERN WASHINGTON AND OREGON LATE FRIDAY AFTERNOON AND
EARLY EVENING. STRONG WESTERLY WINDS WILL DEVELOP IN THE TIGHTER
GRADIENT BEHIND THE COLD FRONT...AIDED BY HIGHER WINDS ALOFT MIXING
TO THE SURFACE. WIND ADVISORIES HAVE BEEN ISSUED FOR THE GRANDE
RONDE VALLEY...WALLOWA VALLEY...AND FOOTHILLS OF THE BLUE MOUNTAINS
FOR TODAY AND THIS EVENING...AND THE COLUMBIA BASIN FOR THIS
AFTERNOON AND EVENING. COONFIELD

.LONG TERM...SATURDAY NIGHT THROUGH FRIDAY...RAIN AND SNOW SHOWERS
WILL CONTINUE THROUGH SATURDAY EVENING BUT WILL DIMINISH BY SUNDAY
MORNING. ANOTHER COMPLEX STORM SYSTEM WILL MOVE INTO THE AREA LATE
SUNDAY...WITH PERIODS OF SNOW AND LOW ELEVATION RAIN PERSISTING
THROUGH TUESDAY. WHILE IT IS STILL TOO EARLY TO PIN DOWN SPECIFIC
SNOW ACCUMULATIONS WITH A HIGH DEGREE OF CERTAINTY...THERE IS A GOOD
CHANCE THAT LOCATIONS ABOVE 3500 FEET WILL SEE SEVERAL INCHES OF
SNOW BEFORE THE TROUGH EXITS THE REGION ON TUESDAY NIGHT. DRIER
CONDITIONS CAN THEN BE EXPECTED ON WEDNESDAY AS A RIDGE OF HIGH
PRESSURE BUILDS OVER THE AREA. HOWEVER...ISOLATED TO SCATTERED
SHOWERS WILL REMAIN POSSIBLE IN THE NORTHEAST MOUNTAINS DUE TO
NORTHWESTERLY FLOW ON THE BACK SIDE OF THE TROUGH. THERE ARE
CONSIDERABLE MODEL DIFFERENCES FROM WEDNESDAY NIGHT ONWARD...BUT
BOTH THE 00Z GFS AND THE 00Z ECMWF DEPICT SOME FORM OF A SHORTWAVE
MOVING THROUGH THE PACIFIC NORTHWEST ON THURSDAY...FOLLOWED BY A
STRONGER DISTURBANCE ON FRIDAY OR FRIDAY NIGHT. THEREFORE...HAVE
LEFT CLIMO POPS IN THE FORECAST FOR THE END OF THE WEEK.
TEMPERATURES WILL GRADUALLY RISE WEDNESDAY THROUGH FRIDAY AS
SOUTHWESTERLY FLOW REESTABLISHES ITSELF.  82

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