Wallowa Avalanche Center

Conditions Summary for April 9, 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.

Thank You:

The Wallowa Avalanche Center wishes to thank our Sponsors (home page logos) and all of our individual donors who have contributed kindly to help support this entire effort. It is important to remind everyone that all our staff including our advisors contribute their time to this center voluntarily. None of us receive any compensation for the services we provide to you. All money raised is used to fund our corporate expenditures, website hosting expenses, purchase of educational materials, weather instrumentation, support materials for our observers network and so forth. We hope as we gear up for another fabulous backcountry winter in the fall, that all of you will continue to provide the same level of support for us as you did this season.

General stuff to watch for ‘til The End:

Wet loose or Wet slab type activity: During forecasted sunny or warm spells think about skiing around the compass. Start on east aspects and work through southerly, then westerly and finishing up on northerly. You can enjoy the cream as it softens in step with the suns movement. Remember to allow yourself an exit strategy that doesn’t require entry onto or under steep sloppy sun baked slopes to get outta’ there. If the mush/slush/goo is nearing ankle deep; better get off quickly. Steeper slopes heat faster than flatter slopes because they are tilted more toward the sun.

A big warning: If it just snowed overnight for instance and the next day is blue-bird and warm, the new snow will be much more sensitive to turning quickly to mush or mash potatoes as some call it. There is much less margin of error in this instance when timing your ski outing in avalanche terrain.

In any of these wet instances look for the start of roller balls, pinwheels, really squishy snow, snow that you can almost wring the water out of, etc.

New snow landing on top of an old surface which hasn’t refrozen: In spring, a classic situation is the mush/slush/goo is still mush/slush/goo when the new snow begins falling. Temps never got below freezing, so you have new snow on a lousy surface layer with poor interlayer bonding properties. Sometimes as daytime heating occurs in this instance water percolates through and into the slush/goo or reaches an impermeable snow layer (like an ice crust), pools and causes a really nice lubricant for an avalanche to get going. This type of concern is not readily obvious from looking at the surface snow. Get down and dig often to check the status of this issue.

Wind transported snow: Springtime can be the same as wintertime when a cold snap occurs with lots of snow and blowing snow. Same principals apply as we have been cautioning about all winter. Pillows, drifts, crossloading, harder wind slabs, but now they may or may not be building on a mush underlayer.

Rain: Bad news. Less so if the snowpack is already a solid concrete mass all the way through. Quite worrisome if the snowpack still has leftover remnants of weak layers. Huge problem if you’ve got new snow falling or has just fallen and now it has changed to a torrential downpour. Yikes!

Falling things: Cornices in spring begin to droop and will break on their own accord without warning. Rockfall. These hazards can certainly trigger an avalanche or hit you outright.

Bridges: As the snowpack melts, snow bridges over creeks and hollows which were convenient in mid winter may not support you any longer. Some depressions, areas around wetlands, cliffs or rock bands and sharply concave chutes and couloirs can have liquid water flowing underneath in a space or void and the snowpack can collapse with the weight of a person.

Water percolation: Especially when the liquid water has reached the ground level. Now the water will saturate the ground surface: grass, mud, and particularly a slick rock surface (like Hurricane Creek’s Slick Rock area). The water then acts as a very good lubricator and natural avalanches can release without warning. Sometimes they travel very fast such as on a slick rock surface and sometimes they travel much slower, called a glide avalanche.

Some additional things to bear in mind: Even with only 2 to 5” of new snow to work with, on continuous steep slopes wet loose or wet slab slides may run far, entraining a lot of snow. So expect slides running full track and surprisingly large, deep debris piles. However, these wet loose sluffs are about as predictable as it gets – and thus avoidable. Watch out for people above and below you, as a triggered slide may run further than expected.

Lingering early season weaknesses that may be trouble:

Buried surface hoar that is isolated mostly (but not only) on northerly and easterly aspects. As of early April this buried weakness has caused at least one confirmed hard slab avalanche.

That depth hoar layer down by the ground has still not fully bonded and can easily cause a VERY BIG and dangerous avalanche should melt water begin to percolate down into this layer and destroy what little bonding that layer currently has. As the season progresses any upper layer failure causing an avalanche to begin and get moving could certainly provide enough weight to step down all the way to ground and involve this depth hoar layer.

The Bottom Line Synopsis:

As it stands today (Friday) there are likely to be several concerns to be watchful for.

From the top snow surface downward:

1. Heavily wind effected terrain from the nasty strong winds of yesterday (Thursday). First from the S and then changing to the NW. The new light density snow that fell early in the week during calm winds will be blown all over. Wind slabs and wind crusts are quite likely to be present and sensitive.
2. Any soft snow which has not been wind effected or soft snow trapped between windslabs and the older surface of late last week is where a weakness may be lurking. Early in the week, before the wind event, the soft snow was not bonding well to the older surfaces on almost all aspects. Some of this is due to recent graupel events, warming or a bit of sun, and off and on squalls of snow with varying density.
3. Buried surface hoar approx. a meter down in the snowpack is still problematic. Additional loading from wind or future storm cycles could easily cause this layer to give way and create a very BIG and dangerous avalanche. Although most likely isolated to N -> E aspects, I wouldn’t bet my life on it.

Photo 1: Hard slab avalanche on S. Ridge on NNE aspect at 8000 feet. This photo taken 4-3-10 and since then the crown and flanks have filled in quite a bit. The crown was approx. 3 1/2 feet and the slide ran on the buried surface hoar likely deposited Feb. 6-8.

Recent activity: Soon after the storm cleared enough to see last weekend many new natural’s appeared all along the Chief Joseph range. A most recent large one possibly released just after the Monday/Tuesday snow storm this week. Although harder to see now, if you look close you’ll count upwards of 10. Some big, some small. These are all generally on NE aspects and somewhere in the range of 7500 to 8500 feet starting zones.

Several other naturals were found on S. Ridge (Salt Creek Summit area). These were at the head of the ridge, lookers left of S. Bowl all on N -> NNE aspects at +/- 8000 feet. Two were sluffs or soft slabs, difficult to tell because they were covered by the most recent snow. Most importantly there was a mid-sized hard slab avalanche spotted on Saturday 4-3-10 on S. Ridge. After a crown profile and crystal study was performed on 4-7-10 it was found that it released and ran on the buried surface hoar deposited long ago. See pictures and crown profile diagram.

Photo 2: Keith performing a crown profile analysis. See PowerPoint diagram for details. Photo taken 4-7-10.

Latest Conditions: One Wing Ridge Client and of course our very own Ken and Charla have provided nearly play-by-play daily weather observations from 4/1 to 4/7 at elevation. Abbreviated they state:

• Gale force winds from the south on Friday (4/2) last week moved the 6” or so we got on Wednesday (3/31) and overnight.
• During the day Friday it began snowing with most falling heavily in the afternoon and evening. Winds played a huge part this day in moving snow.
• Winds shut down on Saturday and skies cleared. Storm totals: nearly a foot
• Sunday was fair with increasing winds from the SSE.
• Monday brought another 3+ inches but winds were nearly calm.
• Tuesday and Wednesday were just splendid, a squall or two but mostly clear and warming up by Wed. afternoon
• Thursday the winds again began in earnest from the SW moving toward NW ushering in a dry cold front. See the weather graph which shows distinctly the cold front with a rapid change of temperature, a shift in wind, and a noticeable smooth dip in barometric pressure.

Figure 1: Weather Graph from Thursday morning at Sheep Ridge showing frontal passage around 7am.

Photo 1: This photo was taken 4-7-10 from the uppermost flank of the larger hard slab slide that occurred on S. Ridge of Wing Ridge. Photo is looking toward S. Bowl; avalanche is behind photographer.

Concern Summary:

• Wind driven snow
• Very sensitive new snow bond to older snow surfaces (may involve a crust or graupel layer)
• Somewhere around 30” down as of Wednesday is a layer of quite sensitive high energy weakness. Maybe associated with buried surface hoar, or something else.
• Generally around a meter below the surface on Wednesday is a buried surface hoar layer but is dependent on elevation and aspect.

Figure 2:  Crown profile of S. Ridge (Wing Ridge)  natural avalanche. Entry point to perform crown profile was  from uppermost right hand side of photo above the tall evergreen on right hand side.

Figure 3: Ken provided this profile from Monday 4-6-10

General Announcements:

General Announcements: Over summer we’ll be planning many things. Every so often we will update the home page blog with what we’re up to. Each of us involved with the avalanche center has other pursuits in the summer. Keith bicycle tours all over the country and we’ll get others to share what they’re up to in summer.

Weekly summaries will start up once again in the late fall. We’ll keep you posted on the home page blog. Enjoy the remainder of the ski season and travel wisely and safely.

Current Regional Weather Synopsis:

.AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
220 AM PDT FRI APR 9 2010

.SHORT TERM...TODAY THROUGH SUNDAY...A ZONAL WESTERLY ONSHORE FLOW
WILL ALLOW SCATTERED SHOWERS TO PERSIST OVER THE CASCADES TODAY.
ELSEWHERE...DRIER AND WARMER CONDITIONS ARE EXPECTED AS A RIDGE OF
HIGH PRESSURE BEGINS TO BUILD IN FROM THE SOUTH. EVEN WARMER WEATHER
IS ANTICIPATED ON SATURDAY. WITH THE RIDGE AXIS STRENGTHENING OVER
THE AREA...DAYTIME HIGHS WILL RANGE FROM THE MID 50S TO MID 60S.
HOWEVER...A DEEP LOW APPROACHING THE CALIFORNIA COAST IS FORECAST TO
SEND A WEEK SHORTWAVE INTO SOUTHERN OREGON ON SATURDAY
AFTERNOON...THUS ISOLATED SHOWERS WILL BE POSSIBLE IN AND AROUND
DESCHUTES COUNTY. THIS SHORTWAVE IS EXPECTED TO AMPLIFY AS IT
PROGRESSES NORTHWARD ON SATURDAY NIGHT FOR A CHANCE OF RAIN SHOWERS
AND MOUNTAIN SNOW SHOWERS IN NORTHEAST OREGON. PRECIPITATION WILL
THEN SPREAD INTO SOUTH CENTRAL WASHINGTON BY SUNDAY MORNING. AS THE
LOW MOVES CLOSER TO THE NORTHERN CALIFORNIA COAST ON SUNDAY
AFTERNOON IT WILL ADVECT ADDITIONAL MOISTURE AND ENERGY INTO THE
REGION AND ANOTHER ROUND OF SCATTERED SHOWERS WILL BE POSSIBLE
ACROSS CENTRAL AND NORTHEAST OREGON.  82

.LONG TERM...SUNDAY NIGHT THROUGH THURSDAY...AN UPPER LEVEL LOW OR
TROUGH WILL REMAIN OFF THE COAST THROUGH THE LONG TERM WITH
SOUTHERLY UPPER LEVEL FLOW OVER THE PACIFIC NORTHWEST. A SERIES OF
WAVES IN THE SOUTHERLY FLOW WILL KEEP A CHANCE OF PRECIPITATION OVER
EASTERN WASHINGTON AND OREGON.