Wallowa Avalanche Center

Conditions Summary for February 26, 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.

Amended at 20:45 on 2010-02-26: Added additional information for southern Wallowas including a photo.

The Bottom Line Synopsis:

We are limited this week to data from the northern Wallowas, where the widespread melt/freeze crust that formed early last week and the spring-like weather in the Valley have started many folks thinking of warmer weather pursuits. For those willing to work a bit, good skiing was still had early this week above 7,000 feet the north and northeast slopes. More adventurous folks have been skiing the biggest alpine north and north east lines, reporting good “old powder” on the shaded slopes.  Tracks could be seen on nearly all alpine and higher timbered north and northeast slopes from the Stock Drive Saddle to Little Sheep Creek.  We needed some new snow.

It arrived as forecast.  Tuesday night brought an inch or so of new cold snow and Wednesday delivered another 3-5 inches (depending on elevation) of warmer, denser snow. This improved the riding a lot and brought in another weak layer to keep an eye on. Hand sheers showed an easy failure at the interface between these two new snow layers, as the denser upper 3” slid off of the lighter colder 1” beneath it.  Wednesday night brought another 3” of new.  With the milder temperatures we have had that 7” – 9” has settled down to about 3” of new snow below 7,000’ and 4”-5” up higher.

Temperatures in the northern Wallowas started off the week cold, with highs staying below freezing and nights dropping down to single digits.  Starting on Tuesday more mild temperatures prevailed, with highs Tuesday, Wednesday and Thursday reaching above freezing and nighttime lows in the twenties. Early in the week the winds were out of the north with enough energy to redistribute last weeks 1” of snow on open slopes. Since the snow started Tuesday night winds have been southerly and mostly light.

Recent activity: No avalanche activity reported this week from the northern Wallowas.

Photo 1: Southbowl gets a facelift:  Several inches of new snow this week on top of that crust has improved the skiing, but keep an eye on weaknesses in that upper foot of snowpack.

Concern #1: Buried crusts with associated weak layers located in the top 30 cm of the snowpack are of primary concern. Of most interest is a moderate density crust buried between 15 and 25 cm down on most aspects in the higher terrain.  This crust is two cm thick, with over 10 cm of lighter density snow above and below it.  Easy to Moderate compression tests are failing on the thin weak layer on top of this crust. Near the top of Phat Ridge at 8300’ (east aspect) that layer produced easy sheers in compression tests and an extended column test on Wednesday.   Thursday on South Ridge at 8,000’ (north aspect) the same layer sheared easily in multiple compression tests.

A second crust/weak layer combination to watch is found most everywhere except the protected north aspects.  Last week’s 2 cm thick MF crust on the snow surface developed a layer of facets underneath it.  As of Thursday afternoon it had 3”- 5” of new snow on top of it and was failing easily in hand shears on all aspects.    Keep an eye on this layer as additional loading from new snow and winds forecast through the weekend is applied. 

There is one more layer interface to keep an eye on. It is between the few inches of new warmer snow that fell on Wednesday and the colder snow from Tuesday night. These layers appear to be weakly bonded and failed easily with hand sheers on Wednesday afternoon. Additional loading from new snow and/or wind may also trigger this layer to fail too.

Concern #2: The buried layer of faceted grains near the ground is still there.  Hard CT scores have been found that far down in the snowpack, but it still holds potential for large consequences should you be able to get it to fail.

Figure 1: Idealized profile northeastern Wallowas.

Concern Summary:

The upper foot of the snowpack has been the most reactive in field tests this week.  All open slopes available to wind and any sun have a couple of buried crust layers that are reacting to hand shears and compression tests with easy shears.  Also, Tuesday night’s inch or so of cold snow has a few inches of heavier snow on top of it. This interface was shearing easily in hand shears on Wednesday afternoon.

The higher elevation shaded north-facing slopes have been tested and skied with good results for the past week.  However, these slopes too have a crust between 6” to 8” down with a weak layer on top of it.  On Thursday easy failure on compression tests were experienced at 8,000’ on a South Ridge north aspect.  Additional loading is expected on these slopes from this coming weekend’s storm cycle.

Be aware of the recent new heavy snow atop an inch of drier snow, and the buried crusts below which have faceted grains above and/or below them. These all will warrant attention as new snow and wind add additional loading.  Evaluate these layers of the snowpack carefully before entering avalanche terrain and steep slopes.

Average snowpack depths at 8000 feet this week:

Northern and Eastern Wallowas: 7-8 inches new, 56 inch total depth
Southern Wallowas: As of last week data indicated high 90 inch total depths
 

Figure 2: Phat Ridge Pit Data

The Juicy, Techie Stuff:

Observations this week are limited to the northern Wallowas in the area accessed by Salt Creek Summit Sno-Park. 

Hand shears and compression tests agreed this week that the top 30 cm of snow contains the most reactive layers. The widespread MF crust formed last week developed some faceted grains on its lower surface during the high pressure dominated weather late last week and over the weekend.  This interface was shearing easily with hand shears over a variety of aspects and elevations on Thursday after several inches of new snow had fallen on it. The next crust layer about 20 – 30 cm down also has weak grains associated with it and has been producing easy shear scores in compression tests and extended column test.  On Wednesday, near the top of Phat Ridge at 8300’, that layer produced easy sheers in two compression tests (CTE (1, 4), Q2) and an extended column test (ECTP (4), Q2).  The next day on a north aspect at 8,000’ on South Ridge, easy compression test scores (CTVE, CTE (7), Q2) were also experienced.

The deep layer of facets near the ground took a lot of pounding to make it fail (CTH (23), Q2) but, as has been noted numerous times in our summary, if it fails the consequences would be large.

Figure 3: South Bowl pit data.

Southern Wallowas Latest: Yesterday, 2/25, the skies cleared slowly and  temps ramped up quickly in the area. Wet loose slides were heard rumbling down on the lower flanks of the E aspect of Cornucopia Peak. Only very small fresh  soft slabs were spotted: SS-R1-D1 on mid elevation 6500-8000', easterly aspects.  ECTN4 on old/new interface on  east aspect of Simmons Ridge, CTE  Q2.

Photo 2: NE facing bowl in the Middle Fork of Pine Creek, the  SS-N-R4-D2 was suspected to be a result of the 2/12 storm.

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.

We have a strong southerly flow lining up to bring a warm front into the Wallowa Mountains. Current models indicate this front should reach us around Friday evening and bring with it some strong winds in the valleys and ridge tops. Mt Howard winds are modeled at gusting to around 40 MPH for most of the day Friday. The bulk of the precipitation should arrive Friday evening and could leave us with a few inches at the higher elevations. Snow levels are forecast at 4500 - 5800 ft. Saturday should bring unsettled weather and drying conditions on Sunday.

Early Friday morning, winds at Sheep Ridge were gusting to almost 40 MPH. Forecast winds aloft are in the 25-35 MPH rand from the southwest.

Current Regional Weather Synopsis:

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE PENDLETON OR
352 AM PST FRI FEB 26 2010

UPDATED AVIATION DISCUSSION

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT.  A MOIST SOUTHERLY FLOW
ALOFT IS SPREADING WIDESPREAD PRECIPITATION ACROSS THE WESTERN HALF
OF WA/OR.  DUE TO AN ALMOST STRAIGHT SOUTHERLY MERIDIONAL
FLOW...PRECIPITATION HAS BEEN A LITTLE SLOW TO SPREAD ACROSS THE
FORECAST AREA.  RAIN WITH SNOW ABOVE 6000 FEET IS LIKELY AS THE DAY
PROGRESSES.  A WIND ADVISORY IS CURRENTLY IN EFFECT FOR THE GRANDE
RONDE VALLEY THROUGH SATURDAY MORNING.  THE PRESSURE GRADIENT
BETWEEN ONTARIO TO MEACHAM IS 6.3 MB AND IS EXPECTED TO REMAIN ABOUT
AS STRONG THROUGH THE DAY SO THE WIND ADVISORY IS ON TRACK.  THERE
IS A 3.0 MB DIFFERENCE BETWEEN MEACHAM AND PENDLETON...AND WINDS
ALONG THE BASE OF THE BLUE MTNS ARE IN THE 20-30 MPH RANGE.  BELIEVE
THAT THERE IS A GOOD POTENTIAL FOR DOWNSLOPE WINDS GUSTING TO 45-50
MPH BY EARLY THIS EVENING OVER THE OREGON PORTION OF THE BLUE MTN
FOOTHILLS SOUTH OF PENDLETON AS WINDS ALOFT INCREASE.  MODELS ARE
SHOWING THE POTENTIAL FOR A MOUNTAIN WAVE OVER THE SOUTHERN BLUES
WITH WINDS AROUND 5500 FEET BETWEEN 45-55 KNOTS.  WILL ISSUE A WIND
ADVISORY FROM 2 PM TO 10 PM FOR THIS AREA.

PRECIPITATION WILL GRADUALLY DECREASE OVERNIGHT AND SATURDAY MORNING
WITH ISOLATED TO SCATTERED SHOWERS EXPECTED BY SATURDAY AFTERNOON.
THE UPPER LEVEL TROUGH WILL MOVE INLAND LATE TONIGHT THEN WILL SHEAR
APART AS IT MOVES EAST OF THE CASCADES ON SATURDAY WITH THE
STRONGEST ENERGY DIRECTED OVER CALIFORNIA.  PRECIPITATION WILL
DIMINISH SATURDAY NIGHT.  THERE ARE NO WINTER HIGHLIGHTS ANTICIPATED
IN THE SHORT TERM...BUT THE MTNS ABOVE 4000 FEET COULD SEE BETWEEN
1-3 INCHES LATE TONIGHT AND SATURDAY.  JOHNSON

.LONG TERM...SUNDAY THROUGH THURSDAY...A FEW SHOWERS MAY LINGER OVER
WALLOWA COUNTY SUNDAY...OTHERWISE A TRANSIENT HIGH PRESSURE SYSTEM
WILL KEEP CONDITIONS DRY OVER MUCH OF THE AREA SUNDAY THROUGH
MONDAY. ANOTHER DEEP...UPPER LOW WILL THEN APPROACH THE PACIFIC
NORTHWEST TUESDAY INTO WEDNESDAY. MODEL DATA INDICATES THAT THIS
UPPER TROUGH WILL SPLIT WITH SOME ENERGY SWINGING TOWARDS BRITISH
COLUMBIA AND THE REST OF THE ENERGY DRIVING INTO CALIFORNIA. THIS
MAY LIMIT THE AMOUNT PRECIP THAT MAKES IT EAST OF THE CASCADES. FOR
NOW WILL CONTINUE CURRENT LOW TO MID RANGE CHANCE POPS. MAY SEE A
BREAK BETWEEN SYSTEMS WEDNESDAY NIGHT AND THURSDAY. THERE IS ENOUGH
UNCERTAINTY IN THIS BREAK THAT WILL CONTINUE CURRENT SLIGHT CHANCE
TO LOW CHANCE POPS. 90