White Pass
Issued: 9:05 AM PST Friday, February 7, 2014 by Dennis D'Amico
NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts
do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest
level. Updated Fri AM to add youtube link to west slopes of Cascades forecast zones. The Bottom Line: New wind slab formed over the last few days on lee aspects presents the greatest
avalanche danger Friday. Avoid wind loaded slopes near and below ridgelines, mainly on SW thru N aspects. Elevation
Saturday
Outlook for Sunday
Above Treeline
Considerable
Dangerous avalanche conditions. Careful snowpack evaluation, cautious
route-finding and conservative decision-making essential.
Considerable
Near Treeline
Moderate
Heightened avalanche conditions on specific terrain features. Evaluate
snow and terrain carefully; identify problem features.
Moderate
Below Treeline
Moderate
Heightened avalanche conditions on specific terrain features. Evaluate
snow and terrain carefully; identify problem features.
Moderate
Avalanche Problems for Saturday
Wind Slab
Wind slabs can take up to a week to stabilize. They are
confined to lee and cross-loaded terrain features and can be
avoided by sticking to sheltered or wind scoured areas.
Avalanche
Problem
Aspect/Elevation
Likelihood
Size
Snowpack Analysis
Very cold temperatures this week have produced strong temperature gradients in the upper portion of the snowpack. See WSDOT Snoqualmie pit profile below
for an example of the steep temperature gradient in the upper 10 cm and a more moderate temperature gradient (~ 10C/m) over the top meter. The strong
temperature gradient has lead to near surface faceting and a breakdown on recently buried crusts. The preservation of weak layers buried about a week
ago from the last storm cycle in and near the Cascade Passes continues during this cold spell as well. Public and NWAC observers continue to report
buried surface hoar on sheltered non-solar aspects in the below tree-line zone, most widely reported near the Cascade passes and just east of the crest under
about 25-50 cm of the most recent and settled storm snow. However, near surface facets on or just above the late Jan crust show the most sensitivity in
snowpack tests with high quality shears but low sensitivity to ski triggering and have not been linked to any recent natural avalanches. These layers are not
likely on solar aspects and the most recent storm snow has been reported as better bonded to the old crust on solar aspects. A thin sun crust has formed on
some steeper solar slopes.
Click to enlarge pit profile by Mark O'Geen, Snoqualmie DOT 2-5-14
Outside the passes these layers are not widely found south of Snoqualmie Pass and only to be reactive as of last weekend in isolated areas near Mt. Baker on
Shuskan Arm. Not to be overlooked...the cold temperatures have also preserved good surface snow conditions this week (read powder or recycled powder!). Newly formed wind slabs on lee slopes have been sensitive near Snoqualmie Mountain including 2 different skier triggered wind slab avalanches from
Wednesday. One slide caught a skier and was classified as a hard slab with a 16 in crown and entrained the settled storm snow down to the old crust. Shooting
cracks and Watch for recent wind transport even into lee below tree-line zone to put additional load on recently buried weak layers. Less snow was available for transport near Crystal, Mt. Rainier and Mt. Baker and should make for smaller but not necessarily less sensitive wind slabs. On Thursday NWAC observer Tom Curtis (see picture left and video link below) reported easy shears and good propagation in the upper snowpack
near treeline on a WSW aspect near White Pass. He observed the new wind slab to be sensitive with widespread cracking as he moved through the terrain. http://youtu.be/xzK8fCV2xfw
Photo by Tom Curtis near White Pass 2-6-14
Note: The forecast discussion is the same across the west slopes but the persistent slab concern is relegated to near Stevens and Snoqualmie Passes and the
Considerable rating extends into the near treeline zone due to potentially larger wind slab in those areas. Detailed Avalanche Forecast for Saturday
A weather system passing into Oregon should only bring light amounts of new snow from Snoqualmie Pass and south. Moderate to strong east winds are
expected to peak Thursday evening and ease on Friday with temperatures remaining quite cold. Until we receive more significant load...the increasing avalanche danger on Friday across the west slopes will be tied to new wind slab formed mainly on SW
thru N aspects, although potentially cross-loaded on other aspects, by east to southeast winds at Cascade Pass to crest level seen Tuesday through Thursday. East facing ridges may be scoured down to the late Jan crust.
Manage the terrain by avoiding freshly wind loaded lee slopes near and above tree-line. Watch for shooting cracks as a sign of instability. Even a small wind
slab in the wrong terrain, i.e. above cliffs or very steep slopes, could have unintended consequences. Continue to test for the presence of recently buried weak layers near Stevens and Snoqualmie Passes, mainly below tree-line and choose conservative slope
angles and avoid terrain traps if unsure about stability. Mountain Weather Synopsis for Saturday & Sunday
A low pressure system off the central Oregon coast Saturday afternoon is tracking towards the north OR coast. An associated frontal band has spread
increasing moderate to heavy precipitation to the Mt Hood and this band is gradually lifting northward over the southern WA Cascades. Increasing pressure
gradients ahead of the low pressure system offshore have caused easterly winds to increase across the Cascade crest and lower passes, keeping cold air in
place. The precipitation band is expected to make it northward to about Stevens Pass overnight Saturday with only light amounts of precipitation expected
there with less expected to reach the north Cascades. Another disturbance now well west off the Oregon coast Saturday afternoon should lift over the entire
forecast area Sunday to spread increasing light to moderate precipitation with increasing westerly winds and slightly moderating freezing levels by late Sunday
and early Monday. 24 Hour Quantitative Precipitation ending at 4 am
Location
Sun
Mon
Hurricane Ridge
lt .10
lt .25
Mt Baker Ski Area
lt .10
lt .25
Washington Pass
lt .10
lt .25
Stevens Pass
lt .10
.25 - .50
Snoqualmie Pass
lt .25
.50
Mission Ridge
lt .10
lt .25
Crystal Mt
lt .25
.25 - .50
Paradise
lt .25
.50
White Pass
lt .25
.50
Mt Hood Meadows
1.00
.50
Timberline
1.00
.50
LT = less than; WE or Water equivalent is the liquid water
equivalent of melted snow in hundredths of inches. As a
rough approximation 1 inch of snow = about .10 inches WE,
or 10 inches of snow = about 1 inch WE.
Snow Level/Freezing Level in feet
Day
Easterly
Northwest Northeast Central
South
Flow in
Olympics Cascades Cascades Cascades Cascades Passes
Saturday Afternoon Sunday Afternoon
500'
None'
None'
500'
1000'
Sunday Evening Sunday Night
3500'
None'
None'
2500'
4000'
*
Cascade Snow / Freezing Levels noted above refer to the north (approximately Mt Baker and
Washington Pass), central (approximately Stevens to White Pass) and south (near Mt Hood).
Freezing Level is when no precipitation is forecast.
* Note that surface snow levels are common near the passes during easterly pass flow and
may result in multiple snow / freezing levels.