Download Stevens Pass - Northwest Avalanche Center

 Stevens Pass
Issued: 8:52 AM PST Wednesday, January 18, 2017 by Garth Ferber
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.
Forecast day corrected at 850 am.
The Bottom Line: Heavy sleet or freezing rain should add to a thickening crust, limiting the avalanche danger in the
passes, but also causing very poor travel conditions.
Elevation
Outlook for
Thursday
Wednesday
Above Treeline
Considerable
Dangerous avalanche conditions. Careful snowpack evaluation, cautious
route-finding and conservative decision-making essential.
Moderate
Near Treeline
Considerable
Dangerous avalanche conditions. Careful snowpack evaluation, cautious
route-finding and conservative decision-making essential.
Moderate
Below Treeline
Low
Generally safe, watch for unstable snow on isolated terrain features.
Low
Avalanche Problems for Wednesday
Wet Slabs
Wet slabs occur when there is liquid water in the snowpack, and
can release during the first few days of a warming period. Travel
early in the day and avoiding avalanche paths when you see
pinwheels, roller balls, loose wet avalanches, and during
rain-on-snow events.
Avalanche
Problem
Aspect/Elevation
Likelihood
Size
Avalanche
Problem
Aspect/Elevation
Likelihood
Size
Loose Wet
Loose wet avalanches occur where water is running through the
snowpack, and release at or below the trigger point. Avoid
terrain traps such as cliffs, gullies, or tree wells. Exit avalanche
terrain when you see pinwheels, roller balls, a slushy surface, or
during rain-on-snow events.
Snowpack Analysis
Weather and Snowpack
A pair of warm fronts moved across the Northwest last weekend, with fluctuating temperatures to near or above freezing in areas away from the Passes with
milder Pacific air scouring the colder air in the Passes late Sunday night 1/8. This allowed a rain or melt freeze crust to form in some areas below treeline before
5-15 inches of snow accumulated through Monday 1/9. A weak low pressure system, tracking across Oregon Tuesday morning through Wednesday afternoon, deposited another 3-6 inches of snow with light winds
from Stevens Pass and southward, with Paradise adding another 12 inches. A period of strong E to NE winds occurred Tuesday afternoon through early
Wednesday.
An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday through Sunday, with moderating temperatures at higher elevations. Several
NWAC stations warmed into the lower 40's by Monday while strong temperature inversions and breezy cold east winds persisted in the Passes. Warm, wet and windy weather has returned to the Cascades Tuesday with the rain intensities really increasing Tuesday afternoon, 1/17 to all elevation bands. In the Stevens and Snoqualmie Pass areas, strong east winds have maintained a thick layer well below freezing air with very mild air aloft. This has caused
sleet or freezing rain in these pass areas. The sleet and freezing rain have caused a thickening surface crust to form over the entire snowpack, yuck.
Recent Observations
NWAC pro-observers Dallas and Ian Nicholson were in the Crystal backcountry Wednesday to assess the distribution and sensitivity of fresh wind slabs from
recent strong E-NE winds. They found that wind slabs were becoming larger as they moved into the near tree line band and exposed ridges. NE-E-SE facing
slopes were stripped of recent snow with new reactive wind slabs on N-W-S facing terrain and cross loaded features. In wind sheltered terrain below treeline,
the storm snow was right side up and lacked slab structure with excellent skiing conditions.
Some reports are available via the NWAC Observations page for Wednesday and Thursday. in summary skiers triggered wind slab layers on Mt Herman near
Mt Baker and in the Slot Couloir on Mt Snoqualmie on Wednesday. Potential wind slab layers were noted at Stevens and at Snoqualmie on Thursday, but no
avalanches were triggered.
NWAC pro-observer Jeremy Allyn was out in the Alpental Valley on Friday and mainly found that recently formed wind slab in the near and above treeline was
settling and strengthening.
Lee Lazarra was taking observations for NWAC in Glacier Creek, NW of Mt. Baker proper on Saturday. Lee found generally good riding conditions in the trees
on non-solar aspects. Solar aspects were softening and likely to form sun crusts overnight. Snow surfaces just below and along ridges were variable from last
week's winds and Lee found recent wind slabs to be unlikely to trigger in his specific area. Widespread surface hoar growth was noted in the usual spots like
creek beds, but surface hoar size diminished higher in the below treeline band. Ian was east of Stevens Pass on Monday, MLK Holiday near Lichtenberg Mountain. Any older wind slabs near tree line had stabilized and were not present on
SW facing slopes near treeline where they would be expected, if present. The steep solar exposed slopes produced dozens of small, size 1 loose-wet
avalanches. Dallas was in the Stevens Pass area Tuesday, 1/17 and reported the sleet and freezing rain had already formed a 1/4 inch crust over the snowpack by midday.
Detailed Avalanche Forecast for Wednesday
Expect periods of heavy precipitation of mixed forms, mainly sleet or freezing rain in the passes through Wednesday afternoon. Rain, or heavy wet snow may
fall in the upper elevation bands along with very strong winds through Wednesday.
A thick sleet or freezing rain crust should limit the avalanche danger, especially below treeline through Wednesday.
Near and above treeline, either rain or heavy wet snow should maintain unstable wet snow conditions or build new wind slab layers at the highest elevations. Mountain Weather Synopsis for Wednesday & Thursday
Out with the Arctic and in with the Tropics! The replacement of the Arctic air mass that was over the area will continue one more day on Wednesday in the
Cascades. The heaviest precipitation the 24 hours ending Wednesday morning has been at Mt Baker with over 4 inches. Crystal also had over 3 inches. A mix
of heavy sleet and freezing rain has been seen at Stevens and Snoqualmie Passes. The atmospheric river and stormy weather that moved to the Northwest on
Tuesday will gradually fade over the Northwest on Wednesday. The last weak wave on the front should move south to north over the area Wednesday late
afternoon and evening. Rain or snow will probably be heaviest the volcanoes on Wednesday. More rain, snow, sleet or freezing rain should be seen at Stevens
and Snoqualmie and east of the crest as the surface Arctic air mass continues to erode to the east. A shift to very weak west winds in the Cascade passes
should be seen Wednesday evening which should cause a bump in the snow levels there. The action will be over by Thursday with a weak upper trough and
cooler air mass moving into the Northwest, with the main flow in the south part of the trough moving to south California. There should just be some mostly some
light snow showers in the Olympics and Cascades on Thursday with lower snow levels. 24 Hour Quantitative Precipitation ending at 4 am
Location
Thu
Fri
Hurricane Ridge
.75 - 1.00
.25
Mt Baker Ski Area
2.00 - 3.00
.75
Washington Pass
.50 - .75
lt .25
Stevens Pass
.75
.25
Snoqualmie Pass
1.00
.25
Mission Ridge
.50
lt .25
Crystal Mt
2.00
.25
Paradise
2.00
.25 - .50
White Pass
1.00
.25
Mt Hood Meadows
.50
lt .25
Timberline
.75
.25
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
Wednesday Mid-day
5000'
4500'
1000'
5500'
6000'
*
Wednesday Afternoon
4500'
4000'
2000'
5000'
5500'
*
Wednesday Night Thursday Afternoon
3500'
3000'
2500'
3500'
4000'
Thursday Night
2500'
1500'
1500'
2000'
2500'
*
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.