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Subj: Avalanche Fcst - Central Cascades - Oregon
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Sent: 221227/1024Z 3888@W7WXV.#COR.OR.USA.NOAM BPQ6.0.23
Backcountry Avalanche Forecast
Central Cascades
Issued
Monday, December 26, 2022 - 8:19PM
Expires
Tuesday, December 27, 2022 - 8:19PM
THE BOTTOM LINE
Heavy snowfall and strong winds will create dangerous avalanche conditions at
treeline and above. Wind Slab avalanches will be likely on any North and East
facing slopes that the wind has blown snow onto. Often strong winds can create
wind compacted snow (and potential wind slab avalanches) lower in the terrain
than we are used to.
Avalanche Danger
Tuesday, December 27, 2022
Above Treeline 3 - Considerable
Near Treeline 3 - Considerable
Below Treeline 2 - Moderate
Avalanche Problem #1
The forecast is for 1.5 - 2 feet of new snow at treeline elevations. Combine
that with Strong winds from the SouthWest and we have a good recipe for Wind
Slab avalanches.
As we know here in Oregon, the wind can do a great job of picking up snow and
blowing it onto lee slopes (transport). It also breaks down the snow crystals
as it moves them and can pack them into a consolidated slab that we call a Wind
Slab. When those slabs overlay some type of weak layer we have Wind Slab
avalanches. These avalanches typically occur at treeline and in alpine
elevations where the wind plays the greatest role.
These avalanches exist most often below ridgelines and along features that act
as wind breaks. With Strong and Extreme wind speeds these loading patterns can
be a bit less predictable and sometimes we find wind packed snow lower in the
terrain.
Avalanche Problem #2
Gradually cooling temperatures overnight and during Tuesday should limit the
potential for Storm Slab avalanches. Nonetheless Storm Slabs should be on
everyone's radar with such heavy rates of snowfall in the forecast. The
potential for dangerous storm slabs will increase as the snow piles up. At
nearly all relevent elevations this new snow will overlay very wet snow from
recent rainfall, which may briefly contribute to instability in steep terrain.
Test slopes are really the best tool to evaluate the storm slab problem. If you
see cracking on test slopes or other signs of consolidation in the new snow,
steer clear of unsupported, steep terrain particularly in conjunction with
terrain traps.
Forecast Discussion
Several days with rainfall have put a bit of a damper on some folks riding over
the holiday, and taken a bit of a toll on our snowpack as well. It is looking
promising that the current atmospheric river could be cold enough by Tuesday to
deliver some significant snowfall. Tuesday's avalanche problems will be the
Wind Slabs and Storm Slabs we associate with major winter storms. New snow will
overlay some very wet snow from recent rain. The Dec. 08 buried surface hoar is
still evident in the snow pack (down 25-50 cm.) but is predominently unreactive.
NNNN
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