Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131128

Area Forecast Discussion
National Weather Service Seattle WA
328 AM PST Fri Jan 13 2017

.SYNOPSIS...High pressure and light winds over the region will
allow a cold dry air mass to persist in place through Sunday, with
mid-high clouds passing by at times. Wetter and warmer west flow
aloft will develop Sunday night and Monday, bringing a series of
frontal systems across the area from Monday through next Thursday.


.SHORT TERM...Upper level high pressure will generally persist
through Sunday, at least in terms of 500 mb heights in excess of
550 decameters persisting. Weak shortwaves in the upper flow will
bring temporary increases of mid-high clouds tonight and again on
Saturday night, but the air mass appears too dry for precip. In
fact in recent days, models have been trending drier and drier
with the weather this weekend; they have now eliminated all but
the most scant amounts along the North Coast on Sunday.

Otherwise, the other effect of upper level high pressure is a
stable air mass. When combined with light winds, this means that
the cold and dry low-level air mass will be left to very slowly
modify and moisten in place until an increase in wind can finally
swap out the low-level air mass around Sunday night. So that
means a continuation of freezing nights and cooler-than-normal
afternoons through the weekend. As advertised, some freezing fog
has developed in the fog-prone Chehalis Valley early this morning,
and this is likely again on Saturday morning. Fog could even
venture beyond the Chehalis Valley and cover more of the
Southwest Interior on Saturday morning.  Haner

.LONG TERM...The weather pattern will undergo a big change on
Monday. Very low upper heights over Alaska will support the
development of a large, broad longwave trough over the eastern
Pacific, with a consolidated zonal jet stream developing along its
southern periphery. This will aim an atmospheric river type rain
event at portions of the U.S. west coast from Monday night through
next Thursday. All models show wet and windy weather over western
Washington on Tuesday and especially Wednesday. The ECMWF has
been most consistent in recent days in keeping flood-producing
rain to our south over Oregon and northern Cal. However, the
operational GFS and the Canadian show a lot of rain over the area
on Tue night and Wed (6+ inches in 24 hours over the Olympic Mtns)
concurrent with snow levels of 6000-7000 feet, which would
certainly cause river flooding. Yet the GFS ensembles reveal the
operational run as an outlier at the wet end of the scale.
Ensemble mean rainfall over Seattle is about half of the
operational run from Mon night through Wednesday; in fact the
operational run exceeds the rainfall shown by the wettest ensemble
member by a large margin. The situation bears watching for
rainfall and flooding potential, yet the odds still tilt away from
anything more than minor flooding on a river or two.

Rain will continue next Thursday. Rainfall amounts should be more
moderate, and snow levels will come down to around 2500-3000 feet.


.AVIATION...An upper ridge will will remain over the area
through the end of the week along with high pressure inland and
light offshore flow. The air will be dry and stable with cirrus
in northwest flow aloft.

KSEA...Cirrus, light and variable wind---mostly northeast.


.MARINE...Surface high pressure east of the Cascades will give
light offshore flow through Saturday. A weak front will dissipate as
it reaches the area this weekend. A series of stronger fronts will
reach the Pacific Northwest next week.


.HYDROLOGY...River flooding is not expected through Monday. A period
of heavy rain next Tuesday/Wednesday will cause rises on area
rivers. Minor flooding on a few rivers is possible. Note: Snow is
not a significant contributor to flood flows on western Washington
rivers. Even when there is snow on the ground, river flooding is
almost exclusively caused by runoff from heavy rain.




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