Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 261010

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PDT Tue Sep 26 2017

.SYNOPSIS...A strong ridge of high pressure aloft combined with
developing low-level offshore flow will give warming and drying
to the area today through Thursday. Weather will become cool and
unsettled Friday onward as a cold front moves across the area and
an upper level trough settles into the region.


.SHORT TERM...Satellite imagery shows abundant cloud cover along
the coast and over the southern portions of the interior while
breaks in the clouds are seen over the interior from about the
Seattle area northward. Some patches of fog have formed over the
coastal zones and in the area that has cleared over the interior.
The lower levels of the atmosphere will rapidly dry out later
today as a sharp upper level ridge over the offshore waters shifts
gradually eastward into the area and surface high pressure
offshore builds into southern British Columbia inducing low level
offshore flow. Temperatures with increasing sunshine this
afternoon will rise into the mid 60s to mid 70s, quite nice for
the end of September.

While offshore flow continues to slowly ramp up tonight into early
Wednesday, good diurnal cooling and moisture trapped below an
increasingly shallow inversion should result in some patchy fog
again on Wednesday morning. Most of the fog will be seen over
typically foggy river valleys and around the Strait of Juan de
Fuca. Any fog that does develop will rapidly burn off late in the
morning leaving a dry, sunny, and warm day across the area with
highs getting well into the 70s across the area. Some lower 80s
are possible through the Chehalis Gap and in the interior from
about Shelton and Olympia southward.

Thursday promises to be the warmest day of the upcoming warm spell
as the upper ridge shifts off to the east and offshore flow
continues. Most locations in the interior will see temperatures
rise into the upper 70s to mid 80s for what will likely be the
last day of summer-like weather for the year. Enjoy this last gasp
of summer!  Albrecht

.LONG TERM...Onshore flow ahead of a frontal system will increase
Thursday night. Then a weak front will move across the area on
Friday giving a chance of some light rain to the area, mainly over
the Olympic Peninsula. With the increase in moisture and the rapid
decrease in insolation this time of year, maximum temperatures
were cooled to a few degrees below normal starting Friday.

Forecast models are generally consistent for Saturday showing an
upper trough moving across the area giving showers.

The ECMWF and Canadian model ensemble means, as well as a number
of GFS ensemble members show general upper troughing continuing
over the Pacific Northwest Sunday through at least the middle of
next week, while the 00Z GFS operational solution shows
northwesterly flow aloft and drier conditions overall starting on
Sunday. The forecast for Sunday onward will lean in the direction
of the wetter and cooler ensemble means and operational ECMWF per
guidance and discussions issued by the Weather Prediction Center
in College Park MD. Albrecht


.AVIATION...An upper level ridge will shift inland today for dry
and stable weather. The low level air mass will remain moist with
areas of low clouds and fog - LIFR to MVFR ceilings possible this
morning. The air mass will dry out this afternoon for VFR
conditions. Skies will clear tonight with a return of fog and/or
low clouds near the surface. 33

KSEA...Light/variable wind becoming northerly by 15-18z. MVFR
cigs possible early this morning with moist low levels. VFR
conditions expected this afternoon. 33


.MARINE...The flow will turn northerly today then offshore on
Wednesday as a thermal trough forms along the coast. Onshore flow
will increase on Friday as a cold front moves inland. Onshore flow
will persist through the weekend. 33





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