Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202152

Area Forecast Discussion
National Weather Service Seattle WA
252 PM PDT Wed Sep 20 2017

.SYNOPSIS...A large upper level low will maintain a chance of
showers and cooler temperatures over Western Washington through
Thursday. Drier north flow aloft from an offshore upper level ridge
will bring more sunshine and a little warming Friday and Saturday.
The weakening ridge will move over the area Sunday and Monday and
but could allow transient weak weather systems to bring some spotty
light rain. The ridge will strengthen on Wednesday for dry weather
and more sunshine.


.SHORT TERM...A large upper level low remains over the PacNW with
the main low center over Alberta. Somewhat moist and weakly unstable
northerly flow coming around the low and S over W WA will maintain a
mostly cloudy skies and a chance of showers into Thursday evening.
The flow aloft will still be drier than the air mass the past couple
of days so there will probably be some breaks in the clouds at
times on Thursday.

The northerly flow will dry out on Friday with areas of morning
clouds giving way to a mostly sunny afternoon. The clearing will
also set up good radiational cooling Friday night so that patchy fog
will be around Saturday morning. The upper level ridge will tilt
inland over W WA on Saturday but there will be quite a bit of flow
through the ridge, enough so that mid and high level moisture from a
weak warm front moving inland over B.C. may brush the area with some
clouds in the afternoon. The air mass will still be a little cooler
than normal. Kam

.LONG TERM...The upper level ridge will remain over the area with a
positive tilt Sunday and Monday night. This has the ridge axis
extending NE from the WA offshore waters across central B.C. There
will still be quite a bit of flow through the ridge with mainly mid
and high level clouds crossing W WA. Both the GFS and ECMWF have one
or two systems embedded in the flow aloft brushing mainly the N part
of W WA with some light rain. Unfortunately, the models don`t really
agree on which systems could bring the area some light rain, so the
Sunday-Monday night forecast is more of a broad brush with slight
chance POPs.

Both models briefly build the ridge around mid-week with 500 MB
heights rising to around 5820 meters. This should allow temperatures
to warm back to near normal. Unfortunately, once again the models
don`t quite agree on whether this will be Tuesday or Wednesday, so
confidence is not very high. Kam


.AVIATION...Scattered showers will dry up this evening and there
should be some partial clearing. Clouds set up again overnight as
the air becomes more stable and the area of cloudiness over
British Columbia moves down over Western Washington in
northerly flow aloft.

KSEA...Winds look like they will be variable in some light
convergence through this evening. There could be some clearing this
evening and then clouds will set up again overnight.


.MARINE...Weak low pressure over the area through Thursday will be
replaced with weak high pressure for the end of the week as an upper
ridge replaces the upper trough over the region.


PZ...Small Craft Advisory outer coastal waters and Strait of Juan de
     Fuca this evening.



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