Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210411
AFDSEW

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

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

&&

.SHORT TERM...An upper level low oriented northeast to southwest
over the Pacific Northwest will slowly move southeast tonight and
Thursday. Flow aloft will become northeast Thursday and more
northerly Friday. The air mass will remain weakly unstable, giving
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 on Friday with areas of morning clouds
giving way to a mostly sunny afternoon. The clearing will also allow
radiational cooling Friday night so that patchy fog will be around
Saturday morning. The upper level ridge will move inland Saturday.
There will be quite a bit of flow through the ridge, perhaps enough
that mid and high level moisture from a weak warm front moving
inland over British Columbia could brush the area with some clouds
in the afternoon. The air mass will still be a little cooler than
normal. Kam/Burke

.LONG TERM...Previous discussion...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...Thu upper level trough over the Pacific
Northwest will dig southward into Northern California overnight.
Light northerly flow aloft will become northeasterly as the trough
shifts southward. Weak low level onshore flow tonight with some
partial clearing this evening. Clouds will fill back in with MVFR
ceilings by around 10-12Z over most of the interior and coast and
linger into the afternoon before lifting and scattering out to VFR
levels.

KSEA...South to southeasterly winds 4 to 6 kt with MVFR ceilings
developing around 10 to 11Z.

&&

.MARINE...Light onshore flow with small craft northwesterlies over
the outer coastal waters this evening. Weak low pressure over the
area 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.



&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory outer coastal waters this evening.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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