Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201040
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT TUE SEP 20 2016

.SYNOPSIS...An upper level trough over the Pacific Northwest will
gradually shift south the next couple of days. The air mass will
become somewhat drier but weak instability will produce a few
showers, mainly in the afternoon and over the mountains. A weak
front will bring a chance of rain Friday and Saturday, with dry and
sunnier weather likely Sunday and Monday.

&&

.SHORT TERM...Latest obs indicate areas of stratus and isolated
patchy fog around Western Washington this morning. Only a few spotty
showers can be seen on radar over the north coast, Strait, and north
interior but nothing measurable noted. Most the area is dry and this
will likely be the case for the next few days as the strong jet with
moist westerly flow has shifted south of Washington. The upper low
will be over the area today and Wednesday as it slowly shifts
southward. Residual low level moisture and diurnal instability could
trigger a few isolated showers but mostly over the mountains.
Just a some patchy night and morning low clouds and fog possible in
the lowlands, mainly in prone valleys. Thursday should be dry and a
bit warmer as the closed upper low will be well south of our area
and the next approaching system still offshore. Highs will mostly be
in the 60s through Thursday.

.LONG TERM...Latest global models are in better agreement in
showing a frontal system reaching Washington on Friday. The
GFS/ECWMF/and Canadian all show the front bringing rain to the coast
Friday morning but quickly weaken the system as it pushes inland.
Since the front is encountering a large scale split in the flow and
heights rising above 5700m, it`s still unclear if rain will make it
into the area. The chance pops currently in the forecast will
remain.

Some past model runs were showing a second trailing front to
possibly arrive Saturday with a better shot at some rain. 00z models
have trended toward building a ridge and pushing any moisture
heading our way up to the north through British Columbia. Models
have been in flux with with pattern over the weekend. Will not make
any changes at this time and keep a chance of rain into Saturday
with drier weather Sunday and Monday. Mercer

&&

.AVIATION...The upper level trough that has been working over
British Columbia will shift south over Western Washington today.
Westerly flow aloft will become northerly this afternoon and evening
as the upper level low works south into Oregon. Light easterly low
level flow will become northerly during the day today, then ease
again Tuesday evening. The air mass remains somewhat moist with
showers possible today as the upper level trough moves over the
area. Expect VFR conditions today with MVFR ceilings possible at
some locations this morning as well as in rain showers.

KSEA...VFR conditions will persist this evening with a few hours of
MVFR are possible around daybreak. Southeast winds 6-8 KT will
become northeasterly by late morning.

&&

.MARINE...Light southerly winds over the inland waters will
become northerly over the interior waters. Increasing northerly
winds over the coastal waters this evening will likely reach small
craft advisory levels before easing after midnight. Winds will
strengthen again Wednesday afternoon and evening over the coastal
waters. Light onshore flow will develop Thursday as higher pressure
settles offshore. Weak fronts will dissipate over the coastal waters
Friday and Saturday.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small craft advisory coastal waters late this afternoon through
     late tonight.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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