Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 212240
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
340 PM PDT Thu Sep 21 2017

.SYNOPSIS...An offshore upper level ridge approaching Western
Washington will bring more sunshine and a little warming Friday and
Saturday morning. The weak ridge will remain over the region Sunday
through Tuesday but will probably allow transient weak weather
systems to bring more clouds and some spotty light rain to mainly
the coast and north part of Western Washington. The ridge will build
Tuesday or Wednesday for mostly sunny and warmer weather.

&&

.SHORT TERM...A large upper level trough centered over E Oregon
will drift slowly E tonight allowing an offshore upper level ridge
to tilt eastward over southern B.C.  Drier N flow aloft ahead of the
ridge has already allowed clouds to diminish over especially the
north part of W WA. However, RADAR is still picking up scattered
showers over mainly the mountains this afternoon.

Clearing overnight should set up a mostly sunny day on Friday,
however the air mass will still be a little cooler than normal with
highs in the 60s. The upper level ridge will be over southern B.C.
on Saturday, but it will be weak enough to allow transient weather
systems to pass through the ridge and brush mainly the N part of W
WA with some light rain. Mid level clouds from the first system will
probably spread over the area from the NW Saturday morning, but this
system should be pretty much dry. The models have been inconsistent
with the system on Sunday, especially with the coverage of light
rain, ranging from zero to some spotty light rain over the N part of
the area. What is more certain is that it will be cloudy with highs
remaining a little below normal. Kam

.LONG TERM...The weak upper level ridge over southern B.C. will
remain in place Sunday and Monday, still wide open to weak transient
weather systems. The GFS brings another system Monday through Monday
night with a bit more coverage of light rain. The ECMWF is in better
agreement but still a little slower, bringing some light rain Monday
afternoon through Tuesday morning. Despite the better model
agreement, confidence is still low.

Models are in better agreement that the upper level ridge will build
enough by Wednesday for mostly sunny weather Wednesday and
Thursday with temperatures near or slightly above normal. Kam

&&

.AVIATION...A broad upper level low over the interior west will
migrate further eastward through Friday. High pressure centered
along 140w will persist, maintaining light northerly flow aloft. The
air mass will become stable tonight and somewhat moist in the lower
and mid levels. Northerly flow will allow the air mass to become
mostly dry Friday into Saturday except at the surface. Patchy fog or
low stratus is possible late tonight and Friday morning, mainly in
the most prone valleys such as the southwest interior, lower
chehalis, coast, and river valleys. Any fog or status should mix out
before noon. Otherwise VFR conditions.

KSEA...Light or west-northwest wind to 5 kt becoming northeast to 5
kt this evening. Scattered clouds tonight. Patchy low stratus or fog
will develop in place around parts of the central/south Sound. A few
hours of IFR cigs/vis mainly between 12-16z Friday morning cannot be
ruled out (20-25 percent chance). Most likely, stratus or fog will
just be in the vicinity of the KPAE/KSEA/KBFI terminals and will
indicate this in the TAFs.

&&

.MARINE...High pressure over the NE Pacific will build over the
waters later tonight and Friday. Winds should remain fairly light,
less than 15 kt or so through the weekend. A weak front could bring
a little increase in southerly wind Monday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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