Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 150403
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
902 PM PDT Mon Aug 14 2017

.SYNOPSIS...Northwest flow aloft coupled with low level
onshore flow will maintain typically mild summer weather through
this weekend. The northwest flow will bring a couple of weak
decaying cold fronts to the area Wednesday and Friday. An upper
level trough will maintain the mild late summer weather next
weekend.

&&

.SHORT TERM...Dry northwest flow aloft will persist over W WA
through Tuesday. A surface ridge offshore will maintain weak low
level onshore flow. This will keep W WA mostly sunny and mild,
although there will be high clouds embedded in the NW flow crossing
the area at times. Low level onshore flow looks to be pretty weak
tonight so there shouldn`t be much stratus around over the interior
Tuesday morning, so sunny weather will rule. Afternoon highs will be
in the 60s in coastal areas and in the 70s over the interior.

A broad low amplitude upper level ridge will move across the area
Wednesday through Friday, but the several degrees of warming from
the ridge will likely be offset by increased cloud cover from a
couple of weak decaying cold fronts. The 18Z GFS agrees with the 12Z
ECMWF in bringing a couple of weak decaying cold fronts, embedded in
the NW flow through the ridge, to W WA. The air mass will still be
dry above 700 MB/10Kft but the lower levels below 700 MB and
particularly below 850 MB/5Kft should be quite a bit more moist.
These decaying fronts will bring clouds over W WA but just about no
precip at all except for possibly some light drizzle, mainly on the
coast.

The first decaying cold front should bring low/mid level clouds to
the coast Wednesday morning, with SCT-BKN layers spreading over the
interior Wednesday afternoon and night. Onshore flow will crank up
Wednesday evening, and when combined with the frontal clouds, should
produce a deep marine layer Thursday morning. The GFS still
indicates a breakout Thursday afternoon, but it could be a little
later than normal for this summer.

Clouds from the next decaying cold front should reach the coast
Thursday night, spreading inland by Friday morning. Clouds should
continue spreading inland Friday afternoon, so the day looks a lot
more cloudy than previously forecast. High temperatures also look to
be cooler than previously forecast, more like right near normal than
several degrees above normal. Kam

.LONG TERM...The upper level ridge will shift E of the area on
Friday, but W to NW flow aloft will continue. An upper level trough
moving mainly across B.C. will bring some cooling to the air mass
over W WA this weekend. 500 MB heights will lower back down to
around 5760 meters Saturday afternoon and low level onshore flow
will continue. This should keep highs in the lower to mid 70s.

The GFS and ECMWF both have a frontal system approaching the region
on Monday. The faster GFS suggests some clouds may reach the coast
before midday, but ECMWF still has the system a little farther
offshore. Per the GFS, there could be a few showers over the NW part
of W WA in the afternoon. But there is lots of time for the models
to wobble since Monday is still a week away. Kam

&&

.AVIATION...Light northwest flow aloft will prevail through Tuesday
with an upper ridge offshore and a trough east of the region. Air
mass stable and somewhat dry, except for residual low level
moisture. IFR stratus may develop near the coast overnight and
Tuesday morning but mainly just patchy scattered stratus over the
interior lowlands with predominately VFR conditions.

KSEA...VFR. Winds will generally be northeasterly 4-7 kt, then
northwest 5-9 kt after 18z Tuesday.

&&

.MARINE...Surface high pressure over the coastal waters and lower
pressure east of the cascades will maintain varying degrees of
onshore flow through Saturday. Typical late afternoon and evening
westerlies will increase in the central and eastern Strait each day.
Expect solid small craft conditions, with a slight risk of low-end
gales Tuesday and Wednesday evening. Winds below 20 kt elsewhere.

&&

.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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