Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 230414
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PDT Sun Oct 22 2017

.SYNOPSIS...Isolated showers are still possible overnight,
otherwise mostly dry conditions are expected over Western Washington
through Tuesday. Flooding continues on a few rivers but waters
continue to recede. A weak system is expected to dip into the area
Wednesday for some light rain before dry conditions resume for the
remainder of the week.

&&

.SHORT TERM...Current radar imagery is mostly clear with only an
isolated patch of showers over portions of Whatcom and Skagit
counties...mainly over the mountains. The remainder of the area is
generally dry. Satellite trend showing a downward turn with regards
to cloud cover...although some lingering low to mid level clouds
remain at the time of this writing. With the threat of any sort of
precip pretty much over...probably safe to take down the inherited
flood watch for Lewis and Thurston counties.

Models remain consisted on upper level ridging taking hold of not
only the area but much of the Western US as well Monday and Tuesday
allowing for gradually clearing skies and increasing high
temperatures into the lower to mid 60s in many lowland locations.
Lingering low level moisture Monday night and Tuesday morning will
likely give rise to fog...particularly in prone locations.

Each successive model run has not be kind to the system for
Wednesday...as the current GFS 00Z run has it looking like little
more than a blip on the radar. While increased clouds...and thus
somewhat cooler temps...can be expected...current solutions not
really favoring much in the way of precip. Will be curious to see
what newest ECMWF shows and will wait for that input before altering
inherited forecast.

That being said...no need for forecast updates at this time.  SMR

.LONG TERM...From previous discussion...After the weak front
Wednesday, an upper ridge will build off the coast, then slowly move
inland through the weekend. This should give clear skies, dry
weather, and highs mostly in the 60s Thursday through Sunday. Burke

&&

.HYDROLOGY...With rain pretty much ending over the area...local
rivers experiencing flood conditions continue to trend downward and
the threat of any additional new flooding is essentially nil. Check
the latest Flood Statement (SEAFLSSEW) to monitor which Western
Washington rivers remain in flood stage. The only river that
continues to trend upward is the White River near R Street. Current
forecast suggests that it is yet to crest...although some pause
about whether or not it will meet forecasted peak which would still
fall short of flood stage. As most recent observations do not offer
a clear picture on this...will pass decision as to whether or not to
cancel this particular warning on to next shift.  SMR

&&

.AVIATION...The flow aloft over W WA will become WNW tonight as a
low amplitude upper level ridge builds over the region. The air
mass will gradually dry, especially at mid and upper levels, with
low level onshore flow turning weakly offshore tonight.

A few isolated light showers were still hanging around at 04Z but
they should be gone in the next few hours. The mid and upper
levels of the air mass have dried out, but the lower levels are
still moist enough from the recent rains that patchy IFR fog or
low stratus will form 06Z-18Z. Otherwise conditions will be VFR.

KSEA...Good VFR conditions are expected tonight. Model guidance
has temp-dewpoint spreads down to 2 or 3 degrees 06Z-16Z. The
models maintain VFR conditions, but this is borderline for
possibly some low stratus to form, so confidence is not that high.

Surface winds will be S 6-10kt through 06Z then weaken to SE 4-5kt
overnight. Kam

&&

.MARINE...Residual onshore flow this evening will turn weakly
offshore late tonight and continue through Tuesday, as a surface
ridge builds over the Pacific Northwest.

A SCA for hazardous seas will remain in effect for the coast and
W entrance to the strait through Monday evening. Westerly swell
10 feet or higher generated by recent storms will continue through
Monday evening.

A weak cold front will move southeast across the area on
Wednesday, with small craft advisory winds over the coastal waters
and in the Strait of Juan De Fuca. Northerly offshore flow will
follow the front Thursday and Friday as a ridge builds over
southern British Columbia. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Monday morning for Cascades of Pierce and
     Lewis Counties-Southwest Interior.

PZ...Small Craft Advisory for hazardous seas until 11 PM PDT Monday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory for rough bar until 11 PM PDT Monday for
     Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 5 PM PDT Monday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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