Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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020
FXUS66 KSEW 271621
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Tue Jun 27 2017

.SYNOPSIS...A very typical onshore flow pattern will persist into
the weekend with varying amounts of late night and morning clouds.
The weak upper level trough over the region today will move east on
Wednesday allowing an upper level ridge to build into the area
Thursday and Friday.  The rising heights with the ridge will result
in less morning cloud cover and warmer high temperatures. Another
weak trough will arrive late in the weekend.

&&

.SHORT TERM...Satellite imagery and surface observation this morning
showing marine stratus has worked into most of the interior...save
the far north and central Strait of Juan de Fuca. Like yesterday,
expect the marine layer to burn back to the Coast by early
afternoon...albeit a bit later than yesterday. High temperatures
will near normal, reaching into the lower to mid 60s on the Coast
and 60s to mid 70s for the interior. Low level onshore flow will
bring morning clouds again to the interior again late tonight into
Wednesday morning with afternoon sunshine as the stratus burns back
to the coast. The upper level trough over the region today will
begin to shift east Wednesday afternoon setting the stage for
heights to begin to rise as an upper level ridge builds behind it.
Afternoon highs will reach into the lower to mid 60s on the Coast
and 60s to mid 70s inland. Lows will mostly be in the 50s.

The building ridge and weaker onshore flow will help thin the marine
layer and result in less morning cloud cover Thursday morning and an
earlier burn off. Warmer temperatures aloft and more morning
sunshine will help boost highs on Thursday by around 5 degrees with
the warmer locations pushing 80 degrees.

.LONG TERM...from previous discussion...Extended models in good
agreement Friday with the upper level over the area Thursday night
slowly moving east on Friday. Low level onshore flow continuing
which will keep high temperatures within a couple of degrees of the
temperatures on Thursday, upper 60s Coast and upper 60s to near 80
inland. 00z extended models trending weaker with the trofiness over
the weekend. The GFS still has a very weak shortwave approaching the
Coast Saturday night/Sunday evening. Will keep the low pops in the
forecast for the Coast and Southwest Interior. For the remainder of
the weekend with the weaker trough have gone ahead and taken the low
pops out of the forecast for the northern sections of the area. Low
level onshore flow increasing leading to a deeper marine layer in
the morning and a little less sunshine in the afternoon. Highs will
be near normal, in the 60s and lower 70s. Both the GFS and ECMWF
have a little more trofiness on Monday with the 500 mb heights
lowering back to the lower 570 dms. Models are generating little if
any precipitation with this feature so for now will continue with
the idea of a little more cloud cover and high temperatures
remaining near normal. Felton

&&

.AVIATION...A weak upper level trof will remain over the area
today. The flow aloft will be from the NW. Contd low level onshore
flow. MVFR CIGs blanketed much of the lower elevations this morning.
Expect VFR conditions to return over the interior lowlands by 2100
UTC. The stratus will slowly break up over the coast this afternoon
but areas of MVFR CIGs will persist.

KSEA...CIGs will lift (but remain in the MVFR category) late this
morning. The clouds will scatter out early this afternoon or by 2100
UTC (2 PM PDT). Low clouds will return early Wed morning. The cloud
bases early Wed will probably be lower than this morning but the
stratus should scatter out earlier than today. Winds will generally
be SW 5-10 knots, becoming light NW late this afternoon.
Expect light and variable winds tonight.

&&

.MARINE...
High pres offshore with lower pres E of the Cascades will result in
onshore flow of varying strength this week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

$$

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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