Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271044
AFDSEW

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

.SYNOPSIS...High pressure offshore will maintain low level onshore
flow into the weekend with varying amounts of late night and
morning clouds. A weak trough aloft will move east on Wednesday
with an upper level ridge moving through the area Thursday and
Friday resulting in less morning cloud cover and warmer high
temperatures. Another weak trough will arrive late in the weekend.

&&

.SHORT TERM...After a month that included a top 10 rain day and an
all time record high tying temperature in Seattle the last few
days of June are going to be pretty normal.

Satellite imagery shows the stratus is already covering most of
the area this morning with just the central Strait of Juan de Fuca
and the Northwest Interior and San Juans in the clear. Temperatures
at 3 am/10z were in the upper 40s to mid 50s.

Surface high pressure centered well offshore will maintain low
level onshore flow today with weak trofiness aloft. Model cross
sections show the marine layer is thin enough over the interior to
burn back to the Coast by early afternoon. High temperatures will
be within a degree or two of normal with lower to mid 60s on the
Coast and 60s to mid 70s inland.

Little change in the pattern tonight into early Wednesday with
the upper level trough drifting east Wednesday afternoon. Low
level onshore flow still intact which will keep the morning clouds
afternoon sunshine scenario for the interior going through
Wednesday. Highs will be similar to today with lower to mid 60s on
the Coast and 60s to mid 70s inland. Lows will mostly be in the
50s.

Upper level ridge offshore moving east Wednesday night and
Thursday. 500 mb heights in the lower 570 dms Wednesday morning
rise to near 580 dms by 00z Friday. Low level onshore flow weakens
and becomes more northwesterly on Thursday which will help thin
the marine layer resulting in less morning cloud cover Thursday
morning. With less cloud cover and warmer temperatures aloft highs
on Thursday will be 5 degrees or so warmer than Tuesday and
Wednesday with the warmer locations pushing 80 degrees.

.LONG TERM...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 very typical pattern of morning clouds and afternoon
sun is expected for the interior of Western Washington over the next
few days. Morning clouds will give way to afternoon sun, except
perhaps on the coast where it may not burn off again today in the
south part. The air mass is stable. Southwest flow aloft will
become northwest today.


KSEA...Morning clouds and afternoon sun  and the southerly breeze
will probably turn around to the northwest for the late afternoon
and evening hours.

&&

.MARINE...High pres offshore with lower pres E of the Cascades will
result in onshore flow--decreasing toward the end of the week. There
is a marine layer over Western Washington and the stronger gradients
now will be east of the Cascades. The gradient in the Strait should
only require a small craft advisory today.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 3 AM PDT Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     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

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


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