Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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024
FXUS66 KSEW 181026
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Fri Aug 18 2017

.SYNOPSIS...A weak system will brush the area today bringing clouds
and slightly cooler conditions into Saturday. This will also result
in a slight chance of light rain or drizzle mainly for the north
coast and north Cascades today. High pressure will begin to rebuild
over the region Sunday afternoon, reinforcing dry and warmer weather
into early next week.

&&

.SHORT TERM...Marine stratus has had a hard time working into the
area this morning...contrary to inherited forecast...but also
starting to see some mid to high level clouds associated with a weak
system traversing Vancouver Island at the time of this writing. So
have pared back some on sky forecast for this morning.

Models have remained pretty consistent with regards to general
trends and weather patterns...which makes for great forecasting but
does not help with regard to writing forecast discussions. Weak
system to the north may result in a slight chance for showers over
the north coast and north Cascades...but main impact over the area
will be slightly cooler daytime highs today and again Friday with
some clouds today and tonight. Saturday will see the trough axis
exit the area making it east of the Cascades by late afternoon as
upper level ridging builds over the Pacific. This will result in
gradually increasing 500mb heights over the area...bringing daytime
temps up a degree or two for Sunday and kicking off a bit of a
warming trend as Monday will see temps return to the lower 80s in
the interior lowlands. Speaking of Monday...models remain in
lockstep as the ridge starts to make its eastward voyage over WA
resulting in pretty clear skies for much of the area for the
eclipse. The outlook along the coast...however...still looks a
little dicey as morning marine stratus may not burn off until late
morning...possibly after the event.  SMR

.LONG TERM...On the back side of the ridge...temps will not be too
much different Tuesday than what is expected Monday. An upper level
low off the Canadian coast will spell some troughiness for the area
starting Wednesday. Cooling temperatures are a sure bet...but models
really are not coming up with an agreed solution for the path of
this low and thus the degree to which WA will see precip. Both
solutions (ECMWF and GFS) show precip for Wed and Thu...but with the
ECMWF showing a more northerly track...that would likely result in
lower POPs whereas the GFS has the low actually dipping south enough
to pass over the northern third of the CWA producing higher and more
widespread POPs. With no discernible agreement...simply went middle
of the road and ran with slight chance to chance POPs for that
period. Beyond that...looks like another Pacific ridge will
influence next weekend with models in disagreement as to the
amplitude of that ridge.  SMR

&&

.AVIATION...Westerly flow aloft today with weak disturbance
moving by to the north this afternoon. Low level onshore flow
continuing into the weekend.

Stratus early this morning confined to the coast. Possibility some
stratus forms in place over the interior but for the most part
will have to wait for sc deck later this morning out ahead of
weak system moving by to the north for ceilings to develop over
the interior. Ceilings in the 4000-6000 foot range developing late
morning and continuing into the evening hours.

KSEA...Some patches of stratus around the terminal but no not
expect any ceilings at the terminal. Ceilings near 4000 feet
arriving at the terminal late morning. Light southerly winds this
morning with the winds trying to go northwesterly late this
afternoon. Felton

&&

.MARINE...A typical August pattern will prevail through the
middle of next week. Expect onshore flow of varying strength to
persist due to higher pres offshore with lower pres east of the
Cascades. Weak system moving by to the north this afternoon with
increasing onshore flow behind the system later this afternoon
into this evening. Gale warnings have been issued for the central
and eastern Strait of Juan de Fuca along with small craft
advisories for Admiralty Inlet and the Northern Inland Waters for
tonight.

Small craft advisory westerlies are possible in the central and
eastern Strait of Juan de Fuca in the evening and early morning
hours beginning Saturday evening and continuing into the middle of
next week. Felton

&&

.CLIMATE...Thursday was the 50th day in a row with a high of 70
degrees or more in Seattle. This is the 3rd longest streak on
record surpassed only by the 62 day streak from June 13 to August
13, 2015 and the 61 day streak from July 8 to Sept 6, 2003. The
last time Seattle had a high below 70 degrees was June 28 with a
high of 68 degrees. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning from 6 PM this evening to 3 AM PDT Saturday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 3 AM PDT Saturday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 6 PM PDT this evening for Central
     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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