Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230340

Area Forecast Discussion
National Weather Service Seattle WA
840 PM PDT Tue May 22 2018

.SYNOPSIS...Onshore flow will increase through the end of the
week. Areas of morning clouds will be mainly on the coast
Wednesday, then spread farther inland on Thursday. A few showers
might develop over the Cascades Wednesday afternoon. An upper
trough will bring a chance of showers Friday through Saturday as
it moves inland. A weak ridge should bring dry weather Sunday.


.SHORT TERM...An upper level ridge remains over the Pacific
Northwest this evening, its axis tilting positively from offshore
of California across Western Oregon and Washington into southeast
British Columbia. With 500 mb heights around 5750 meters and the
850 mb temperature around +14 C, afternoon highs in the Western
Washington interior were in the 70s to lower 80s with light
onshore flow.

The upper ridge will weaken over the area tonight and Wednesday,
and light southerly flow aloft will convey some high level
moisture and upper level instability over the area late in the
day. That should result in some high-based showers -- and possibly
thunderstorms -- over the Cascades Wednesday afternoon and
evening. At low levels, increasing onshore flow will bring marine
stratus onto the coast later tonight and Wednesday morning. For
the interior lowlands, Wednesday should be a mostly sunny day and
just slightly cooler than today.

The upper ridge will continue to weaken and shift slowly eastward
Wednesday night through Friday, while a cutoff upper low slowly
approaches California from its offshore waters and a northern
stream upper trough slowly approaches British Columbia. So Western
Washington will be in a region of fairly weak southwest flow
aloft. The air mass at mid and upper levels will remain fairly
dry, though some showers could develop late Friday as the two
troughs come into phase and begin to move inland.

At lower levels, stronger onshore flow will develop and
deepen the marine layer. Thursday will probably begin with a layer
of marine stratus more or less covering the Western Washington
lowlands, which should give way to sunshine over the interior in
the afternoon. Friday could remain mostly cloudy, though, due to
marine stratus in the morning and more general cloud cover
associated with the upper troughs in the afternoon. Highs should
be in the mid 60s to mid 70s on Thursday, and in the 60s on
Friday. McDonnal

.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- The hint of a little upper trough Saturday
morning with a deep marine layer might result in a few areas of
drizzle of light showers and then some afternoon sunbreaks.
Whatever happens, the mostly cloudy/chance of showers type of
forecast should give way to a good chance of warmer and sunnier
weather for Sunday and Monday. That weak upper trough early
Saturday should give way to a weak ridge Sunday--with 500mb
heights rising about 100m into the upper 570s. But there wont be a
ridge axis over the area--the models show a front nearing the
region and an upper trough offshore early next week--but so far
the front is only forecast to effect British Columbia while upper
level heights stay high enough for pretty good weather for Western
Washington. So, precip is not likely over our area from that
front--but the tail end of the front and onshore flow might very
well result in marine layer cloudiness at times.


.AVIATION...Light southerly flow aloft over Western Washington
this evening will continue through Wednesday, with a weak upper
level ridge axis just east of the Cascades. High level moisture
and weak upper level instability should produce some showers --
and possibly thunderstorms -- over the Cascades Wednesday
afternoon and evening.

At low levels onshore flow will bring marine moisture onto the
coast late tonight, and eventually through the Chehalis Gap to the
southern edge of the Puget Sound region. Marine stratus will
result in low-end MVFR to IFR conditions at the coast later
tonight and Wednesday morning, gradually improving in the
afternoon. The stratus should move inland to spots like Shelton
and Olympia for several hours of low-end MVFR to IFR conditions
late tonight and Wednesday morning.

KSEA...Northwest to north wind 4-10 kt tonight, becoming southwest
early Wednesday morning. No significant cloud cover expected below
12,000 ft. McDonnal


.MARINE...High pressure will remain offshore through Saturday with
lower pressure east of the Cascades, maintaining onshore flow. Small
craft advisory westerlies will continue in the central and
eastern Strait of Juan de Fuca through early Wednesday morning.

Stronger onshore flow will develop Wednesday and Wednesday night,
and gale force westerlies are likely in the central and eastern
Strait of Juan de Fuca. The gale watch will be upgraded to a gale
warning with the evening marine forecast. Small craft advisory
winds are likely in the adjacent waters of Admiralty Inlet and
the Northern Inland Waters.

Moderately strong onshore flow Thursday through Saturday will
produce solid small craft advisory winds -- and possibly gales --
in the central and eastern Strait during the diurnal peak each
late afternoon through early morning. McDonnal


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.

     Gale Warning from 3 PM Wednesday to 3 AM PDT Thursday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.


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