Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231525
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
825 AM PDT Wed May 23 2018

.SYNOPSIS...Onshore flow will prevail for the next week for morning
clouds and at least partial afternoon sunshine most days. A weak
upper disturbance will bring a chance of showers and thunderstorms
to the Cascades this afternoon and evening.

&&

.SHORT TERM...There will be a threat thunderstorms over the
Cascades this afternoon and evening with a weak upper low over
Eastern Washington. A few of these might drift west into the
lowlands near the Cascades but they should fizzle as they move
into more stable air. Otherwise, low level onshore flow will
continue with low clouds near the coast and mostly sunny inland.
Highs will be in the 70s for the interior which is upwards of 10
degrees above normal. The San Juans, Strait, and coast will be in
the 60s and closer to normal.

A weak upper ridge will move over the area tonight and Thursday
which will stabilize the air and end the shower and thunderstorm
threat. Low level onshore flow will be stronger which should allow
clouds to spread into the interior tonight and then burn
partially back to the coast later Thursday. Highs will cool a few
degrees but they will still be a little above normal.

A dry upper trough will move over the area on Friday. Low level
onshore flow will continue for another day of morning clouds and
some afternoon sunshine. Highs will cool a few more degrees and be
close to normal. Schneider

.LONG TERM...Previous discussion...A weak upper level ridge or
nearly zonal flow aloft will prevail across much of the Pacific
Northwest during much of this period. This combined with low level
onshore flow of varying strength will result in near normal to
slightly above normal temperatures. The surface onshore pressure
gradient should strengthen on Tuesday as an upper level trough
offshore approaches the region. Chose to compromise between the
GFS and ECMWF solutions during this period (Saturday thru
Tuesday). The Canadian model was the outlier.

&&

.AVIATION...At 8am marine layer stratus is on the coast, through
the Strait of Juan de Fuca and over a good part of Whidbey Island,
and into areas of southwest Washington. Onshore flow will increase
today and the marine layer clouds will fill up most of Western
Washington tonight through Thursday morning. Also, a disturbance
east of the Cascades will bring a chance for showers and
thunderstorms over the Cascades this afternoon and evening. There
might be few towering CU over the Olympics as well.

KSEA...Clear skies--any shower and thunderstorm activity over the
Cascades this afternoon and evening should stay there.  Low clouds
will arrive overnight. Winds will be variable today, probably
westerly to about 5kts, maybe more northerly for awhile then
southwesterly, but vrb04kt is a good forecast too til the onshore
flow kicks in later today.

&&

.MARINE...Strong onshore flow will develop today and tonight,
with gale force westerlies likely in the central and eastern Strait
of Juan de Fuca. A gale warning is in effect for that, with small
craft advisories in effect for the adjacent waters of Admiralty
Inlet and the Northern Inland Waters.

High pressure will remain offshore Thursday through Sunday, with
lower pressure east of the Cascades, for continuing onshore flow
through the period. Solid small craft advisory winds are likely in
the central and eastern Strait during the diurnal peak each late
afternoon through early morning. Gales are possible, especially
Friday afternoon and evening.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 3 PM PDT this afternoon 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 this afternoon 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.

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

     Small Craft Advisory from 6 PM this evening to midnight PDT
     tonight for Admiralty Inlet.

&&

$$

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