Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190445

Area Forecast Discussion
National Weather Service Seattle WA
945 PM PDT Thu May 18 2017

.SYNOPSIS...High pressure aloft will prevail over the region through
Friday. A weak low pressure system will move across the area Friday
night, resulting in increased onshore flow. High pressure aloft will
return on Saturday, and will strengthen early next week for a trend
towards much warmer conditions.


There were probably some spotty sprinkles or light showers falling
from the middle cloud deck over the central part of the CWA;
otherwise, primarily mid and high clouds continued to stream across
the area at this time.

Meanwhile, the upper level ridge of high pressure centered offshore
will move inland while weakening during the day Friday. Models were
indicating that the ridge will remain somewhat dirty during the day,
with some mid and high clouds continuing to drift across the area.
There should be more sunshine Friday afternoon than today because
the day will start off with nearly neutral surface flow. This will
translate to markedly less low clouds, if any, tomorrow morning.

A short wave trough of low pressure aloft will approach the CWA
Friday afternoon and initiate a marine push. This system system will
move quickly across the area Friday night. There may be areas of
light drizzle over the lowlands during the late Friday night through
Saturday morning time frame, with isolated showers possible over
the mountains. The main impact of the short wave trough will be
increased onshore flow along with a deeper marine layer.

Expect high pressure aloft to rebuild over the region during the day
Saturday, continuing through Sunday. The surface onshore pressure
gradient will relax Saturday night and become light northerly on
Sunday. GFS and NAM MOS guidance were quite warm for Sunday,
indicating that much of the lowlands will be in the 70s.

Sunny and much warmer weather is in the extended forecast as the
center of an upper ridge drifts over the region Monday and Tuesday.
Highs should range from the 70s coast to the lower 80s inland Monday
and Tuesday before falling off midweek. The 12z GFS and ECMWF have
different ideas about the latter part of the week--The Euro has a
cold upper trough moving through B.C. and Alberta midweek with a
rapid cool down for Western Washington while the 12z GFS has a much
weaker trough farther north and keeps weak ridging over our area til
late in the week.


.AVIATION...An upper level ridge building into the Pacific
Northwest will maintain a dry and stable conditions across Western
WA tonight. Northwesterly flow aloft will remain light with weak
onshore flow at low levels. VFR conditions will persist tonight with
CIGS 8000 to 1000 feet. Dry weather will continue into Friday with
the ridge in place. Mesoscale models hinting at a few more midlevel
clouds especially in the afternoon ahead of a weak upper level
trough forecast to brush Western Washington Friday night. This weak
feature may trigger isolated showers Friday night.

KSEA...VFR conditions tonight with light northwesterly winds.


.MARINE...A weak surface trough will cross the Coastal Waters
tonight weakening onshore flow. High pressure rebounding over the
offshore waters Friday night will maintain onshore flow through the
weekend. Highest winds and waves expected through the central/east
Strait of Juan de Fuca especially during the afternoon and evening





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