Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 202217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Thu Apr 20 2017

.SYNOPSIS...An upper level ridge will build offshore tonight and
move over Western Washington on Friday bringing sunny and warmer
weather. The break in wet weather will be brief with weak systems
arriving Saturday and Sunday, and a stronger system Monday. Weather
will remain wet and unsettled through the middle of next week.

&&

.SHORT TERM...Scattered showers, mainly over western sections, are
moving east. Most areas should get at least one shower this evening.
The air mass will stabilize rapidly tonight as an upper ridge of
high pressure builds over the offshore waters. A convergence zone in
Snohomish County may keep showers going there a bit longer.

Friday is still likely to be one of the warmest days of the year. An
upper level ridge will move over the area with the ridge axis right
over Western Washington at 00z Saturday. Winds up to 700 mb turn
easterly early in the day and remain easterly all day. Surface
gradients will be offshore but not enough to prevent northerly winds
near the Puget Sound in the afternoon. Most places will be in the
60s with the best chances for lower 70s near the Cascade foothills
and along the west slopes of the Olympics.

The dry weather will be short lived as the upper level ridge moves
east of the area Friday night. An upper level trough will develop
off the coast and rain will spread inland from the southwest
Saturday afternoon. The upper low will move inland on Sunday,
spawning showers.  Highs will be in the mid 50s to lower 60s on
Saturday, cooling a bit to the 50s on Sunday. Burke


.LONG TERM...There are small differences in the model solutions for
Monday with the GFS taking the next system inland a little further
south than the ECMWF. Both models have rain over the area, so have
kept likely pops for Monday. Both models bring an upper trough over
the region Tuesday into Wednesday for showers. Moist westerly flow
aloft continues Thursday. Burke

&&

.AVIATION...An upper level trof of low pres over the area this
afternoon will move over the northern Great Basin overnight. Light w
flow aloft will become strong nw overnight. Light low level sly flow
will become flat tonight. Expect the air mass to stabilize after
sunset.

Meanwhile, expect the threat of isolated tstms to persist until
sunset. There will be MVFR CIGs and/or VSBYs with the heavier
showers this afternoon...and within the Puget Sound Convergence
Zone (PSCZ) that is expected to dvlp near the Snohomish/King County
line early this evening. The PSCZ will dissipate later tonight.
Local shallow fog is anticipated after 0900 UTC.

KSEA...VFR. There is still slight risk of a thunderstorm in the
vicinity this afternoon. The threat will end about sunset.
Southwest winds of 5-10 knots will become light and variable
tonight.

&&

.MARINE...
High pressure will move over the area this evening for the
development of nearly flat flow. The flow will become easterly on
Friday due to high pressure east of the Cascades combined with
falling pressure across the offshore waters. A cold or occluded
front will move across the area on Saturday for increased southerly
flow.

&&

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

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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