Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201617
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PDT Thu Apr 20 2017

.SYNOPSIS...An upper level trough will give showers and a slight
chance of thunderstorms today. 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 around Monday. Weather will remain wet and unsettled through
the middle of next week.

&&

.SHORT TERM...A nice line of showers held together as it moved
inland from the coast this morning, but radar indicates it is
dissipating as it reaches Puget Sound. Models show instability
increasing by 18Z, with lifted indices as low as -2 this afternoon.
500 mb temperatures are near -30C and convective temperatures are in
the lower to mid 50s in the afternoon. There is even a little bit of
CAPE with values as high as 800 J/kg in the middle of the day.
Nothing spectacular in these convective variables but enough to have
a chance of thunderstorms in the  forecast for the entire day in
addition to showers. Highs today will be in the 50s.

Shower activity will end quickly tonight with the end of the daytime
heating and an upper level ridge approaching the area. The exception
to this will be around Snohomish county where a convergence zone
will develop this evening and continue into the early morning hours.
Outside of the convergence zone skies will begin to clear. With
plenty of low level moisture around fog is a possibility over the
Southwest Interior late tonight. Lows will mostly be in the 40s.

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 go
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 with the first of a pair of systems to spin out of the
trough moving over Western Washington on Saturday. The system will
be negatively tilted which will help slow it down a bit. Highest
pops on Saturday in the afternoon. Highs will be in the mid 50s to
lower 60s. Burke/Felton

.LONG TERM...Previous discussion...Extended models continue to show
good agreement in the pattern with an upper level trough moving
through Western Washington on Sunday. Some small differences in the
model solutions on Monday with the GFS taking the next system inland
a little further south than the ECMWF. Both models have rain over
the area. The more direct hit on the ECMWF will result in higher
rainfall amounts. Another system arriving Tuesday into Wednesday
with the GFS slightly faster with this feature versus the ECMWF.
The main message for the extended period is the wet weather will
continue into the middle of next week. Felton

&&

.AVIATION...An upper level trof of low pres over the region this
morning will move slowly e during the day today. Sly flow aloft will
become wly. The low level flow will be light sly. The air mass will
continue to destabilize, resulting in the potential for isold tstms
this afternoon. The mtns will be obscd at times.

KSEA...VFR conditions are expected; however, there will probably be
occasional MVFR CIGs this morning and with the heavier showers this
afternoon. Will need to keep an eye on the possibility of a
thunderstorm this afternoon. The probability is about 20%. Winds
will be sly 6-11 kt, becoming light tonight.

&&

.MARINE...Lower pressure over southwest Canada with higher pressure
over Oregon will result in southerly flow today. The flow will become
flat tonight due to high pressure over the area. Expect the flow to
become easterly or offshore on Friday in response to falling
pressure over the offshore waters.

Meanwhile, a few of the stronger thunderstorms this afternoon could
produce small hail, gusts near 25 knots, and locally higher waves.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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