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000
FXUS66 KSEW 211035
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 AM PDT Thu Sep 21 2017

.SYNOPSIS...An upper level low will maintain a chance of showers and
cooler temperatures over Western Washington today, mainly over the
Olympics and Cascades. Drier north flow aloft from an offshore upper
level ridge will bring sunshine and a little warming Friday and
Saturday. The weakening ridge will move over the area Sunday and
Monday but could allow transient weak weather systems to bring some
spotty light rain.

&&

.SHORT TERM...Current IR satellite shows a mix of high and low
clouds over the area making the case for a good generalized mostly
cloudy forecast. Current radar is pretty quiet with only a few
showers being detected...one over the outer coastal waters...another
in northern Snohomish county and a third running from NW Skagit
county into SW Whatcom. All of these look to have fairly light
precip.

Upper level low pressure and the associated trough will dig in
deeper over the western US today...but as it does so the main focus
for precip does not look to be W WA. While some showers are not
entirely out of the question...coverage looks pretty scattered and
models show the best chances to be in the higher elevations of the
Olympics and Cascades.

The low makes very little headway in traveling east Friday...but
the shift turns out to be enough to bring drier northerly flow over
the area and eliminating any chances for precip both Friday and
Saturday. As the low continues to make its slow eastward
journey...upper level ridging nudges in for Saturday...helping temps
to rebound into the 60s and clearing skies out to a degree. Sunday
remains a bit of a question mark as the 06Z GFS suggests a shortwave
disturbance within the ridge that could bring showers down from the
north while the 00Z ECMWF shows no such feature...keeping a dry
forecast. Given that the 00Z GFS did not have this feature...will
opt for a mainly dry forecast over the area for this time frame
while keeping an eye to see if this solution with the shortwave
feature persists.  SMR

.LONG TERM...The upper level ridge persists Monday and into midweek
keeping the area relatively dry...although each model shows a bit of
moisture and thus the potential for showers to dip south enough to
be right on the doorstep...but never really panning out. Not helping
matters is that there is really not much in the way of agreement as
to which systems or which days this will occur. Feels like a bit of
a cop out...but best course of action is to keep the forecast
painted in broad strokes with slight chance pops for much of the
extended until models can gel on something resembling a singular
solution.

Regardless of precip...should continue to see temperatures gradually
climb into the 70s for many locations for the first half of next
week.  SMR

&&

.AVIATION...Upper level trough over the area shifting east this
morning with northeasterly flow aloft into Friday. Light onshore
flow continuing throughout the day.

Wide variety of ceilings over the area early this morning. Between
KBLI and KPAE and along the coast areas of ceilings near 4000
feet with mid level clouds above. Over the central Puget Sound a
1000 foot stratus deck is forming at 10z. There is also some lifr
ceilings and visibilities near KPWT. Over the Southwest Interior
just some mid level clouds.

South of KPAE and out to the coast expect the stratus deck near
1000 feet to becoming more widespread through 14z with the layer
scattering back out again late morning/midday. The lifr conditions
near KPWT will continue until around 17z. KPAE northward expect
the 4000 foot cloud deck to hang around for a few hours this
morning before dissipating leaving just scattered clouds.

KSEA...Ceilings near 1000 feet through 17z with ceilings lifting
17z-19z. Stratus will scatter out around 19z. Southerly wind 4 to
8 knots becoming northerly around 21z. Felton

&&

.MARINE...Weak surface low tracking southward through the coastal
waters this morning. High pressure building on Friday with high
pressure remaining over the area into the early part of next week.
Winds are forecast to remain below 20 knots across the waters.
Felton

&&

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

$$

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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html