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-- Discussion containing changed information from previous version are highlighted. --
064
FXUS66 KSEW 222219
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 PM PDT Fri Sep 22 2017

.SYNOPSIS...
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A weak weather system will brush Western
Washington tonight and Saturday morning with clouds and a chance of
light rain along the coast. A weak upper level ridge will remain
over the region this weekend but will allow a weak weather system to
bring more clouds and a chance of light rain to the coast and the
northern portions of the area on Sunday. A slightly stronger system
could bring light rain to more of the area on Monday. The ridge will
build Tuesday or Wednesday with a return to mostly sunny skies and
warmer temperatures.
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&&

.SHORT TERM...
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The large upper level ridge just offshore will
tilt inland over southern B.C. this weekend. The ridge will be weak
enough so that any transient weather systems will passing through
the ridge will spread clouds across W WA and some light rain at
times. The first system, a weak warm front, tonight and Saturday,
will remain mostly just offshore, but will be close enough to bring
a chance of light rain to the coast and Olympics and adjacent inland
areas. Over the interior mid and high level moisture will keep skies
partly to mostly cloudy.

The models have finally become a little more consistent and now keep
the area mostly dry Saturday night and Sunday. Another weak system
may brush the far north part of W WA with some light rain on Sunday.

The best chance of rain should be on Monday when another weak warm
front moves across the area. Low level moisture should be plentiful,
but POPs will still be in the chance range, except for the coast.
Rainfall amounts should generally be just a few hundredth of an
inch, except maybe a tenth of an inch for the coast. All the cloud
cover will keep high temperatures in the lower to mid 60s through
Monday, which is a few degrees below normal. Kam

.LONG TERM...Tuesday will be kind of a transition day as the upper
level ridge builds offshore. There still appears to be a lot of
residual low level moisture but this may be clearing out in the
afternoon. The models have been generally consistent with the
building ridge for mid week. It now looks strong enough to keep
skies mostly clear and warm daytime temperatures into the lower 70s
in the warmest spots. The models differ a bit on just how quickly
the ridge will shift E of the area, but for now the forecast remains
dry through Friday. Kam
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&&

.AVIATION...
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High pressure will remain just off the Pacific Northwest
coast through Saturday with light northerly flow aloft. The air mass
will be mostly dry with residual moisture near the surface. Patchy
low clouds and fog will be possible in fog prone locations Saturday
morning such as HQM and OLM.  Mid and high clouds will increase and
hinder any widespread IFR fog/stratus for developing elsewhere. Will
plan to cut back the duration of IFR stratus at most TAF sites
around Puget Sound.

KSEA...VFR conditions. A few hours of IFR or low MVFR possible
Saturday morning. Northerly wind to 5 kt becoming light after sunset.
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&&

.MARINE...
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A large area of high pressure will dominate the northeast
Pacific including the Western Washington waters through the weekend.
A warm front will clip the region on Monday with a chance for small
craft winds, mainly in the coastal waters. Northerly or offshore
flow develops mid week. dtm
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&&

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

$$

www.weather.gov/seattle

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