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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 220358
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Thu Sep 21 2017

.SYNOPSIS...An offshore upper level ridge will bring more sunshine
and a little warming Friday and Saturday morning. The weak ridge
will remain over the region Sunday through Tuesday but will probably
allow transient weak weather systems to bring more clouds and some
spotty light rain to mainly the coast and north part. The ridge will
build Tuesday or Wednesday for mostly sunny and warmer weather.

&&

.SHORT TERM...An upper level trough is still close enough this
evening to keep scattered showers going in the Cascades from about
Stevens Pass south. Temperatures are a bit warmer than earlier
this week; the snow level has risen to around 6000 feet. A big patch
of mid level clouds is moving south from British Columbia and this
might make Friday somewhat less sunny than forecast. The clouds will
probably clear north to south during the day Friday.

The upper level ridge will be over southern British Columbia on
Saturday, but it will be weak enough to allow transient weather
systems to pass through the ridge and brush mainly the northern part
of Western Washington with some light rain. Mid level clouds from
the first system will probably spread over the area from the
northwest Saturday morning, but this system should be pretty much
dry. The models have been inconsistent with the system on Sunday,
especially with the coverage of light rain, ranging from zero to
some spotty light rain over the north part of the area. What is more
certain is that it will be cloudy with highs remaining a little
below normal. Kam/Burke

.LONG TERM...Previous discussion...The weak upper level ridge over
southern B.C. will remain in place Sunday and Monday, still wide
open to weak transient weather systems. The GFS brings another
system Monday through Monday night with a bit more coverage of light
rain. The ECMWF is in better agreement but still a little slower,
bringing some light rain Monday afternoon through Tuesday morning.
Despite the better model agreement, confidence is still low.

Models are in better agreement that the upper level ridge will build
enough by Wednesday for mostly sunny weather Wednesday and
Thursday with temperatures near or slightly above normal. Kam

&&

.AVIATION...There is an upper level trough over the West with
northerly flow aloft over Western Washington. Isolated showers in
the Cascades are diminishing this evening. Satellite and surface
observations show a cloud deck pushing south into W WA with
bases around 8,000 to 10,000 ft. Most areas will stay VFR
overnight but may see patchy fog in the valleys. The trough will
shift toward the Great Basin region on Friday with northerly flow
continuing over WA. Models show isolated showers over the Cascades
with dry weather elsewhere. 33

KSEA...VFR conditions expected this evening with light/variable
wind at the surface. Patchy fog may form early in the morning. Dry
weather on Friday with winds switching to light westerly. 33

&&

.MARINE...Pressure gradients are weak across Western Washington
tonight for generally light winds over the waters. High pressure
over the NE Pac will maintain onshore flow through the weekend. A
warm front will clip the region on Monday. Offshore flow will
develop by midweek as a thermal trough forms along the coast. 33

&&

.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