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000
FXUS66 KSEW 270930
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 AM PDT Mon Sep 26 2016

.SYNOPSIS...A weak front will bring clouds and a chance of light
showers to the interior this morning, with clearing behind the front
this afternoon. Southwest flow aloft will maintain a generally dry
air mass Wednesday and Thursday except for some morning low clouds
and patchy fog. A large upper level trough will bring cool showery
weather Friday through the weekend.

&&

.SHORT TERM...Satellite imagery shows a weak front was over W WA
this morning. So far only the north coast around Quillayute has had
any rain, while all other locations have had just a mix of cloud
cover. The front itself appears to be packing cloud layers from
around 5000 feet and higher. However, weak onshore flow overnight
has allowed areas of lower stratus to form along the coast and
extend inland across central and south Puget Sound. Models indicate
the front will be over the Cascades by 18Z/11 AM with clearing
behind the front. The low stratus may take until mid afternoon to
clear from around Puget Sound. There could also be some isolated
light showers over the mountains this morning, and around Puget
Sound due to weak convergence.

With no weather features of note expected to impact W WA Wednesday
and Thursday, that will leave generally dry SW flow aloft to prevail
over the region. Onshore flow will remain weak, but still strong
enough to produce some patchy low clouds around the interior in the
morning. With longer nights the reduced cloud cover overnight will
allow patchy fog to form, particularly over the SW interior and
south Puget Sound. Temperatures will remain near normal with highs
in the 60s to lower 70s and lows in the 40s to lower 50s. Kam

.LONG TERM...The main weather feature Friday through next Monday
will be a large upper level trough. The trough is well offshore this
morning just S of the Gulf of Ak, but will continue moving E through
Wednesday, then begin digging SE and deepening into a large longwave
trough  as it nears the PacNW. The large trough will eventually end
up centered just offshore near 45N/130W on Friday. Both the GFS and
ECMWF bring a small shortwave trough around the base of the large
trough and up over W WA Friday afternoon for an initial chance of
showers.

The models agree that the large trough will continue to bring mostly
cloudy and cooler weather with at least a chance of showers to W WA
this weekend and into Monday. However after the initial shortwave
trough Friday afternoon the model consensus with details falls
apart. The GFS is wetter, particularly Sunday and Sunday night so
POPs have been left in the likely range for now, but confidence
remains low. The models bring the snow level down to around 5000
feet or so this weekend, with the GFS slightly lower. Kam

&&

.AVIATION...Low level onshore flow and a cold front will bring
some moisture and a chance of some light showers into Western
Washington this morning. The cold front may stir things up however
and make for variable conditions. By midday skies across the region
should have scattered out again--although the mountains might still
be obscured at times into the afternoon.

KSEA...Light and variable winds this morning with clouds setting up
before daybreak after the push of marine air. This weak cold front
may give us light showers or drizzle around daybreak. Northerly
winds will pick up this afternoon.

&&

.MARINE...Westerly onshore flow will turn more northerly and
decrease later today. There is a cold front over Western Washington
this morning, but there is not much precip with it, just a good
westerly push of marine air. High pressure will build in behind it
from offshore today. Another weather system should reach the area
late in the week but might not have much in the way of a low
pressure system. The GFS just shows the high offshore gradually
being replaced by weak low pressure over the course of the week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory coastal waters and central and eastern
     Strait of Juan de Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html