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000
FXUS66 KSEW 231603
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
903 AM PDT Sat Sep 23 2017

.SYNOPSIS...A weak north to south moving weather system will bring
a threat of light rain for the coast and Olympics today. Another
weak system will bring clouds and a chance of rain Sunday night
and Monday, mainly northern portions of the area. Higher pressure
aloft is expected to build offshore Tuesday into mid week for a
return to sunshine and warmer temperatures.

&&

.SHORT TERM...An upper ridge will remain just offshore through
Monday with northerly flow aloft for Western Washington and weak
low level onshore flow. The ridge, however, will be dirty with
weather systems coming over the top and bringing clouds and a
chance of rain at times.

The first such threat is a short wave moving south through the
Coastal and Offshore waters this morning. This will bring a
chance of rain to the coast this morning. Based on current radar
however, the rain threat will probably remain offshore this
morning for pretty much a dry day all areas.

Sunday will be a dry day in between weak weather systems. Another
weather system will bring a threat of rain Sunday night and
Monday. This system looks wetter than the system today,
especially toward the coast where rain will be likely. Rain
chances will be less further inland as the system moves onshore
and falls apart - a chance of rain at best.

Morning clouds today and Sunday should give way to partly to
mostly sunny weather each afternoon. Monday looks to be mostly
cloudy all day as the weather system moves onshore. Highs will be
near normal today and Sunday and then cool a few degrees Monday
with additional clouds and spotty light rain. Schneider

.LONG TERM...Previous discussion...It is pretty safe to say that
Tuesday is where the ridge gets its groove back and sheds the
dirty moniker. While clouds will gradually erode throughout the
day...the standout will be the high temperatures as they rise once
more to around the 70 mark for many locations. With clear skies
holding court over W WA for the remainder of the work
week...conditions will continue to warm with temps peaking on
Thursday as many locations find themselves in the upper 70s. The
ridge exits soon afterward and W WA as models start to coalesce on
a solution for Friday and into the weekend of an upper level
trough starting to enter the area. This will bring temps back down
to around 70 in most spots. Models still disagree with whether or
not this will result in precip though...as the GFS proves the
wetter solution bringing precip area-wide while the ECMWF limits
any POPs to the northernmost portions of the area. Leaning toward
ECMWF solution at this time. SMR

&&

.AVIATION...An upper level ridge centered offshore will maintain
northerly flow aloft over the area today. The low level flow will
be light onshore. Local MVFR CIGs should dissipate by 1900 UTC.

KSEA...VFR. Winds will be light and variable.

&&

.MARINE...High pressure over the NE Pacific will maintain light
onshore flow this weekend. A warm front will clip the area on
Monday with possible Small Craft Advisory winds over the Coastal
Waters. The flow will turn offshore 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