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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 211631

Area Forecast Discussion
National Weather Service Seattle WA
931 AM PDT Sat Sep 21 2019

.SYNOPSIS...Upper level ridge over the area will shift east later
today. A cold front will spread rain through the area Sunday
morning. A weaker system will move through Monday night. After a
break a cool upper level trough will arrive late in the week 
keeping unsettled weather in the forecast.


.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows areas
of low clouds over Western Washington this morning with plenty of
breaks already showing up in the imagery. Temperatures at 9 am/16z
were in the mid 50s to lower 60s. 

Looks like a split for the weekend weather wise with a dry day
today and a wet day on Sunday. Upper level ridge over the area
this morning shifting east this afternoon and evening. Light flow
in the lower levels combined with a shallow marine layer this 
morning will result in the lower cloud cover scattering out this 
afternoon. With some sunshine highs will be near normal, in the 
60s and lower 70s.

Increasing clouds this evening as the cold front, currently back 
near 134W, approaches the area. Rain out ahead of the front
starting along the coast around midnight with the leading edge of
the rain near the Seattle area by 12Z Sunday. Lows tonight in the

Cold front moving through Western Washington Sunday morning/midday
with the upper level trough axis right behind the front for Sunday
afternoon. Rain changing to showers forecast on track for Sunday.
Highs will only be in the lower to mid 60s. 

Upper level ridge trying to rebuild Sunday night into Monday with
a weak system moving over the top of the ridge into Western
Washington late Monday. The ridge will take a toll on the system 
with the best chance for light rain over the northern sections. 
Highs once again in the lower to mid 60s.

Will increase the rain chances 06z-12z Sunday. Remainder of the
forecast unchanged this morning. Felton

.LONG TERM /TUESDAY THROUGH FRIDAY/...Previous discussion follows. There
are hints at a break in the action Tuesday as a large scale 
trough shifts more toward the center of the US...the ECMWF showing
this a bit better than the current GFS...but models come back in 
agreement that another large scale trough will begin to set up 
shop over the area starting Wednesday night with a series of upper
level lows dipping down into the area and making for a generally 
wet remainder of the week. Models do disagree on timings and paths
of these lows a break or two in the action is 
certainly possible. For example...the GFS tries to push an upper 
level low south of the area to the CA/OR border Friday which would
make for a potentially dry day...but the ECMWF keeps the system 
further north close to the WA/OR border which would keep the 
potential for rain going. Opting to split the difference and thus 
advertise a generally wet pattern until a more concrete consensus 
emerges. Temps will continue their cooling trend through this 
period. Lowland highs will start off Tuesday in the lower to mid 
60s but fall into the mid to upper 50s Thursday which is cooler 
than what would normally be seen this time of year. SMR


.AVIATION...Northerly flow aloft will become more westerly and 
southwesterly today as an upper level ridge starts to exit the 
area, making way for an incoming upper level trough. Current 
satellite imagery indicates areas of patchy fog and low stratus 
across portions of the Sound, yielding LIFR ceilings for KPWT and 
KPAE. Otherwise, there's a mixed bag of MVFR/IFR conditions for 
other terminals across the Sound, coast, and southwest interior. 
Only KBLI is experiencing VFR conditions at this hour. Expect 
gradual improvement of conditions throughout the morning hours to 
VFR this afternoon.

The next frontal system will approach western Washington tonight 
and move inland during the overnight hours, brining rain and lower
ceilings to the region. Expect winds to be southerly around 4 to 
8 knots today.

KSEA...Current conditions MVFR. Expect stratus to clear by midday
and improve to VFR conditions by this afternoon. Winds southerly 
4 to 8 knots. SB


.MARINE...Lighter onshore flow will persist through the day 
today. A frontal system will then move through the area late 
tonight through Sunday. Small craft advisory southerlies will be 
likely for the coastal waters ahead of the front, with small craft
advisory westerlies expected in its wake. SCA level westerlies 
will also be possible in the central and eastern portions of the 
Strait of Juan de Fuca on Sunday night following the frontal
passage. Another system will slide through the region Monday into
Monday night. SB


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


PZ...Small Craft Advisory from 2 AM Sunday to 2 AM PDT Monday for 
     Coastal Waters From Cape Flattery To James Island 10 To 60 
     Nm-Coastal Waters From James Island To Point Grenville 10 
     To 60 Nm-Coastal Waters From Point Grenville To Cape 
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 2 AM to 8 AM PDT Sunday for Coastal 
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal 
     Waters From James Island To Point Grenville Out 10 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out 
     10 Nm.