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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 141609
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
909 AM PDT Sat Sep 14 2019

.UPDATE...Made some minor adjustments to the rain chances this
morning. Recent MRMS imagery suggests light scattered showers
generally extend from a Grays Harbor to Thurston to Pierce County
line northward. Browsing some of the higher-res guidance in
comparison to current conditions suggests this activity will
continue into the afternoon hours ahead of a cold frontal boundary
that continues to lie across Vancouver Island and SW into the
eastern Pacific. So while a wash out is not anticipated today,
have decided to increase POPs for most of the area to account for
current scattered light showers.

Rest of the forecast remains on track for today. Aforementioned
cold frontal boundary will make slow progress southeastward thru
the day as an upper level trough digs southward from the Gulf of
Alaska. Rain chances are still on track to increase noticeably
tonight.

Kovacik

&&

.PREV DISCUSSION... /issued 333 AM PDT Sat Sep 14 2019/ 

.SYNOPSIS...A cold front moving down from the northwest will 
reach Western Washington on Sunday. Rain out ahead of the front 
starting Saturday night. Upper level trough over the area Sunday 
night and Monday. Another frontal system will arrive Tuesday 
followed by another trough Wednesday. 

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows
cloudy skies over Western Washington early this morning. Doppler
radar still has some areas of light rain from Snohomish county
northward. With the cloud cover temperatures at 10z/3 am were in 
a narrow range, mid 50s to lower 60s.

Light rain in the north moving out of the area early this morning
as weak warm front moves east. Trailing cold front still north of
the northern tip of Vancouver Island will slowly move southeast
today but still be well offshore by late afternoon. Rain out ahead
of the front moving into the North Coast and Northwest Interior
this afternoon while the remainder of the area stays cloudy. Highs
in the 60s and lower 70s. 

Front continuing to move slowly southeast tonight with rain
spreading over most of the area overnight. By 12z Sunday the front
will be just west of Quillayute. Lows tonight in the mid and upper 50s.

Cold front drifting through Western Washington Sunday morning with
the steady precipitation changing to showers behind the front in 
the afternoon. Highs will only be in the lower to mid 60s.

Cool upper level trough behind the front approaching the coast
Sunday night and moving inland on Monday keeping showers in the
forecast. Jet stream and the best dynamics are well south of the 
area but the models are showing some increased instability in the
afternoon over the Southwest Interior so will add a slight chance
of thunderstorms to the forecast for that area. Lows Monday 
morning mostly in the 50s. Highs on Monday remaining in the 60s.
Felton

.LONG TERM /TUESDAY THROUGH FRIDAY/...Models are in good 
agreement for the first half of the extended with another well 
organized front moving through Western Washington Tuesday followed
by a cool upper level trough Wednesday with the jet will south of
the area. Model inconsistencies show up for Thursday and Friday 
with the GFS building an upper level ridge to the northwest with 
dry northwesterly flow aloft over the area. The ECMWF is much 
weaker with the ridge allowing a weak shortwave to move through 
the area Thursday night/Friday morning with a warm front 
approaching Friday night. Ensembles are not much help with both 
not far off from the operational runs. With the lack of 
consistency and the ECMWF verifying a little better lately will 
not go dry for Thursday and Friday and include slight chance pops 
in the forecast. Temperatures will remain cool with highs in the 
60s through the period. Felton

&&

.AVIATION...A front will approach the coast today, then move 
through the area tonight. Flow aloft will be southwesterly. At the 
surface, winds will be southerly, and will increase tonight. The air 
mass is moist and stable. Ceilings will be mostly low end VFR today, 
with some pockets of MVFR. Spotty light rain could fall at times 
today and this evening, but steady rain will develop on the coast 
around 00Z and over the Puget Sound area around 06Z.

KSEA...Discussion above applies. Mostly VFR conditions today and 
this evening. Rain developing around 08Z ceilings probably falling 
to 1k ft by Sunday morning. CHB

&&

.MARINE...A front will move through the region tonight, bringing 
small craft advisory strength southerly winds to most areas later 
today into tonight. After lighter winds Sunday and Monday, another 
system will arrive around Tuesday with more advisory level winds. CHB

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Sunday 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 until 2 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-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Sunday 
     for Admiralty Inlet-Northern Inland Waters Including The 
     San Juan Islands.

     Small Craft Advisory from 2 PM this afternoon to 5 AM PDT Sunday 
     for Puget Sound and Hood Canal.

&&

$$

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