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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 200351
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Wed Jun 19 2019

.SYNOPSIS...An upper trough will move south over Western Washington 
tonight into Thursday bringing showers. Weak high pressure will 
build Friday and Saturday. Another trough will approach the area 
late Saturday into Sunday and settle over the region early next week.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...No changes to the forecast 
this evening. Puget Sound Convergence Zone formed over Snohomish 
County and moved into north King County. There is also a band of 
rain over western Whatcom and Skagit counties. Overall, shower 
activity will become more widespread through the night across the 
interior and Cascades. The coast will only get spotty showers at 
best with this system. 

Showers will be dissipating on Thursday and should be confined to 
the Cascades by afternoon. Weak high pressure aloft begins nosing 
into the region on Friday with temperatures returning to something 
that resembles normal for the time of year. CHB

.LONG TERM /SATURDAY THROUGH TUESDAY/...Dry conditions will continue 
for the majority of Saturday but upper ridging flattens quickly as 
next upper trough shifts southward along the British Columbia coast. 
The 12Z runs of the GFS and Euro are in remarkably good agreement 
with the path of the upper low through the middle of next week. By 
later Tuesday, the trough looks to be parked just offshore of the 
OR/WA border and doesn't move much as a series of disturbances 
rotate into the region producing periods of showery weather and 
keeping temperatures below normal much of the time. 27

&&

.AVIATION...An upper trough will move into the area from the 
northwest tonight. An upper low will remain inland on Thursday.
West to northwest flow aloft will become northerly later tonight
and Thursday. At the surface, onshore flow will continue with high 
pressure offshore and lower pressure inland. The air mass is moist 
and relatively stable.

5000-8000 foot ceilings this afternoon and evening will lower
1000-2000 feet later tonight. A Puget Sound Convergence Zone is
currently over mainly north King County. It is forecast to move 
south tonight and dissipate. Local ceilings below 1000 feet are 
possible in the PSCZ.

KSEA...PSCZ currently just north of the terminal. It will probably 
reach the area then dissipate, with a few hours of northerly wind 
resulting. Otherwise southerly wind 8-12 kt. CHB

&&

.MARINE...Onshore flow will continue through early next week with 
high pressure offshore and lower pressure inland. At least small 
craft advisory strength west winds are likely most days in the 
strait. Gales are occurring in the strait tonight and are possible 
tomorrow night and again Saturday night. Have hoisted a gale watch 
for Thursday afternoon and night as the only change to the marine 
forecast. On the coast there will be small craft advisory strength 
northwest winds at times through at least Thursday. There will also 
be small craft advisory level steep west swell through Thursday. 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 Friday 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 11 PM PDT Thursday 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.

     Gale Watch from Thursday afternoon through late Thursday night 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 5 AM PDT Thursday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 8 AM PDT Thursday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 5 AM PDT Thursday for West Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 AM PDT Thursday for Puget Sound and 
     Hood Canal.

&&

$$

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