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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 241543

Area Forecast Discussion
National Weather Service Seattle WA
843 AM PDT Mon Jun 24 2019

.SYNOPSIS...Low level onshore flow will weaken today. An upper
level trough will approach the area Tuesday and be over the
Pacific Northwest Wednesday through Friday. The trough will then 
move east of the area on Saturday.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
cloudy skies across much of the interior this morning with recent
onshore push. Some clearing is taking place across the Southwest
Interior into portions of the San Juan Islands. Expect clouds to
continue to scatter for some afternoon sunshine as low level 
onshore flow weakens with flow aloft turning more southwesterly. 
Temperatures will rise into the mid to upper 60s this afternoon 
with some low 70s possible.

An upper level low will slide south along the coast of British
Columbia through today. The trough will dig well offshore tonight
into Tuesday which will keep low level onshore flow light through
tonight. This will reduce the amount of low clouds around on
Tuesday, though increasing mid to high level clouds are expected
given the location of the upper trough offshore. A few showers 
may develop late Tuesday afternoon ahead of the system. 
Temperatures on Tuesday will be a bit warmer given less morning 
stratus around with highs generally in the upper 60s to low 70s.

The upper low will begin to slide east over the area Tuesday 
night with showers around for Wednesday into Wednesday night.
Temperatures on Wednesday will likely be in the upper 50s for the
coast with mid 60s elsewhere given generally cloudy skies. JD

.LONG TERM /THURSDAY THROUGH SUNDAY/...Good agreement in the
extended models for Thursday and Friday with both the GFS and the
ECMWF keeping the trough in the area through Friday. Previous 
runs had the trough moving out of the area on Friday. Some 
differences in the models over the weekend with the ECMWF bringing
a weak upper level trough over the area while the GFS has Western
Washington in a lull with light flow aloft over the area. Both 
models have low level onshore flow in the lower levels. Will keep 
showers in the forecast through Friday with a continuation of 
below normal high temperatures. For the weekend morning clouds 
and afternoon sunshine with highs warming back to near normal, 60s
and lower 70s Saturday and a couple of degrees warmer Sunday. 


.AVIATION...Westerly flow aloft over Western Washington today 
will become southwesterly tonight as another upper trough digs 
southward over the offshore waters. The air mass remains moist below 
10,000 feet this morning with widespread MVFR ceilings around Puget 
Sound as the remnants of an overnight convergence zone dissipate. 
The air mass will gradually dry out this afternoon and the low level 
flow will turn weak northerly by late afternoon. This will lead to 
improving ceilings this afternoon with VFR expected across much of 
the area from mid or late afternoon through late tonight. Some 
stratus is expected to reform near the coast tonight and push 
locally inland overnight.

KSEA...MVFR ceilings continuing through midday, but gradually 
lifting this afternoon. As low level northerly flow develops this 
afternoon, clearing over the north interior should expand southward 
with ceilings scattering out near or a little after 00Z. Surface 
winds light southerly this morning veering to northwesterly this 
afternoon and to northerly less than 10 knots this evening.  27


.MARINE...As a broad trough of low pressure develops over the 
offshore waters and remains in place through much of the week, 
the strength of onshore flow will be held in check over the coming days. 
Onshore flow will be strong enough to produce small craft advisory 
westerlies in the central and eastern Strait of Juan de Fuca most 
afternoons and evenings, but winds look otherwise rather light 
across area waters.   27


.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 PM this afternoon to 3 AM PDT 
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.