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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 232138

Area Forecast Discussion
National Weather Service Seattle WA
240 PM PDT Sun Jun 23 2019

.SYNOPSIS...Onshore flow will continue. An upper trough will move 
over the Pacific Northwest by the middle of the week with showers 
increasing. The trough will move out of the area late in the week. 


onshore flow will continue through tonight. The NWAC sensor at 
Paradise Mount Rainier shows a half inch of rain so far--in what 
looks like heavy drizzle averaging 0.03" an hour since late last 
evening. Meanwhile up high on the mountain the webcams at Schurman 
and Muir were sunny. Hurricane Ridge also had sunshine today and 
most of the central Olympics had sun--as did the San Juans and 
Bellingham area, and also an area just south of the Olympics. 
Elsewhere most of Western WA remains cloudy this afternoon. A few 
areas of late afternoon clearing or sunbreaks will be about all we 
can expect on Monday too. We might see a PSCZ drizzle area develop 
again overnight, but most of the drizzle or 'showers' will be on the 
west slopes of the Cascades tonight and Monday morning. Tuesday will 
probably be a little warmer and brighter--with less low level 
moisture in lighter onshore flow. An approaching upper trough will 
increase the chance of showers Tuesday afternoon or evening as the 
flow aloft becomes southerly and the air destabilizes. Showers are a 
good bet Wednesday as the low nears and unstable southerly flow 
aloft picks up while temps cool aloft.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The upper low center will 
move ashore Thursday and then on Friday it should be east of the 
Cascades. Another vort center might wrap around the low and start a 
new low center near Vancouver Island--the GFS shows that happening 
and then a weak low peters out over northwest WA through the 
weekend. The ECMWF is similar in that it takes the main low up into 
Alberta but peels a weaker vort max off the B.C. coast--but that 
drifts westward and there doesn't look like much shower activity 
over Western WA--but there isn't a ridge either. 


.AVIATION...CIGS are expected to continue their slow improvement
this afternoon, with many of the terminals likely seeing low-end
VFR conditions late this afternoon/early evening. CIGS will drop 
again this evening, reaching low-end MVFR to high-end IFR 
overnight. Another window for a potential convergence zone will 
develop this afternoon across King/Snohomish County, potentially 
bringing some north winds through the central Puget Sound as far 
south as KSEA. Outside of that, winds will generally be west to 
southwest 6-12 kt with some isolated higher gusts possible at 

KSEA...CIGS will continue to rise into low-end VFR this
afternoon/early evening then falling back down to low-end MVFR 
overnight. Another convergence zone will bring back precipitation
chances this evening and overnight and may result in a period of 
north winds at the airport this evening (most likely between 01z- 
07z). Winds generally 6-12kt through the period. -Wolcott-


.MARINE...Onshore flow continues this afternoon with borderline
gale conditions through the central/eastern Strait of JDF where a
Gale Warning remains in effect through 06z followed by a SCA
through 15z as gradients relax. SCA also in place through tonight 
for the adjacent waters.

Guidance continues to suggest that gradients will relax during 
the first half of the upcoming week. Periods of precipitation are
possible as a slow moving upper level low settles over/near the 
area. Daily westerly pushes through the Strait of JDF may result 
in at SCA conditions at times. -Wolcott-


.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 11 PM this evening to 8 AM PDT Monday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 11 PM PDT this evening for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Monday for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands.