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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 260940

Area Forecast Discussion
National Weather Service Seattle WA
240 AM PDT Sat May 26 2018

.SYNOPSIS...A weak upper trough and strong onshore flow overnight
will bring a cloudy start to the day, with clouds giving way to a
partly sunny afternoon. A flat upper ridge will be over the area 
for sunny and warmer weather Sunday, with marine clouds pushing 
inland again for Memorial Day. Upper level troughiness will keep 
the weather cloudier and a bit cooler much of next week.


.SHORT TERM...A weak upper trough is moving into the Pacific
Northwest early this morning, helping the onshore low level flow 
to deepen the marine layer over Western Washington. Cloud cover
has been increasing during the night, and it should fill in by
sunrise as the air mass becomes moist below about 7000 ft. There
will be spotty drizzle or light rain this morning as well --  
mainly at the coast, in the southwest interior, or in the area of
weak convergence over the Puget Sound region.

The upper trough will move across the Cascades in the next couple
hours and continue eastward today, while a flat upper ridge moves
into the region. Onshore flow will weaken a bit, and the marine
layer will dry somewhat. The cloud cover should break up enough
for partly sunny skies. Highs should be mainly in the 60s across
the interior lowlands of Western Washington, close to normal.

The upper ridge will build slightly over the region tonight and
Sunday, while the lower reaches of the air mass continue to dry
and warm. It should be a sunny and warmer day, with highs across
the interior lowlands in the mid 60s to mid 70s.

The ridge axis will shift east of the Cascades late Sunday, and a
broad upper trough/decaying front will approach the Pacific
Northwest from the offshore waters. That should again trigger
stronger onshore flow, a deeper marine layer, and more clouds 
moving into Western Washington Sunday night and Monday morning.
The arrival of the frontal remains should limit the weather to
partly sunny and somewhat cooler for Memorial Day. McDonnal

.LONG TERM...The models are in fairly good agreement Tuesday
through Thursday, showing a sharper upper trough moving through
the Pacific Northwest Tuesday and Tuesday night, followed by
another upper trough on Thursday. The first of these systems might
just bring some spotty light showers -- mainly to the North 
Cascades -- with the threat of light showers becoming more
widespread with the second trough on Thursday. Model solutions
split on Friday, with the GFS showing a ridge starting to build
and the ECMWF bringing another trough into the region. McDonnal


.AVIATION...Weak upper level trough over eastern Washington will 
continue to move east today. Westerly flow aloft. Onshore flow in 
the lower levels will continue into Sunday morning but weaken 

Wide variety of conditions across western Washington at 09z/2 am. 
North of KPAE just some high clouds. From KPAE to KOLM ceilings in 
the 4000 foot range except near KTIW where ceilings have lowered to 
around 1500 feet. From KOLM southward including the coast ceilings 
are around 2000 feet. Expect the ceilings from KPAE southward to 
become 1500 to 2500 feet across most of the area by 14z with little 
change in conditions to the north and along the coast. Ceilings 
lifting early afternoon into the 3000-4000 foot range with the 
clouds scattering out late in the afternoon. Just partly cloudy 
skies tonight.  

KSEA...Ceilings near 4000 feet lowering to near 1500 feet by 14z. 
Ceilings improving back up to near 2500 feet around 18z and to 3500 
feet early in the afternoon with the clouds scattering out around 
00z. Southwest wind 4 to 8 knots becoming northerly 00z-03z. 


.MARINE...High pressure offshore combined with lower pressure
inland and east of the Cascades will give onshore flow for the
next several days. 

Pressure gradients later today through tonight will not be as 
strong as they were Thursday and Friday, and diurnally driven 
inflow will likely remain at small craft advisory levels. Small 
craft advisories are in effect for early this morning in the 
eastern and central Strait, and late this afternoon and tonight 
for the eastern two-thirds of the Strait, Admiralty Inlet, and 
the Northern Inland Waters (mainly south of the San Juan Islands).
Near gale force inflow is possible Sunday evening with an increase
in onshore flow.

A more northwesterly flow will develop from the west coast of
Vancouver Island southward through the Washington coastal waters 
Monday and continue through the middle of next week. Near gale
force winds are expected, especially over the outer coastal waters,
and the longer fetch of northwesterly winds will likely generate 
a steep northwesterly short-period swell. Albrecht


PZ...Small Craft Advisory until 8 AM PDT this morning for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to 3 AM PDT Sunday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to midnight PDT 
     tonight for Admiralty Inlet-Northern Inland Waters 
     Including The San Juan Islands.