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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 261634

Area Forecast Discussion
National Weather Service Seattle WA
935 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...Clouds are clearing a bit faster than forecast over 
the interior of Western Washington this morning. Northwest King 
County has cleared though it may fill in again, and there are 
numerous other clear patches around the area. Will keep the forecast 
of partly sunny this afternoon, though it may be mostly sunny in 
some places and cloudy in others. Highs today will be in the 60s.

Onshore flow will weaken tonight, and Sunday in general looks less 
cloudy than today. Fewer clouds and building heights will allow 
temperatures on Sunday to rise into the low to mid 70s, with mid 
60s on the coast. Stronger onshore flow Sunday night will deepen the 
marine layer, with more clouds moving into Western Washington Sunday 
night and Monday morning. The weather will be partly sunny and 
somewhat cooler for Memorial Day. Burke

.LONG TERM...Previous discussion...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...Flat upper ridging will be over Western Washington
through Sunday, with light to moderate northwest flow aloft. Low-
level onshore flow will keep a moister marine layer in place, with
dry and stable air above the marine layer. Cigs on Sunday morning
should be similar to this morning, mainly in the 020-030 range. 

KSEA...Lifting and scattering of the clouds expected this 
afternoon and evening. Development of light westerly seabreeze off
Puget Sound expected late this morning, very gradually becoming 
northwesterly this afternoon. Northerly winds should briefly 
exceed 6 knots early this evening. Winds becoming southerly again 
later tonight. Haner


.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 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 westerlies are possible in the Strait on Sunday 
and Monday evenings.

A stronger northwesterly wind will develop from the west coast of
Vancouver Island southward through the Washington coastal waters
on 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. Haner


PZ...Small Craft Advisory from 1 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.