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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181617

Area Forecast Discussion
National Weather Service Seattle WA
917 AM PDT Tue Sep 18 2018

.SYNOPSIS...Dry weather pattern continues today but becoming
increasingly active with increasing shower chances through the


A zonal flow is present across the west coast this morning in the
mid levels. At the surface, it has been a very cool morning 
across Western Washington. Several sites dipped into the mid 30s 
this morning. As is more typical of the fall season, lots of fog 
developed in river valleys and along the sound. This will continue
to slowly dissipate by late morning and early afternoon. With 
sunny skies following the fog, temperatures will rebound and climb
into the mid 60s for the most part. Wednesday should also be a 
similar day to today with temps in the 60s and dry weather 
expected with just a weak mid level trough present. A system 
moving into southwest BC by Thursday will bring increasing 
cloudiness and shower chances later this week. Temperatures will 
cool in response to an onshore flow on Thursday. There is not an
impressive amount of moisture present, however, and it does not 
appear to be a significant amount of rainfall. LAJ

Next system will move across the area Friday into Saturday. This 
system looks to be wetter and stronger than first system with a 
better shot of more widespread precipitation and moderate 
accumulations. In addition, as gradient strengthens, breezy to 
locally gusty winds will accompany the frontal passage. Final 
shortwave looks to swing through later Saturday into Sunday with a
few more showers. 

Models came into better agreement overnight for Monday/Tuesday of 
next week. Both GFS and ECMWF show ridging developing offshore, 
though differ on strength. Regardless, given drier trend, have 
cut back precipitation chances and bumped up temperatures a few 
degrees. CEO


.AVIATION...The air mass across Western WA is dry and stable with
onshore flow. The flow aloft is westerly. Low level clouds are
over the interior for MVFR ceilings. Low clouds will lift and 
scatter early this afternoon for VFR conditions. Low level clouds
will likely reform overnight for a return of MVFR conditions. 33

KSEA...Low clouds and MVFR cigs this morning. Low clouds will lift
and scatter 19-21z for VFR conditions. N/NW winds at the surface
to 10 kt. Low clouds will likely reform overnight by 12z. 33


.MARINE...Onshore flow will prevail across Western Washington
today. Winds through the Strait of Juan de Fuca will be in the 
10-20 knot range this evening. Onshore flow will increase on 
Wednesday behind a cold front with Small Craft Advisory winds 
likely through the strait. A stronger Pacific frontal system will 
arrive Thursday night and Friday for another round of Small Craft
Advisory winds over the Coastal Waters, and possibly the entrances
to the strait. 33