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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181709

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PST Sun Nov 18 2018

.SYNOPSIS...High pressure aloft will maintain cool but dry weather 
through Tuesday. Patchy night and morning fog or freezing fog will 
give way to afternoon sunshine with high reaching into the low or 
mid 50s. A series of weather systems will bring wet weather 
beginning about Wednesday and continuing through the holiday weekend.


.SHORT TERM...A strong upper ridge will maintain the dry offshore 
weather pattern through Monday. Southerly flow develops Tuesday with 
the flow becoming onshore, although rain the next elongated front 
will hold off until Wednesday when the ridge shifts further inland. 
Low dew point air has infiltrated most of Western Washington due to 
the northerly or offshore flow the last couple days. Dew points are 
in the 20s across most the area while temperatures have dipped into 
the 30s, and below freezing in colder pockets. Only a few prone 
valley locations are reporting fog, while enough dry air has mixed 
down to the surface, inhibiting fog formation elsewhere. Most the 
area will have another mostly sunny day, and any morning fog will 
dissipate over the next couple hours.

High pressure aloft and a relatively dry air mass, even near the 
surface, will limit fog to patchy coverage Monday morning in prone 
sheltered valleys. Clear skies and a dry air mass will again allow 
for strong radiational cooling with lows down to the 30s, except mid 
to upper 20s in colder valleys. Mercer

.LONG TERM...Previous discussion...Wednesday through Saturday will 
be a wet period. Rain will spread inland Wednesday, with likely or 
better pops everywhere by afternoon. A front will move through 
Wednesday night, and another one will arrive Thursday afternoon. An 
upper trough will keep at least showers going on Friday. Models 
differ about Saturday, with the Euro fairly dry but the GFS 
distinctly wet, so have kept likely pops Saturday. 

QPF is not especially heavy with these fast moving systems, with 
perhaps an inch of water per day in the mountains and half that in 
the lowlands. The wettest period currently looks to be 18Z Thursday 
to 18Z Friday. The snow level will fall to 4000 feet on Wednesday, 
and even lower by the weekend. This means there will be snow at 
times at the ski areas and in the passes. Burke

.AVIATION...High pressure and offshore flow will lead to dry and
stable conditions across Western WA today and Monday. The flow aloft 
is northerly with E/NE winds at the surface. Skies are mostly clear 
for VFR conditions. Low clouds an/or fog may be more widespread 
Monday morning with less mixing. 33

KSEA...VFR today with N/NE wind to 10 kt. 33


.MARINE...Offshore flow will prevail across Western WA through
early this week due to strong high pressure east of the Cascades.
Winds are N/NE over the waters. The next frontal system will 
approach the region on Tuesday then split and weaken as it moves 
inland on Wednesday. A stronger Pacific frontal system will impact 
the area on Thursday with gales possible over the Coastal Waters.


.HYDROLOGY...River flooding is unlikely over the next seven days.