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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 190030
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 PM PST Sun Nov 18 2018

.SYNOPSIS...High pressure aloft will maintain cool dry weather 
through Tuesday. Patchy night and morning fog or freezing fog will 
give way to afternoon sunshine with highS 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...High pressure aloft building over the Pacific 
Northwest will start to shift inland Tuesday night. An elongated 
upper trough will eject northeast into the region on Wednesday with 
some light rain, the first of several systems expected to affect the 
area.

Skies will remain clear the next couple days with chilly nighttime 
temperatures due to strong radiational cooling. Offshore flow has 
dried the air mass, even in the lower levels. This will limit fog 
development, with patchy fog expected to be mainly confined to prone 
river valleys and around the southwest interior and far south 
interior Sound. lows will dip into the 30s, and below freezing in 
colder pocket locations. 

High clouds increase Tuesday night, then deeper moisture arrives 
Wednesday as a positive tilt trough splits as it approaches the west 
coast. The heaviest rainfall will head south into CA with the 
southern stream, while the northern stream brings lighter 
precipitation to WA/Oregon Thursday. Snow levels start off around 
5000 to 6000 feet but will fall some as the front passes, down to 
around 3000 to 4000 feet by Thursday night. Only light accumulations 
at the higher passes with the first system.

.LONG TERM...Models show a series of systems moving across the area 
Thursday through the weekend. The next fast moving wave will bring 
cooler and wet weather to the region on Thanksgiving and another 
trailing wave arrives Friday. Snow levels will fall, with some light 
accumulations possible at the higher passes Thursday night and 
possibly more significant snowfall on Friday. Snow levels could be 
down to the 3000 to 4000 foot level. Potential snow amounts in the 
mountains is somewhat uncertain but the trend toward a cooler wet 
pattern and lower snow levels means anyone planning travel over the 
Cascade passes this holiday weekend should monitor forecasts. 
Locally windy conditions may cause some visibility issues, 
especially by Thursday. Other travel such as the ferry crossings or 
travel over bridges may be especially difficult.  

Timing of additional systems next weekend is tricky with the GFS 
and ECMWF becoming out of phase. The GFS shows a weaker system on 
Saturday, and the ECMWF shows stronger split flow with little or no 
precipitation. Will leave chance pops in place for now.  Mercer

&&

.AVIATION...High pressure and offshore flow will lead to dry and
stable conditions across Western WA tonight 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 tonight with N/NE wind to 8 kt. May see patchy/shallow 
clouds in the vicinity early Monday morning. 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, 
and Small Craft Advisories elsewhere. 33

&&

.HYDROLOGY...Flooding is not expected of area rivers the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle













































.LONG TERM...
        
&&

.AVIATION...High pressure and offshore flow will lead to dry and
stable conditions across Western WA tonight 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 tonight with N/NE wind to 8 kt. May see patchy/shallow 
clouds in the vicinity early Monday morning. 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, 
and Small Craft Advisories elsewhere. 33

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

www.weather.gov/seattle