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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 190515

Area Forecast Discussion
National Weather Service Seattle WA
915 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 low pressure systems will bring wet 
weather beginning about Wednesday and continuing through the


The forecast was updated to add fog for tonight through Monday
morning. Fog has already developed at a few places. It looks like
fog will be more extensive Monday morning compared to the previous
two morning. Otherwise, no major changes to the previous forecast

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.

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.

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...No major changes to the previous forecast thinking.
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 and/or fog in the vicinity early Monday morning. 33


.MARINE...No major changes to the previous forecast thinking.
Easterly (offshore) flow will continue through Tuesday because of  
high pressure east of the Cascades with lower pressure offshore.
The next frontal system will approach the region on Tuesday then 
split and weaken as it moves inland on Wednesday. A stronger 
frontal system will impact the area on Thursday with gale force
winds possible over parts of the area. 33/05


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