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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181100

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

.SYNOPSIS...High pressure aloft will maintain cool but dry weather 
through Tuesday. Areas of night and morning fog or freezing fog will 
give way to afternoon sunshine. A series of weather systems will 
bring wet weather beginning about Wednesday and continuing through 
the holiday weekend.


.SHORT TERM...A sharp upper ridge will slowly move across Washington 
today through Tuesday. Skies will be clear today and Monday, with 
areas of night and morning fog and freezing fog. Temperatures at 2 
AM were mostly in the 30s, with some lower 40s and upper 20s. Under 
clear skies temperatures are several degrees lower than last night 
at this time. Highs today will be in the lower 50s, and on Monday 
they will be a few degrees warmer. Overnight lows will be around
freezing tonight and Monday night. The mountains will remain clear, 
with the freezing level around 10,000 feet.

By Tuesday the upper ridge axis will be over Idaho, and southerly 
flow aloft will develop over Washington as a front moves toward the 
state. Clouds will increase during the day. Rain will probably reach 
the coast Tuesday evening, then spread to the Olympic peninsula 
later Tuesday night. Highs Tuesday will be in the mid 50s. Lows 
Tuesday night will be above freezing, mostly 35-40. Burke

.LONG TERM...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...Light northwest flow aloft continues over Western 
Washington this morning as an upper ridge axis remains over the  
coastal waters. The air mass is dry and stable with low level 
offshore flow. Except for patchy low clouds on the north side of the 
Olympics and a little river valley fog this morning, VFR conditions 
are expected today. The upper ridge axis will shift over the region 
tonight with the flow aloft becoming southwesterly on Monday. A 
surface ridge east of the Cascades will weaken today and tonight. 
This will weaken the offshore flow somewhat and allow stronger 
nighttime inversions to develop. As a result, more widespread 
coverage of fog is expected on Monday morning...especially over the 
Southwest Interior and South Sound.

KSEA...VFR today. Surface winds N-NE 5 to 10 knots. May see some fog 
develop in the vicinity of the terminal after 10Z Monday morning, 
but confidence in details not high at this time.   27


.MARINE...High pressure over Idaho with lower pressure over the 
coastal waters will result in easterly (offshore) flow through 
Tuesday. A weakening frontal system will bring the possibility of 
gale force winds to parts of the coastal waters (beyond 10 NM) on 
Wednesday. The cold front will probably dissipate as it moves 
onshore late Wednesday. A stronger system is expected to impact the 
area on Thursday for the possibility of widespread gale force winds.


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