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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 191103

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PST Mon Nov 19 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 to 
mid 50s. A series of low pressure systems will bring wet weather 
beginning Wednesday and continuing through the weekend. 


.SHORT TERM...A sharp upper ridge axis is right over Western 
Washington this morning. Surface gradients are light offshore. 
Satellite imagery shows a mass of low clouds well off the coast, as 
well as areas of fog over the land, especially the southwest 
interior. Today will be like the last couple of days, with sunny 
weather after some morning fog. Highs will be in the low to mid 50s. 
The freezing level in the mountains is around 10,000 feet.

The upper ridge will move east Tuesday, giving southerly flow aloft 
over Western Washington, and allowing clouds from an approaching 
weather system to leak into the area. Models have slowed this system 
slightly, so most inland areas will probably still have partly sunny 
skies Tuesday afternoon. The coast and Olympics will become cloudy, 
though. Any rain will hold off until Tuesday night. Highs will again 
be in the low to mid 50s.

Rain will develop on the coast Tuesday night, then spread inland on 
Wednesday. By Wednesday afternoon rain is likely nearly everywhere, 
and it should definitely be raining Wednesday night. This 
system has to push through strong high pressure, so a low QPF 
event is expected. Precipitation amounts will be several tenths of 
an inch in the mountains, with perhaps half that over the lowlands.
Highs will be in the low 50s. The snow level will fall to 4000 feet 
or so Wednesday night, but with the low QPF, there will probably be 
only a few inches of snow in the passes. Burke

.LONG TERM...Models have been consistent in showing another, wetter 
system arriving Thursday, followed by an upper trough for showers 
Friday. High temperatures will fall to around 50 on Thursday and 
into the upper 40s on Friday. The snow level will fall to around 
3000 feet on Friday. During the wettest period, about 18Z Thursday 
to 18Z Friday, several inches of snow are likely in the passes--
possibly even advisory amounts.

Models diverge somewhat after Friday. The GFS brings another system 
into the region Saturday, while the latest Euro run builds a ridge 
and is pretty dry both Saturday and Sunday. For now have disregarded 
that solution and kept likely pops Saturday with chance pops Sunday. 
Highs both days will be 45-50, with lows around 40--pretty close to 
average for this time of year. Burke


.AVIATION...Upper ridge axis over Western Washington this morning 
will shift east of the Cascades today with increasing south to 
southwest flow aloft. The air mass remains dry and stable except for 
some patchy low level moisture. Low level flow is still offshore, 
but weaker than 24 hours ago. This has allowed some patchy fog to 
develop. This should burn off by late morning, but will likely 
redevelop again Tuesday morning.

KSEA...VFR. May see patchy/shallow fog in the vicinity early this 
morning. Surface winds light and variable becoming northerly 5 to 10 
knots in the afternoon. 27


.MARINE...High pressure over ID with lower pressure over the WA 
coastal waters will keep the flow mainly easterly through Tuesday. 
Expect a weakening cold front to approach from the west Tuesday 
night. This front will probably dissipate before reaching the coast 
late Wednesday. 

A stronger frontal system may impact the area the latter part of the 
workweek. Confidence in the forecast beyond Wednesday was low 
because of lack of consistency in and agreement between the weather 
models. After collaboration with adjacent marine forecast offices, 
we decided to stick with the stronger solution for now in regards to 
wind speeds. However, we also chose to not bite off on the 20+ foot 
waves being forecast for the central WA coastal waters on Friday. 
There is a good chance that this will disappear in the next wave 
model run or cycle.


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