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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 191714
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 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 ridge of high pressure will maintain dry conditions 
through Tuesday. Areas of fog or freezing fog is likely again later 
tonight and Tuesday morning. Highs will reach the low to mid 50s 
today and Tuesday, with lows Tuesday again dipping into the 30s with 
sub-freezing temperatures in colder valleys. 

High clouds will start to increase Tuesday as high pressure shifts 
inland ahead of a split trough approaching the region. Models will 
no doubt have difficulty handling this split flow pattern as the two 
streams battle for dominance. Most the energy typically splits off 
with the southern portion, which models show tracking into Oregon 
and CA. The northern stream will be still be active, but the 
potential for heavy precipitation will be less in this pattern. 

Rain with the first split front will reach the coast early Wednesday 
morning and quickly spread inland. QPF in the mountains looks to 
around a few tenths with less much less in the lowlands. Cooler air 
arrives with the front and snow levels should steadily drop to 
around 3500 to 4000 feet Wednesday night into Thanksgiving. A few 
inches of snow could fall at higher passes Wednesday night and 
Thursday. Higher passes like Stevens and While pass are most likely 
to see these amounts, while Snoqualmie pass may only see a couple 
inches. Motorists planning travel through Cascade passes should pay 
close attention to the forecast through the holiday weekend as the 
an active weather pattern is anticipated. Mercer

.LONG TERM...Previous discussion...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...A strong upper ridge over the region will continue 
through Tuesday, then shift east. Light southerly flow will become 
moderate Tuesday. The air mass remains dry and stable except for near 
surface moisture, trapped by the subsidence inversion. Areas of fog 
or freezing fog this morning will develop again later tonight and 
Tuesday morning. Fog should dissipate by midday today and again 
Tuesday. Local IFR fog is likely again tonight/Tuesday morning. 

KSEA...Patchy IFR fog in the vicinity of KSEA should dissipate by 
midday, with VFR prevailing this afternoon and evening. Surface 
winds light and variable becoming northerly 5 to 10 knots in the 
afternoon. DTM

&&

.MARINE... High pressure will remain in control with mainly easterly 
flow through Tuesday. A series of low pressure systems will move 
across the area from late Tuesday night through the end of the week. 
Stronger onshore flow is likely with periods of small craft advisory 
conditions from time to time. It's possible there could be a period 
of gales over the coastal waters Thursday into Friday with second 
stronger system but will have to monitor the forecast going forward. 
CEO

&&

.HYDROLOGY...A pattern change will get underway late Tuesday with a 
series of low pressure systems moving across the area Wednesday into 
the end of the week. Snow levels will be lowering with these 
systems, down to around 3000 feet by Friday. At this time, rainfall 
amounts do not look high enough to cause any river flooding but 
river levels will be on the rise. Still some uncertainty with the 
model precipitation forecasts so will continue to monitor. CEO

&&

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

&&

$$

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