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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 180430

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PST Sat Nov 17 2018

.SYNOPSIS...High pressure aloft will maintain the cool dry weather
pattern 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 by the middle of next week 
through the holiday weekend.


No major changes to the previous forecast thinking: An upper 
level ridge centered along 130w will shift directly over the 
Pacific Northwest through Tuesday. Dry weather will persist along 
with some patchy fog night and morning fog or freezing fog in 
prone sheltered valleys. 

The SEA-EAT gradient may reach -10 mb overnight so there could be
some brief localized gap winds, but gusts to 30 or 35 mph will 
likely remain near the pass summits. 

The upper ridge finally shifts inland by Tuesday allowing deep 
southerly flow to develop over the west coast. This flow direction 
could allow wildfire smoke from CA to lift north again into 
Oregon/Western WA. This potential for a brief smoke intrusion 
around Tuesday of next week could cause some hazy conditions with 
worsening air quality for roughly 24 hours. A front on Wednesday 
should create a lot of mixing and should improve air quality issues 
if any arise. 

Models indicate a pronounced pattern change by around Wednesday 
of next week, through the holiday weekend. A series of stronger 
systems will bring rain at times to the region, but the progressive
flow and pronounced split will likely reduce rainfall amounts. 
See the hydrology section below for details. Temperatures will 
cool slightly by next Friday and Saturday, but not far off the 
average for this time of year. Highs mostly upper 40s to low 50s 
with lows in the 30s to around 40. Mercer

No major changes to the previous forecast thinking: High pressure
and offshore flow will lead to dry and stable conditions across 
Western WA. The flow aloft is northerly with E/NE winds at the 
surface. Skies are clear but may see patchy/shallow fog near the 
sound early Sunday morning. Otherwise, expect VFR conditions. 33

KSEA...N/NE winds to 8-15 kt. Patchy low clouds may form near the
Sound Sunday morning but will be shallow and short- lived. 33


No major changes to the previous forecast thinking: Offshore
flow will prevail across Western WA through early next week due 
to strong high pressure east of the Cascades. Winds are N/NE over 
the waters with Small Craft Advisories in effect, specifically for
the West Entrance and adjacent Coastal Waters. Winds will ease 
this evening. The next frontal system will approach the region on 
Tuesday then split and weaken as it moves inland on Wednesday. 33


River flooding is not anticipated the next 7 days; however, there
is a slight risk the Skokomish River could flood by the end of 
upcoming week.