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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 180028
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 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.

&&

.SHORT TERM...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 northerly PDX-BLI gradient 
peaked close to -5.7 mb but winds have remained below 20 mph so far. 
Expect locally breezy winds to ease tonight as surface high pressure 
shifts east of the Cascades, creates easterly flow. The SEA-EAT 
gradient may reach -10 mb 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. 

.LONG TERM...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
        
&&

.AVIATION...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 8-12kt. Dry conditions tonight. Patchy low
clouds may form near the sound Sunday morning but will be shallow
and short-lived. 33

&&

.MARINE...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

&&

.HYDROLOGY...Dry weather is likely to persist through Tuesday. A 
series of fronts will bring rain at times beginning around the 
middle of next week. The progressive flow pattern and split stream 
will tend to weaken systems more than models will pick up on. It 
still looks rather wet with light rain and breezy conditions. 
However, 24 hour rain amounts appear to light to cause river 
flooding. The Skokomish river is the exception, with 12z runs 
wetter over the Olympics with a lot more flow which would favor 
heavier rain on the southwest slopes. The Skokomish river is not 
forecast to flood, but there is a slight risk it could flood by the 
end of next week. Mercer

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PST this evening for Coastal 
     Waters From Cape Flattery To James Island Out 10 Nm-West    
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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