Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 080506
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
906 PM PST Thu Dec 7 2017

.SYNOPSIS...A strong upper ridge will bring dry, or nearly dry,
weather to Western Washington through next Thursday at least. The
air mass is stable and should become increasingly stagnant Friday
through the weekend. High clouds could increase this weekend as a
frontal system passes by well to the north.

&&

.SHORT TERM...An upper ridge will remain over the Pacific
Northwest through the weekend for dry weather with low level
offshore flow. A frontal system will approach from the west
Saturday and skirt the area to the west and north Sunday. The only
expected impact is to bring some high clouds. There will be some
night and morning fog each day, especially in the south interior
and the more fog prone valleys.

A note on air quality -- the network of sensors shows that air
quality has deteriorated somewhat during the last 24 hours, but
most stations are still showing good or moderate air quality this
afternoon. Ventilation will likely worsen a bit tonight and
Friday, as the northeast pressure gradient turns easterly and
weakens, and air quality could begin to deteriorate Friday through
the weekend. There will be a statewide coordination call between
NWS offices and state and local partners Friday morning; at that
point we will make a collaborative decision with regard to the
issuance of an air stagnation advisory. Schneider

.LONG TERM...Previous discussion...While they differ considerably
on timing and other details, the models all show another weak
frontal system moving up and over the ridge (from the offshore
waters into British Columbia) Monday and Tuesday. They even show
some light precipitation clipping our forecast area, but we will
leave that out of the forecast until it looks more convincing. At
most that looks like a brief interruption in the current pattern,
as the models agree that the upper ridge will rebound again
Wednesday and Thursday. McDonnal

&&

.AVIATION...A strong upper ridge axis will slowly drift across
Western Washington for the rest of tonight and into Central
Washington on Friday. Light to moderate southwest flow aloft. The
air mass will be dry with persistent low-level temperature
inversions. This will lead to stubborn persistent fog in a few
places, most notably over South Puget Sound and the OLM/CLS area.
Fog in these areas unlikely to dissipate on Friday. Also some low
marine stratus north and south of the San Juans.

KSEA...Dry easterly flow over eastern King County will again help
avoid fog development at the terminal. However, will go with VCFG
in the TAF to account for fog to the S-SW of the terminal. TIW is
socked in with fog. Light and variable wind for the next 30 hours,
favoring a light SE wind at times.  Haner

&&

.MARINE...Easterly gradients between a surface ridge over the
Rockies and lower pressure offshore will maintain low level
offshore flow through this weekend. Small craft advisory strength
winds are likely at times near the West Entrance to the Strait of
Juan De Fuca. Haner

&&

.HYDROLOGY...River flooding is not expected during the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 9 PM PST this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 9 AM PST Friday 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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