Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 070505
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
905 PM PST Wed Dec 6 2017

.SYNOPSIS...A strong upper ridge will bring dry weather to
Western Washington for at least the next week. 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...A strong upper ridge over the area tonight will
shift slowly inland over the next few days. It will be dry
through the period although some models show a few spots of rain
at the far north coast by late Saturday as a frontal system
offshore approaches. Northeasterly low level offshore flow will
become more easterly with time and weaken as inland high pressure
shifts south and weakens.

It will be another clear and chilly night tonight followed by
another sunny day Thursday. There will be some fog over the south
interior and in the usual fog prone valleys tonight and Thursday
morning but nothing too widespread. Clear areas should be frosty
again. More of the same is expected Thursday night through
Saturday night although there will probably be high clouds at
times Starting Friday.

A note on air quality concerns -- Air quality is showing some
early signs of worsening, but still remains generally good due to
moderate northeast low level flow. As the flow becomes more due
easterly and weakens Thursday night and Friday, ventilation will
worsen and air quality could begin to deteriorate more seriously.
In the meantime, we are holding off from the issuance of an air
stagnation advisory; this is in coordination with local and state
agencies, and the issue will be revisited daily as the dry
stagnant weather continues. Schneider

.LONG TERM...Previous discussion...The upper ridge will begin
rebuilding and shifting slightly westward Sunday and Monday. As a
result, northeast low level flow should increase again. While they
disagree on details, the longer range models are all showing a
small upper low moving up the west side of the upper ridge around
Tuesday/Wednesday. For now, it looks like the effect of this
feature would be minimal, and we are staying with a dry forecast
through Wednesday. McDonnal

&&

.AVIATION...A strong and amplified upper ridge axis will slowly
drift east across Western Washington through Thursday night. The
air mass will be dry and very stable. Drying low-level offshore
flow will prevail. Flow aloft will be light and variable for the
rest of tonight, becoming light southwesterly on Thursday. Low
stratus will continue near the waters from the San Juans down to
near Port Townsend, with fog on Thursday morning in commonly foggy
spots such as OLM.

KSEA...No fog is expected at SEA through the day on Thursday
thanks to a strong, drying easterly pressure gradient across the
Cascades. A northeast surface wind will prevail through early
Thursday afternoon, then a gradual change to east-southeast wind
is expected by Thursday evening. Clear skies for next 30 hours.
Haner

&&

.MARINE...Easterly gradients between a surface ridge over the
Rockies and lower pressure offshore will maintain offshore flow
across Western Washington through this weekend. Small craft
advisory strength winds are likely at times near the West
Entrance, near Grays Harbor, and over much of the coastal waters.
Haner

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PST Thursday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

&&

$$

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