Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 111317 AAA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
515 AM PST Mon Dec 11 2017...updated aviation section

.SYNOPSIS...A strong upper ridge will maintain dry and stagnant
conditions across Western Washington through Wednesday. A weak
system will dissipate over the Coastal waters Tuesday morning. The
ridge will break down by Thursday with rain returning Thursday
night and Friday. Wet weather will continue through the weekend
with strong moist flow over the area.

&&

.SHORT TERM...Satellite imagery shows clear skies over Western
Washington early this morning. Areas of fog have formed in the
last few hours but most of the visibilities in the foggy areas are
above a half mile. With the clear skies temperatures are on the
cold side with mid 20s to mid 30s at 11z/3am. Air quality readings
remain in the moderate or unhealthy for sensitive groups category
at most locations.

Little change in the forecast today with the upper level ridge
firmly in place over the area. Inversion still intact with 3 am
temperatures at Hurricane Ridge ( 5250 feet ), Mount Baker (4200
feet ) and Paradise on Mount Rainier ( 5400 feet ) all in the
50s. Sandpoint profiler shows light winds up to 2000 feet. There
is a good chance the air stagnation advisory, which currently
expires at noon today, will get extended. The fog that has formed
in the area is shallow so expect another morning burnoff leaving
sunny skies this afternoon. Highs in the 40s to lower 50s will be
common. The exception will be along the North Coast where
downslope easterlies off the Olympics will push temperatures into
the upper 50s.

Upper level ridge remaining in place tonight with a dissipating
negatively tilted system moving up from the southwest along the
back side of the ridge. This will add a little bit of cloud cover
to the area overnight especially along the coast. Lows will be in
the mid 20s to mid 30s.

System dying along the Coast early Tuesday morning with the upper
level ridge rebuilding in the afternoon. By 00z Wednesday the
ridge axis will be west of the area. The inversion, which weakens
Monday night into Tuesday morning, will strengthen again Tuesday
afternoon. Cloud cover from the dying system will help limit the
fog formation Tuesday morning with just patchy fog if any forecast
for most locations. Highs Tuesday will be in the 40s and lower
50s.

Upper level ridge remaining over the area Tuesday night and
Wednesday. Low level offshore flow much weaker Tuesday night into
Wednesday so have added more late light and morning fog to the
forecast but coverage will still be less than 50 percent of the
lowlands. Light flow in the lower levels will allow the fog to
hang on a little longer but still have most areas in the sunshine
by Wednesday afternoon. Highs once again will be in the 40s and
lower 50s. With the clear skies Tuesday night lows below freezing
will be common with the colder locations dropping into the mid
20s.

.LONG TERM...Extended models remaining consistent with the ridge
breaking down and a series of systems moving through Western
Washington. Timing on the last few runs for the first system has
been a little slower than the current forecast. Will go ahead and
dry the forecast out for Thursday. With the models showing good
consistency have raised the pops into the likely or higher
category for Friday through Sunday. Rainfall amounts in the
central Puget Sound might not amount to much for the middle of
December with model solutions showing the area somewhat
rain shadowed over the weekend. Felton

&&

.AVIATION...A strong upper level ridge axis will persist over
Eastern Washington through tonight, though it will flatten some.
A dissipating front will brush the coast tonight. Light southerly
flow aloft today will become moderate southwest tonight. The air
mass will be dry and very stable. Shallow dense fog will be the
main forecast challenge this morning over the Puget Sound
lowlands. It will mainly favor sheltered valleys and the South
Sound Region. In contrast, dry east wind off the Cascades will
minimize fog over King County. Looking ahead to tonight,
conditions for fog development will be less favorable due to
thicker cirrus and a little more mixing in the boundary layer.

KSEA...Airport on the northern edge of an area of fog that
extends south to near Tacoma and east over to Auburn. The layer is
shallow and does not go much further north than Sea-Tac so a midd
morning breakout is expected. Clear skies and light south
wind by late morning. Tonight appears unfavorable for
fog development at SEA due to thicker cirrus and better dry air
entrainment from aloft into the surface layer. Felton/Haner

&&

.MARINE...A 1042 mb high over Eastern Washington and lower
pres offshore will result in moderate offshore flow through Monday
evening. A dissipating front will brush the north coast on Monday
night, causing easterly gradients to weaken. Light northerly
offshore flow will prevail on Tue and Wed. A cold front will
sweep through the waters on Friday, leading to a turn to onshore
flow.  Haner

&&

.HYDROLOGY...River Flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Air Stagnation Advisory until noon PST today for Admiralty Inlet
     Area-Bellevue and Vicinity-Bremerton and Vicinity-East
     Puget Sound Lowlands-Eastern Strait of Juan de Fuca-Everett
     and Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Whatcom
     County.

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.

     Small Craft Advisory until 3 PM PST this afternoon 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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