Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 150420

Area Forecast Discussion
National Weather Service Seattle WA
820 PM PST Thu Dec 14 2017

.SYNOPSIS...A weak cold front will spread light rain across
Western Washington late tonight through Friday morning as it moves
southeast across the area. An upper level trough trailing the
front will bring a few showers Friday afternoon and evening. A
frontal system will bring light rain to the area this weekend.
After a minor lull on Monday another system will bring rain and
possibly windy conditions on Tuesday. Drier cool weather may
return around mid week.


.SHORT TERM...IR imagery shows a ragged looking cold front just
offshore and moving slowly toward the WA coast. No echoes yet on
RADAR. Models have been quite consistent in bringing light rain to
the N coast and N interior around midnight, then spreading it
slowly SE across the area. Rain may just be moving over the
Seattle area around 12Z/4 AM. The front should be through the
interior by around midday Friday. Precip amounts should be
modest, only up to a quarter inch over the N interior and up to
two tenths elsewhere.

The lower air mass has a bit more moisture this evening as
dewpoints are a little higher. Dense fog in the Olympia area
lifted briefly this evening before starting to return at KOLM.
Patchy fog has also developed at KPWT. However, highway cams don`t
show any fog around. Mid-level cloud cover will begin spreading
across the area after midnight, which should pare back the threat
of fog.

After the cold front moves E midday Friday, the trailing upper
level shortwave trough will follow Friday afternoon. It doesn`t
look very convective offshore behind the front, but never-the-less
there could be a few showers around.

The GFS and ECMWF are in good agreement that a slow moving wetter
frontal system will bring rain to the area, likely through the
weekend. An approaching warm front will bring light rain to the
area on Saturday, but accumulations should be 0.15 or less. Rain
should pick up a bit Saturday night as the warm front moves
inland. The trailing cold front, sliding S along the B.C. coast
will approach the area on Sunday. A strong westerly jet stream
will slide down over the region on Sunday, causing the cold front
to slow to a crawl as it crosses the area. This should keep the
rain going pretty much all day Sunday. The coast, N interior, and
mountains should get plenty of rain, but low level W flow will
put the central Puget Sound region in the Olympic rain shadow, so
precip amounts there may end up being lighter. The front should
slide S of the area Monday night. Kam

.LONG TERM...Monday appears to be a transition day, with a chance of
showers. A strong weather system is forecast to arrive Tuesday and
impact the region through Tuesday night. This could bring heavy
rain to the mountains, along with windy conditions in the lowlands.
After that, cool northerly flow aloft sets up, giving dry conditions
with highs in the low 40s and lows possibly below freezing. Burke


.HYDROLOGY...River flooding is not expected in the next 7 days.


.AVIATION...High clouds and patchy fog this evening, but a weak low
pressure system and cold front will move over Western Washington
Friday morning. The air will become moist with areas of rain
developing on the coast after midnight and spreading inland a little
after daybreak. Southwest flow aloft. Onshore flow Friday afternoon
and evening will keep the air moist with areas of light rain or
showers tapering off into Friday night.

KSEA...Cirrus this evening and then clouds fill in and rain
develops 12Z-18Z Friday. MVFR conditions will likely continue in
onshore flow behind the front Friday afternoon and evening. Light
winds will become southerly and then southwest Friday afternoon--
but only up to about 10kt.


.MARINE...A 1020mb low and cold front will move over Western
Washington Friday morning. Small craft advisory northwest winds will
develop over the coastal waters behind the front and then
spread into the Strait of Juan de Fuca by evening.

A warm front will move through the waters late Saturday night into
Sunday. Some small craft advisory conditions are likely across the
waters, especially Sunday, as flow aloft interacts with the area

A deepening low will move onto Central Vancouver Island on Tuesday
then move rapidly eastward along the Canadian border. Widespread
small craft advisory conditions are expected with this system and
gale force winds are possible over some waters as it moves by to
the north of the waters.


PZ...Small Craft Advisory from 6 AM Friday to 3 AM PST Saturday 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 James Island To Point
     Grenville Out 10 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 from 4 PM Friday to 3 AM PST Saturday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca-West Entrance U.S.
     Waters Strait Of Juan De Fuca.


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