FXUS66 KSEW 111137

Area Forecast Discussion
National Weather Service Seattle WA
337 AM PST Mon Dec 11 2023

.SYNOPSIS...Upper level trough moving south out of the area this
morning. Upper level ridge building into the area this afternoon
will strengthen tonight and Tuesday. The ridge will shift east
Tuesday night and Wednesday. Next front arriving Wednesday night
dissipating over the area Thursday morning. Another upper level
ridge will build Thursday night into Friday with a warm front in
the vicinity of the coast. The ridge will move east and the warm
front will dissipate Saturday. Another system will approach from
the southwest Sunday.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
cloudy skies over Western Washington early this morning. Lower
layer of the air mass is saturated with many locations reporting
reduced visibility in fog and or light drizzle. Temperatures are
in the lower to mid 40s.

A grey and soupy day in store for Western Washington today. Weak upper
level trough moving south into Oregon this morning. Low layers of
the air mass saturated so even a little lift from the weak trough
will result in some morning drizzle or light showers. Surface
gradients are light and will remain light through the day. This
will keep the low level moisture intact making for a cloudy
afternoon as well. With the cloud cover there will be little
diurnal spread in the temperatures with highs only a few degrees
warmer than the current temperatures, in the mid and upper 40s.

Weak upper level ridge building over the area tonight will
strengthen a little Tuesday. Light flow in the lower levels
continuing into Tuesday. Subsidence associated with the ridge will
compress the low level moisture but not get rid of it. The weak
December sun will be little help during the day keeping skies
cloudy at least through the morning hours. Some sunbreaks possible
Tuesday afternoon. Fog reforming overnight will continue through
the morning hours before lifting midday. Lows tonight in the mid
30s to lower 40s. Highs Tuesday in the mid to upper 40s with lower
50s possible along the coast.

Upper level ridge shifting east Tuesday night into Wednesday.
Increasing offshore flow Tuesday night into Wednesday will help
dry out the lower layers just in time for the middle and high
level cloud deck out ahead of the next front to move over the
area. Rain reaching the coast Wednesday afternoon with a chance of
rain inland. Lows Tuesday night in the mid 30s to lower 40s. Highs
Wednesday near 50.

consistent the last few runs with the front moving into Western
Washington Wednesday night. Large upper level trough digging south
in the Eastern Pacific between 140-150W will stall the front over
Western Washington Thursday morning with the front dissipating
overhead. Models continue to show a race between an upper level
ridge building just to the east and a warm front approaching the
area from the southwest Friday. The GFS operational run and
ensembles support the ridge winning pushing the warm front west
and north of the Western Washington. The ECMWF is a different
story with the ridge east enough to allow the warm front to brush
the area Thursday night into Friday. ECMWF ensembles have about a
half of the solutions with precipitation as far east as Seattle.
With the lack of consensus will keep at least a slight chance of
rain in the forecast for Thursday night into Friday. Both the
ridge and the warm front weakening over Western Washington Friday
night into Saturday keeping a chance of rain in the forecast. For
Sunday the ridge rebuilds while another front spinning out of the
upper level low off Northern California approaches the area. Will
go for a mostly dry forecast Sunday. Felton


.AVIATION...An upper level ridge will continue to build into the
region today, supporting W/NW flow aloft. Flow aloft will then shift
more N/NE tonight into Tuesday as the ridge axis starts to move
inland. Radar shows a few light showers continuing to linger
across the central Sound early this morning. Conditions at the
terminals are a mixed bag. With abundant low level moisture and
weak surface gradients, expect fairly widespread IFR to LIFR
ceilings across much of the area to continue through early this
morning with visibilities restricted in fog and low clouds. Expect
some gradual improvement by late morning with more widespread
MVFR ceilings developing in the afternoon. MVFR ceilings then look
to persist through much of the day, however another round of low
clouds looks to return tonight and looks to bring another round
of IFR to LIFR conditions. Winds remain light and out of the north
for the majority of the terminals today, persisting at 6 kt or

KSEA...Current conditions IFR, with drizzle at the terminal.
Expect ceilings and visibilities to improve towards MVFR late
this morning into early this afternoon. Winds will remain light
from the north at 6 kt or less. Another round of low clouds and
reduced cigs/visibilities is possible overnight into Tuesday. 14


.MARINE...Surface high pressure will build across the coastal
waters today and then shift inland on Tuesday, allowing for
offshore flow to develop across the region. A weak frontal system
will then move across the area waters Wednesday into Thursday and
may bring another round of headlines to the region. Another frontal
system may move up into the local waters late in the week.

Seas across the coastal waters will continue to subside today
from 7 to 9 ft to 4 to 6 ft by tonight. Seas look to hover at
between 4 to 6 ft through Tuesday, but look to approach 10 to 11
ft again on Wednesday as the next swell train arrives. Seas then
look to subside to 7 to 9 ft Friday and look to continue to
subside through the weekend. 14


.HYDROLOGY...The Newaukum River in Lewis county and the Chehalis River
near Grand Mound in Thurston county have both crested below flood
stage. This means the lower reaches of the Chehalis in Thurston
and Grays Harbor county will also remain below flood stage. The
flood watch was cancelled around 230 AM.

The Skokomish River near Potlatch has receded below flood stage
and will continue to recede today. The flood warning for the
Skokomish will be cancelled with this forecast package.

Rivers will continue to recede through the first half of the week.
Hydrologically significant rainfall not expected with the front
Wednesday night.

No river flooding is expected for the next 7 days. Felton