Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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622
FXUS66 KSEW 131200
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
400 AM PST Wed Dec 13 2017

.SYNOPSIS...A strong upper level ridge will maintain dry and
stagnant conditions across most of Western Washington through
Thursday afternoon. The ridge will break down on Thursday allowing a
cold front to bring rain late Thursday night into Friday morning.
A couple of weather systems will brush the area over the weekend
keeping a chance of showers in the forecast.

&&

.SHORT TERM...Satellite imagery shows upper level ridge axis
offshore. Skies have cleared over Western Washington early this
morning. So far the fog formation has been patchy with Bremerton,
Arlington and Forks the only stations reporting visibilities less
than a mile a 3 am/11z. Temperatures ranged from the mid 20s to
near 40.

The clearing skies have given a good view of the Geminid meteor
shower early this morning with multiple meteors in a couple of
brief trips to the roof. The shower peaks tonight into Thursday
morning.

Upper level ridge remaining in place off the coast today. Surface
gradients are light and the Sandpoint profiler shows light winds
up to 1000 feet. Expect a little more fog to form before sunrise
but with temperature dew point spreads still 3 to 4 degrees and
little if any haziness to report from the roof obs will keep the
coverage of the fog in the patchy category this morning. Offshore
gradients have weakened with the cross Cascade gradient down to -5
mb. even with the sunshine with little mixing in the lower layers
highs will once again be in the mid 40s to lower 50s for most
areas. The light winds in the lower levels will keep the air
stagnation advisory going through Thursday afternoon.

Upper level ridge axis shifting to the Coast overnight and into
Eastern Washington by late Thursday afternoon. First system in
a week and a half still well to the northwest by 00z Friday. With
the clear skies it will be another cool night with many locations
dropping below freezing. High on Thursday will be similar to today
with mid 40s to lower 50s common.

00z model runs continuing the trend of previous runs slowing down
and weakening the next system to move into the area. Have taken
the pops out the area south of a Hoquiam to Everett line for
Thursday night. Increasing cloud cover will keep most places above
freezing Friday morning but a few locations from Tacoma southward
could drop a couple of degrees below freezing before the clouds
arrive.

Weak cold front moves through the area Friday morning followed by
an upper level trough Friday afternoon. The combination of the two
will bring some light precipitation to the area. Snow levels will
drop quickly with the front but at this point it looks like the
lower passes ( Stevens and Snoqualmie ) precipitation type will be
rain. Highs on Friday will be in the 40s.

.LONG TERM...Confidence not real high in the extended forecast.
Models waffling around on the position and intensity of a warm
front on Saturday. The ECMWF keeps the chance of rain over the
northern portion of the area while the GFS has spotty light rain
over most of the area. On Sunday the ECMWF has the trailing cold
front reaching the northern portions while the GFS has this
feature further north. The models have another warm front for the
first part of next week with little consistency in the timing and
intensity of the system. With the uncertainty in the models will
go with just chance pops for the extended period for most of the
area. Temperatures will be near normal with highs in the mid 40s
to lower 50s and lows in the mid 30s to lower 40s. Felton

&&

.AVIATION...High pressure is over the region and the air is stable.
Skies early this morning are clearing after mid and high clouds
passes through overnight with a weak upper trough. Areas of fog
should form this morning--mainly near the water since it isn`t
especially damp and dewy yet early this morning. Winds are mainly
calm however, so that should help some fog patches set up around
daybreak.

KSEA...A little fog this morning seems a good bet with clear skies
and calm winds, but at 2am it wasn`t yet very damp and dewy here at
NOAA so I cut it back on a 10z TAF amd.

&&

.MARINE...A ridge of high pressure extending from south-central
British Columbia into Eastern Washington will give light offshore
flow to the waters tonight through Thursday. The light pressure
gradients and a strengthening temperature inversion aloft will
result in areas of fog over the southern portions of the inland
waters during the late night and morning hours. A cold front will
move east through the waters late Thursday night into Friday morning.
A warm front will brush the coastal waters on Sunday.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Air Stagnation Advisory until 2 PM PST Thursday 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...None.
&&

$$

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