Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 161027
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
327 AM PDT Sun Jul 16 2017

.SYNOPSIS...An upper level trough combined with low level onshore
flow will keep Western Washington mostly cloudy and cooler than
normal today. A weak upper level trough will remain over the
region this week and onshore flow will continue, which will
maintain this typically mild summer weather pattern.

&&

.SHORT TERM...Water vapor imagery shows an upper level shortwave
trough over W WA at 09Z/2 AM this morning, which should be over E
WA this afternoon. Moisture with the trough has been lacking, but
it has managed to deepen the marine layer to around 5000 feet this
morning. Onshore flow overnight has been un-impressive, peaking
early Saturday evening and has been slowly weakening all night. As
a result low level moisture has not increased nearly as much as
earlier expected and the deeper marine layer means that much of
what is present has collected up near the top in a BKN040-050
layer. IR imagery shows most of the cloud cover extends from
around central Puget Sound up over the N interior, with large
breaks over the SW interior, coast and strait of Juan de Fuca.

There is still a slight chance of very light rain or drizzle this
morning but it is starting to look even more unlikely especially
since weakening pressure gradients mean a convergence zone would
be much weaker and less likely. The overall cloud cover across W
WA is currently less than expected, but NAM bufr soundings show
weak low level instability later this morning and this afternoon
which could allow some stratocu to develop and fill in some of the
above mentioned breaks.

The cooler air mass from the upper level trough with minor
support from the onshore flow will still keep highs today mainly
in the upper 60s to lower 70s.

Drier northerly low level flow will develop this evening with only
a weak component of onshore flow. Skies will begin clearing this
evening and there should be little stratus around Monday morning
for a sunny day. There could still be a little stratus along the
coast and over the lower Chehalis valley, but hardly any over the
interior.

Models show the PacNW under a broad upper level trough through
about mid week with 500 MB heights averaging around 5740 meters,
even though there will be some minor shortwave ridging over the
area on Tuesday. This means that even with mostly sunny skies
and weak onshore flow both Monday and Tuesday, the cooler air mass
from the trough should cap max temperatures in the 70s to near
80. Kam

.LONG TERM...On Wednesday, W WA will be in-between the large upper
level ridge over the Western U.S. and a large upper level trough
just offshore near 48N/132W. This puts W WA under NW flow aloft
with weak low level onshore flow from a weak surface ridge over
the WA offshore waters. Wednesday should be another mostly sunny
and mild day with minimal morning stratus.

The GFS and ECMWF have been kicking the offshore upper level
trough, mentioned above, eastward sometime later this week, but
had disagreed on just when this would happen. The new 06Z GFS has
finally migrated to the ECMWF solution and both models now show
the trough ejecting east across Vancouver island on Thursday. This
could generate a few light showers over W WA, especially over
northern areas and the mountains. The ECMWF shows an upper level
shortwave trough crossing the area on Friday while the 06Z GFS is
slower. Kam

&&

.AVIATION...Dry westerly flow aloft will continue today and into
Monday. Low level flow will remain onshore. While some clouds are
present over W WA...the threat for MVFR cigs is getting to be non-
existent. Thus...TAFs will need a bit of a tweak this morning. Best
chances for BKN cloud cover looks to be north of a line from HQM to
PWT to PAE...but even that is not absolute...as conditions over CLM
are reporting clear at the time of this writing. These BKN clouds
will break up through the morning...with mostly sunny skies expected
from late morning on.

Prospects for stratus for Sunday night into Monday morning do not
look terribly convincing...although coastal locations such as HQM
might see some.  SMR


KSEA...VFR persist with FEW-SCT clouds around 3-4k ft. Southwest
wind 5-10 kts turning northerly by late afternoon/early evening.
SMR

&&

.MARINE...Strait made gale for a little bit but winds continue to
ease over the areas with headlines. Have downgraded Strait to an SCA
through 5 AM...but have dropped headlines for N Inland Waters and
Admiralty Inlet...as winds there have fallen below criteria.

Onshore flow will continue through next week. Small craft advisory
conditions will prevail each afternoon and evening in the strait.
There may be occasional advisory level northwest wind on parts of
the coast as well. Aside from tonight, none of the next five days
stands out with an obvious gale candidate. CHB/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Central U.S. Waters Strait of Juan De Fuca and
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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