Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 140408
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
909 PM PDT Sun Aug 13 2017

.SYNOPSIS...A few light showers were coming to an end in the Puget
Sound Convergence Zone this evening. A typical weather pattern with
night and morning clouds and near average temperatures is likely for
awhile. Weak weather systems will brush Western Washington at times,
but rain is not expected in the week ahead.

&&

.SHORT TERM...A few light showers fell in a weak PSCZ this evening
and at 9pm it looks like those are just about done. Areas of clouds
through Monday morning will be followed by afternoon sunshine. Highs
will range from the 60s on the coast to the lower and mid 70s inland.
Weak systems and onshore flow will keep morning clouds in the
forecast
at times. The 00z UW MM5NAM suggests fewer clouds Tuesday and then
more clouds pushing into the area midweek as the remnants of a weak
system move ashore.

.LONG TERM...The broad upper level ridge over the Gulf of Alaska and
Eastern Pacific will begin to shift eastward Wednesday afternoon as
an upper level trough begins to dig into the Gulf of Alaska from the
Bering Sea. Heights will begin to rise across Western Washington
Thursday or Friday as the ridge is nudged east. The GFS solution
remains the more aggressive with this shift, with 500-mb height rises
reaching to over 580 DAM by Thursday afternoon, while the ECMWF
remains a bit weaker with the ridge as it shifts onshore. The
current forecast continues to reflect the resulting warming and
drying Thursday into Friday with high temperatures reaching into the
low 80s over parts of the interior and upper 60s to low 70s along
the coast.

Medium range models also remain in general agreement on a weak upper
level trough moving mainly into British Columbia later Saturday. The
current forecast reflects this primarily with increased cloudiness
and cooler temperatures.

&&

.AVIATION...An upper trough will slowly shift east of the region
tonight and Monday. Light northwest flow aloft with stable
conditions. Somewhat dry in the mid and upper levels with low level
moisture persisting through Monday morning. Drying in the low levels
Monday afternoon. Mostly VFR cigs but areas of MVFR stratus are
likely to develop late tonight/early Monday morning. All areas VFR
by 18Z Monday with scattering clouds.

A weak convergence zone may produce spotty light rain north of KSEA
through around midnight. N-NE winds have reached all the way down to
Tacoma and will ease over the next several hours. A shift to light
easterly then light variable to 5 kt will occur later tonight.

KSEA...NE wind 8 to 12 kt becoming E to 5 kt after midnight, then
light/variable toward early Monday morning. Sct-Bkn clouds around 4-
5K ft will lower into an MVFR cloud deck for a few hours late
tonight/early Monday morning. Becoming VFR between 16-18Z then
scattering out. dtm

&&

.MARINE...Onshore flow is still moderately strong this evening.
Gradients through the Strait still support a risk of brief Gales in
the central and east Strait, but winds will generally be on the
decrease. Small craft strength winds should continue after the Gale
ends at 11 PM in the central/east Strait and also in waters
adjacent to the east entrance through 5 AM.

A surface ridge will continue to rebuild over the WA offshore waters
tonight, and will remain in place through this week. The resulting W-
NW onshore flow over W WA will be in the weak to moderate range with
SCA winds expected in the strait during most evenings. dtm/Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning until 11 PM PDT this evening for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from 11 PM PDT this evening until 5 AM
     PDT Monday morning for the central and eastern Strait of Juan
     De Fuca.

     Small Craft Advisory until 5 AM PDT Monday for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands.

&&

$$

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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