Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201555

Area Forecast Discussion
National Weather Service Seattle WA
855 AM PDT Thu Jul 20 2017

.SYNOPSIS...A weather system lifting across Vancouver Island today
will bring cooler temperatures, clouds, and the first chance of
rain for many areas in over a month. Another weak weather system
will brush the northern portions of Western Washington late Friday
through Saturday. The pattern will return to a dry, sunny and
warmer than normal one, with morning clouds and afternoon sun
Sunday through the first half of next week.


.SHORT TERM...A weak disturbance moving northeast across the area
has brought some light showers to portions of the area today.
Northwest radars show what appear to be the heaviest showers over
the Cascades of Whatcom and Skagit counties and on the
northwestern portion of the Olympic Peninsula. Other light showers
are being reported from southeast of SeaTac airport into Thurston
county east of Olympia. Visible satellite imagery and latest 12Z
model time height sections from the NAM12 show these showers
falling from lower clouds below about 6000 feet. Clouds will
gradually decrease from the southeast late this afternoon and this
evening as the disturbance passes off to the north of the area.
While many places will see some sprinkles today, and a few
locations such as Olympia have reported a few hundredths of an
inch of rain this morning, most locations will not receive
measurable precipitation - keeping the dry streak going. With the
clouds and onshore flow, high temperatures today will be below
normal for late July, only be in the 60s to lower 70s in most

500 mb heights will rise tonight through Friday as a weak
shortwave upper ridge moves across the area.

Another short wave trough, this one now seen out along about 140W,
will move east across Vancouver Island late Friday through
Saturday. The 12Z NAM and GFS are slightly more aggressive in
bringing some precipitation into the northern third or so of
Western Washington than previous runs. While 500 mb heights are
expected to be high (in the mid 580 DAM range) and the air mass is
expected to warm aloft, there will be more mid and upper level
moisture associated with this system, and the system will be
accompanied by a stronger jet streak moving by to the north of the
area. The low levels will also be rather moist with dewpoint
temperatures around 60 degrees. The short term forecast for
Friday night through Saturday was updated to increase clouds and
POPs along the coast and in the interior from about Everett or so
northward. Albrecht

.LONG TERM...From the previous long term discussion: The long
term pattern continues to suggest varying degrees of onshore flow.
Early in the coming week, temps will warm well into the 80s for
most spots. Then a cooling trend is anticipated by mid week as
troughing increases in the Pacific just off the coast. This will
support the morning clouds and afternoon sun pattern and
temperatures, while cooler than earlier in the week, still near
normal by later next week. No significant weather or impacts from
weather are expected.

Key Long Term Messages: Following the briefly cooler and showery
weather today, there will be a quick return to warmer
temperatures with highs near to above normal for the rest of the
forecast period. No significant systems in the long term forecast



.AVIATION...A weak upper trough will shift inland today with a
weak upper ridge moving over the area tonight. The flow aloft will
be westerly. At the surface, a weak trough will dissipate over the
Coastal Waters this morning. A ridge will build over the coast
this afternoon with onshore flow through tonight. The air mass
will be slightly unstable today and then become stable tonight.

There is a mix of MVFR and VFR low clouds over the area this
morning with a few sucker holes thrown in. The general trend will
be for the lower ceilings to gradually lift and break up today.
Areas of MVFR low clouds are expected to redevelop later tonight
or Friday morning.

KSEA...MVFR low clouds should lift to VFR early in the afternoon
with ceilings lifting to 5000-6000 feet and perhaps scattering for
a few hours in the evening. MVFR low clouds are expected to
redevelop after midnight tonight. Southerly wind 8-12 knots will
ease in the afternoon and then become northerly 3-6 knots this
evening. Schneider


.MARINE...A weak trough will dissipate over the Coastal Waters
this morning. A surface ridge will build over the coast this
afternoon with small craft advisory strength west winds expected
in the Central and Eastern Strait tonight.

Another weak trough will move into the offshore waters Friday and
dissipate offshore on Saturday. This will be followed by more
onshore flow Sunday and Monday with high pressure offshore and
lower pressure inland. Schneider


PZ...Small Craft Advisory from 6 PM this evening to 3 AM PDT Friday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.



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