Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181002
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
302 AM PDT Tue Jul 18 2017

.SYNOPSIS...A low pressure system offshore will keep the flow
aloft from the southwest through Wednesday. This system will move
across the region on Thursday for a threat of showers. Expect
temperatures to be near or slightly above normal today and
Wednesday but below normal on Thursday.

&&

.SHORT TERM...
There were areas of stratus over the coast at this time while an
area of altocumulus was noted over the northern Puget Sound
Region; otherwise skies were mostly clear. Low clouds will fill in
a bit more over the coast before dissipating late this morning.

A low pressure system will remain offshore through Wednesday,
keeping the region under southwest flow aloft. There may be enough
instability and moisture over the north Cascade crest for a slight
chance of a late afternoon and early evening thunderstorm today
but it appears that the best chance will be east of the crest.
The surface onshore pressure gradient will be a little tighter
this afternoon through tonight; therefore, expect the low clouds
to be more widespread Wednesday morning than this morning.

The offshore low pressure system will move inland Thursday morning
for a threat of rain starting on the coast Wednesday night and
the rest of the CWA on Thursday. It looks like rainfall amounts
will be light or generally less than a tenth of an inch. In other
words, we`re not expecting a nice soaking rain for the area. A
threat of showers will linger over parts of the interior into
Thursday evening; otherwise, expect dry weather to return.

Temperatures will near or slightly above normal today and
Wednesday but below normal on Thursday.

.LONG TERM...
Nearly zonal flow aloft will prevail over the region through
Sunday. Heights/thicknesses will rise during this period;
therefore, expect temps to climb above normal on Sunday.

Both the ECMWF and GFS indicated that the Pacific Northwest will
be under the influence of an upper level trough Sunday night and
Monday. This would result in strengthening onshore flow and a
deepening marine layer Sunday night, leading to a cloudy morning
over the lowlands. Also, it would result in cooler temps across
the CWA. Decided to hold off on making major changes to the Day 7
forecast for now. Would like to see whether the medium range
solutions continue with the idea of cooler weather for early next
week.

&&

.AVIATION...Dry southwesterly flow aloft will continue today and
into Wednesday with onshore surface flow also persisting. Current
obs and satellite imagery show clear skies over much of W
WA...although there is a small patch of mid level clouds around the
north sound. HQM remains clear...and as such 12Z TAF issuance will
need a touch-up. The air mass remains stable...but models remain
consistent with instability developing over the Cascade crest during
the mid to late afternoon...bringing with it a slight chance for
thunderstorms. Conditions will stabilize quickly during the evening
hours.

KSEA...FEW-SCT mid level clouds this morning to go along with the FEW-
SCT cirrus. Mid level clouds expected to burn off by late this
morning. Northerly wind 5-10 KT this morning before turning
northwesterly in the afternoon. Southwesterly winds expected
tonight. SMR

&&

.MARINE...Current observations show that onshore flow continues to
weaken. In the strait...winds have eased in both the central and
eastern portions...but latest Race Rocks ob still a little too close
for comfort to SCA threshold. While canceling the current early
morning SCA is likely...probably will not fully decide one way or
the other until just before sending out morning forecast package.
Stronger onshore flow late this afternoon and tonight will warrant
another SCA for the strait as well as impacting southern portions of
the N Inland waters. Will ship that headline out with the AM package
as well.

This onshore flow is expected to remain in place for the remainder
of the week...with small craft advisory conditions likely each
afternoon and evening in the strait. There may also be occasional
advisory level northwest wind on parts of the coast this week as
well. SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for the central/eastern
 Strait of Juan de Fuca and the Northern Inland Waters.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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