Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171715
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PST Fri Nov 17 2017

.SYNOPSIS...A cool upper trough and showery air mass is shifting
east today. Weak high pressure will be over the area tonight and
Saturday. A strong front will reach the area Sunday--heavy rain
is likely in the Olympics, and windy weather is possible for some
areas. Periods of wet weather are likely next week with windy
conditions at times.

&&

.SHORT TERM...An upper trough is moving east and a weak upper level
ridge is shifting over the area through Saturday. Scattered showers
will taper off through tonight but a little snow shower activity
will persist in the mountains into Saturday. A strong front will be
hung up over the B.C. coast and Vancouver Island Saturday and
Saturday night and prefrontal pressure gradients will pick up over
Western Washington Saturday night. Pressure gradients look strong
enough for wind advisory strength winds Sunday morning. There will be
heavy rain in the Olympics, a strong rain shadow in the lee of the
Olympics, and then as the rain sags south the winds die off in the
afternoon and evening. The front is into Oregon by late Sunday
evening.

.LONG TERM...Looking at the 12z GFS: The front that moves through
Western Washington Sunday afternoon will be over Northern California
and Oregon later Sunday night and Monday morning. The GFS shows a
dry day for Western Washington on Monday. Rain moves back into
Western Washington Tuesday afternoon and evening. The heavier rain
shifts to the B.C. coast and Vancouver Island for Wednesday and
Wednesday night. On Thursday the front sags back south and weakens,
with dry conditions forecast on Friday. Another frontal system
arrives over the weekend while a deep low gets down to 974mb--but
stays far offshore and becomes fairly stationary out around 46N just
outside of 135W.

&&

.AVIATION...A flat upper ridge axis will move eastward across
Western Washington during the next 30 hours. Moderate northwest
flow aloft today, becoming westerly this evening. The low-level
air mass is moist and unstable, but the instability will become
shallower tonight and Saturday, with some drying on Saturday
morning. Scattered convective showers continue today, though they
will be weaker and more anchored to mountainous terrain features
than recent days. A diffuse PSCZ in the Seattle area this morning
will dissipate this afternoon, ending the local focus for cigs
below 020.

KSEA...Southerly surface wind for next 30 hours and beyond.
Currently observed cigs below 020 should dissipate in the next
hour or two, as the supporting Puget Sound Convergence Zone north
of the terminal washes out. Plenty of cloud breaks this afternoon,
maybe even an hour or two of clear skies over the terminal thanks
to shadowing off the Olympic Mtns. Cigs likely to drop below 020
late this evening, probably in the 05z-07z time frame, then
continue overnight. Low-level air mass will experience some drying
on Saturday morning, so clouds below 020 should lift and
dissipate soon after sunrise on Saturday. Haner

&&

.MARINE...A weak ridge will build over the area today, and winds
will ease. The ridge should be strong enough to keep most weather
out of the area on Saturday.

A strong front will slowly approach the area from the northwest late
Saturday then move inland Sunday morning. A gale watch is in effect
during that period for all waters except the central strait.
Forecast KPDX-KBLI gradients are in the 10-12mb range. Another
strong system will arrive around Tuesday. Haner

&&

.HYDROLOGY...Heavy rain will fall in the Olympics on Sunday so
that will drive the Skokomish river in Mason county up again.
Mild wet weather is likely from about Tuesday afternoon through
about Thursday of next week--possibly resulting in flooding on
several rivers. The latest GFS shows the heavy rain shifting
back north into B.C. Wednesday and Wednesday night, so that
pause could help a great deal and possibly prevent any flooding,
although the Skokomish river would be the exception again.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 4 PM PST this afternoon for
     Cascades of Pierce and Lewis Counties.

PZ...Gale Watch from Saturday afternoon through Sunday afternoon for
     Admiralty Inlet-Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

&&

$$

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



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