Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191540
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
840 AM PDT Tue Sep 19 2017

.SYNOPSIS...Low pressure aloft will remain over the region through
Wednesday for cool and unsettled conditions. High pressure aloft
will gradually move over the area from the west the latter part of
the week for a drying and warming trend.

&&

.SHORT TERM...An upper trough over the area will weaken today.
Scattered showers will continue and perhaps increase a bit in the
afternoon. There might be an isolated lightning strike today but
the threat look less than yesterday. Snow levels will be around
5000 feet and there will be some accumulations in the mountains.
It will probably be hard for additional snow to stick to roads
during the daytime hours with the relatively strong September sun
however. Highs will be around 60.

An upper short wave will push onshore tonight and Wednesday
morning. Most of the energy and precipitation will move inland to
the south with just a glancing blow for the south part of the
forecast area. The mountains above 5000 feet will probably pick up
a little more snow tonight. Paradise will probably get the most
with maybe 2-4 inches. Further north, showers will be more
isolated Wednesday. Highs will be near 60 again.

The upper trough will shift inland Wednesday night and Thursday
with drier northerly flow aloft developing over Western
Washington. A few showers will linger but probably mostly over
the mountains. Highs will creeps up a few degrees but still be
several degrees below normal. Schneider

.LONG TERM...Previous discussion...There might be a few light
showers over the interior on Friday under continued weak cyclonic
flow aloft; otherwise, expect an upper level ridge to build over
the area from the west on Friday for a drying trend.

An upper ridge over the northeast Pacific will give dry northerly
flow to Western Washington Saturday through Monday. Highs will
warm back to near normal. 05

&&

.AVIATION...Upper level trough over the area with northwesterly
flow aloft. Air mass moist and unstable with onshore flow giving
scattered showers. Chance of a thunderstorm near the coast. The
mountains will be obscured most of the time.

KSEA...The chance of showers will continue and showers will probably
increase as the low pressure center along the coast shifts over
Puget Sound this afternoon. For now the wind is southerly but the
gradients are light and the wind could become variable as early as
this afternoon or evening.

&&

.MARINE...There is a 1004mb low center just off Destruction Island
at 8am and that low should move east to the Puget Sound area
this afternoon. Pressure gradients are fairly light over
Western Washington in the low pressure area and all the real
gradient is down over the Oregon coast. Another trough will move
through the area tonight and Wednesday morning--and another 1004mb
low center will move ashore, this time near Astoria. The wind should
turn northerly over Western WA on Wed as the low moves in south of
the area.  High pressure will build into the area Friday into
Saturday.

Swell over the coastal waters in the 12 to 14 foot range will
subside below 10 feet tonight and remain below 10 feet into the
weekend.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 5 PM PDT this afternoon for Cascades
     of Pierce and Lewis Counties.

PZ...Small Craft Advisory until 11 AM PDT this morning for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory for hazardous seas until 2 AM PDT Wednesday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory for rough bar until 1 AM PDT Wednesday for
     Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 2 AM PDT Wednesday
     for Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.

     Small Craft Advisory for hazardous seas until 6 PM PDT this
     evening for West 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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