Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182141
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
241 PM PDT Mon Sep 18 2017

.SYNOPSIS...An upper trough will be over the area through
Wednesday for cool and showery weather. Drier northerly flow
aloft will develop late in the week as an upper ridge offshore
moves toward the area.

&&

.SHORT TERM...An upper trough will be over the Pacific Northwest
tonight and Tuesday for cool weather and showers at times. An
upper short wave will move through the area tonight, enhancing
showers, especially to the south. There could also be isolated
thunderstorms this evening, mainly towards the coast.

Another short wave will push onshore Tuesday night. It looks like
the heavier shower activity will be south of the area. The upper
trough axis will shift inland Wednesday and Thursday with drier
northerly flow aloft developing and a decreasing shower threat.
Highs the next few days will be several degrees below normal.

Snow levels will be around 5500 feet the new few days and the
highest elevations could get some snow accumulations. Paradise is
the main threat for heavy snow - either tonight or Tuesday night
as short waves bring increased showers - but temperatures look
just a little too warm for snow to really pile up. Schneider

.LONG TERM...An upper ridge over the northeast Pacific will give
dry northerly flow to Western Washington Friday through Monday.
Highs will warm back to near normal. Schneider

&&

.AVIATION...An upper level trough will continue to bring
scattered showers to the area. The air is moist and unstable with
onshore flow. There will be a chance of thunderstorms through this
evening with the best chance near the coast. The mountains and
higher terrain will be obscured much of the time.

KSEA...Scattered showers and clearing periods with a southerly
breeze.

&&

.MARINE...Low pressure will remain over the region through
midweek. A trough will move through the area tonight and another
trough will reach the area Tuesday night and Wednesday morning--but
the strongest pressure gradient will be down on the Oregon coast.
The winds over Western Washington will be onshore tonight, light and
variable Tuesday and then light northerly on Wednesday as low
pressure passes to the south.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Tuesday 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 until 11 PM PDT this evening 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.

&&

$$

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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