Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 142217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Thu Sep 14 2017

.SYNOPSIS...High pressure and northerly flow will maintain dry
conditions with near or slightly above average temperatures through
Saturday. Rain will return to the area Sunday and continue into
Monday, as a low pressure system moves into the region. A lingering
trough will keep a chance of light showers in the forecast through
mid week.

&&

.SHORT TERM...Northerly flow at all levels is ushering dry air into
Wrn Washington with just a few residual cumulus clouds along the
Cascades. Any fog late tonight and Friday morning should again be
isolated to prone spots in the southwest interior and coast.

The high pressure ridge strengthens on Friday with 500 mb heights
rising to near 5700M. Low level flow will be light with sunny skies
expected. Highs should warm up slightly, reaching the low to mid 70s
over the interior, inland from the water.

A weak short wave without any surface reflection and minimal
moisture will move across on Saturday. No more than a few high
clouds are expected with dry weather continuing. With onshore flow
lacking, and southerly flow aloft developing, Saturday could be
slightly warmer. Boosted the forecast highs to the mid and upper 70s
ahead of an approaching system.

The GFS/ECMWF and other models are in very close agreement with the
next system arriving from the northwest on Sunday. Models indicate a
rather vigorous front bringing rain amounts not seen since June.
Rain will develop at the coast mid to late morning, then quickly
spread inland Sunday afternoon. Clouds and rain will only allow
highs to reach the mid 60s. Forecast rain amounts with this front
look to be roughly a half inch over the lowlands and one inch,
locally higher, in the mountains. Areas north of the Olympics will
likely shadow with lighter rain totals.

.LONG TERM...A broad upper low closed off over the Pacific Northwest
will produce widespread showers Monday. Colder air aloft will arrive
with snow levels falling as low as 5000 to 6000 ft. Some light snow
accumulation could fall in the higher mountains. Depending on
whether we get some sunbreaks, diurnal instability could be just
enough to pop a couple of thunderstorms. At this point, any
lightning looks very isolated with no particular area favored. Will
leave out of the forecast for now but at some point it may be worth
adding an isolated mention.

The low shifts east of Tuesday, but another system arrives in
northwest flow. Rain or showers should develop again Tuesday and
slowly taper off Wednesday. Highs early next week will struggle to
reach the low 60s.

By late Wednesday and Thursday, the GFS and ECMWF differ on the
chance of precipitation. The GFS is slower, showing some lingering
threat of spotty showers into Thursday. A ridge is shown to move the
area by the ECMWF with dry conditions. Left some low chance pops or
slight chance wording in the forecast but it could end up being a
dry day.  Mercer

&&

.AVIATION...Dry northerly flow aloft will continue tonight and
Friday. Weak onshore surface flow will also continue. The air mass
is dry and stable and will remain so through Friday except for
patchy marine stratus along the coast tonight.

KSEA...VFR conditions with northerly winds 9-12 kt, easing after
03Z. CHB

&&

.MARINE...Weak onshore will continue tonight through Friday. Small
craft advisory strength onshore wind is likely for a few hours on
the coast this evening. Elsewhere winds will be less than 20 KT.

On Saturday offshore flow will develop as a weather system
approaches the coast. A strong front will move through the area
Sunday and Monday, bringing at least small craft advisory strength
winds to most waters. Gale force winds are possible on the coast. CHB

&&

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


&&

$$

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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