Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 221326
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Thu Jun 22 2017

.SYNOPSIS...A building upper ridge will provide dry weather and a
warming trend through the weekend. The ridge will shift inland early
next week with onshore flow developing. Temperatures will start to
moderate Monday with a return to more seasonable temperatures
Tuesday and Wednesday. Dry weather will persist but night and
morning clouds will return next week.

&&

.SHORT TERM...Fog/stratus satellite imagery shows clear skies across
Western Washington this morning with the exception of some patchy
stratus along the north coast. High pressure will continue building
over the eastern Pacific the next few days providing dry weather
over our region. Light onshore flow will become northerly on Friday
with gradually warmer temperatures each day. Sea-breeze effects may
hinder potential highs for locations on the water but temperatures
inland from the water could reach the low to mid 80s on Friday.

The ridge migrates eastward directly over the Pacific Northwest by
Saturday. Models remain in good agreement that 500 mb heights reach
near 5880-5900m with the thermal heat low developing north along the
Oregon/Washington coast. Low level offshore flow will develop,
allowing the air mass to warm well into the upper 80s and lower 90s
over most interior areas except cooler along the Strait and north
interior. Typically, the warmest day near the coast will occur
before inland temperatures peak. Forecast temperatures are well
above MOS at the coast with the hottest day likely on Saturday for
places like Forks and Aberdeen. Temperatures across the southwest
interior will be similarly warm, close to 90.

.LONG TERM...Models agree that the upper ridge axis will remain over
Western Washington through Sunday, then begin to flatten and shift
slowly inland on Monday. Thermally induced surface low pressure
along the coast will shift inland over the interior lowlands Sunday
afternoon. Offshore flow will become light in the afternoon with a
lack of sea-breeze effects producing the hottest day of the year
around greater Puget Sound. GFS/ECMWF MOS give 95/92 for Sea-Tac,
either of which would break the record of 88 set in 2006. Given this
is day 4, decided to keep temperatures little changed which mirror
the more conservative ECMWF guidance.

A transition to onshore flow begins Sunday evening into Monday. The
switch looks to occur at the coast first Sunday night, then
gradually develop inland sometime on Monday. Both the GFS/ECMWF
appear to show onshore winds reaching Puget Sound in the afternoon
which would halt afternoon warming from reaching much above 80.
Stayed close to MOS which gives low 80s but did indicate some upper
80s east of Puget Sound near the foothills where onshore flow may
arrive a bit later.

Stronger onshore flow should flood marine air and stratus into
Western Washington Monday night and Tuesday morning, repeating again
on Wednesday. There should be partial afternoon sunshine each day
but highs will fall back closer to average. The dry pattern appears
likely to continue but will need to watch a trough that will brush
the north part of the area Tuesday and Wednesday. Mercer

&&

.AVIATION...A flat but fairly strong upper ridge axis near 135W
will amplify a bit and move to 130W over the next 24-30 hours.
Moderate northwest flow aloft today, becoming lighter tonight and
Friday. The air mass will be stable, with further drying this
morning. Clear skies will prevail, except for some lingering
marine stratus this morning near the North Coast and near the
Strait.

KSEA...Clear skies for the next 30 hours with prevailing north
wind. Haner

&&

.MARINE...A northeast-southwest oriented surface ridge will remain
about 400 nautical miles northwest of Cape Flattery through
Saturday. Meanwhile, a strong heat low over California and
southern Oregon will set up northerly gradients over the waters.
On Sunday gradients will weaken, then onshore flow will quickly
increase on Sunday night. Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 9 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 rough bar until 6 AM PDT early this
     morning for Grays Harbor Bar.

&&

$$

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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