Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211038
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Wed Jun 21 2017

.SYNOPSIS...High pressure will gradually strengthen offshore and
shift closer to the region through the end of the week. Light
onshore flow will become more northerly by Friday with a warming
trend. Offshore flow and hot temperatures are likely for the weekend
as thermally induced low pressure from California shifts north along
the Western Washington coast. Sunday looks to be the hottest day
with some locations reaching the low 90s. A return to onshore flow
will bring moderating temperatures by Monday or Tuesday.

&&

.SHORT TERM...A ridge of high pressure will gradually build over the
eastern Pacific through the end of the week. Light onshore flow
today and Thursday will become more northerly by Friday as the ridge
migrates toward the region just offshore. The air mass will
gradually dry and warm the next several days. Residual low level
moisture will result in some areas of morning low clouds, mostly
this morning. Any low clouds around by Thursday morning will be
patchy and near the coast. Afternoon sunshine today will dominate by
Thursday and Friday with highs warming from the low 70s today to the
low 80s in warmer interior spots by Friday.

.LONG TERM...Models are in good agreement of shifting the upper
ridge directly over the Pacific Northwest Saturday and Sunday. This
allows the thermally induced thermal low along the northern
California coast to build northward along the Washington coast. The
result will be a period of offshore flow and 500 mb heights close to
5900M. GFS MOS gives 93 for Seattle Sunday which would beat the
record of 88 set in 2006 if it occurs. Forecast highs were boosted
for this weekend across most the area. An upper low over B.C. could
suppress the upper heights a bit over Washington and induce onshore
flow by Monday afternoon allowing for some moderation of high
temperatures. Stronger high pressure and a weaker low to the north
would allow hot offshore flow to continue through Monday, mainly the
interior. With the timing of onshore flow uncertain around the
Monday afternoon or evening time frame, forecast highs reflect only
some moderation. Tuesday looks to be cooler across the entire area
with night and morning low clouds likely. Mercer

&&

AVIATION...Moderate westerly flow aloft will become northwesterly
this afternoon and evening as an upper level ridge begins to build
offshore. Low level flow will remain onshore and northerly through
the interior today. Ceilings over the area a mixed bag as of 06Z,
ranging from clear along the central coast and strait to VFR and
MVFR through the interior. Continue to expect stratus will reform
at most locations 12Z-19Z with widespread MVFR conditions and a
few pockets of IFR ceilings. Ceilings will lift to VFR conditions mid
to late morning and scatter out by mid afternoon as the air mass
dries.


KSEA...VFR conditions will lower over the next few hours with MVFR
ceilings settling in over the terminal around 12z. Expect ceilings
to lift by late morning to VFR levels and scatter around 20Z.
Northeast winds 10-12 kt will ease t0 6-8 kt after daybreak then
become northwesterly around 18z.


MARINE...Onshore gradients have eased this morning with KUIL-KBLI
dropping to +1.8 mb as of 10z. Winds have eased but will pick up
again this afternoon through the strait. Have replaced the gale
warning for the central and east strait with a small craft
advisory through this evening. Winds and seas through the west
entrance will also reach small craft criteria this afternoon so
have included it as well. The persistent northwesterlies over the
coastal waters will reach through Thursday keeping the small craft
going there. All other waters will remain below criteria today.

Onshore flow will weaken on Thursday then becoming northerly on
Friday as high pressure builds into British Columbia. The
persistent northwesterly winds over the coastal and offshore
waters will result in the development of steep short period swells
that will be difficult to distinguish from locally generated wind
waves.

A thermally induced trough of low pressure will begin to develop
northward along the Oregon coast Saturday and the Washington coast
on Sunday. This will help boost northerly flow over the interior
waters Friday into Saturday and weak offshore flow on Sunday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT Thursday for 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.

     Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to midnight PDT tonight 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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