Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 212159
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
259 PM PDT Wed Jun 21 2017

.SYNOPSIS...High pressure will gradually strengthen offshore and
shift closer to the region through the end of the week resulting
in a warming trend. Hot temperatures are likely for this weekend
as thermally induced low pressure from California shifts north
along the Western Washington coast. Sunday looks to be the hottest
day of the year so far with some locations in the interior
reaching the upper 80s to mid 90s. A return to onshore flow will
bring moderating temperatures Monday or Tuesday.


&&

.SHORT TERM...A flat ridge of high pressure aloft remains
centered along about 135W this afternoon. Global forecast models
are in very good agreement in showing this ridge amplifying
somewhat as it shifts eastward into Western Washington on
Saturday. As the ridge approaches, expect light to moderate
onshore flow to become lighter on Thursday, then more northerly
on Friday as high pressure builds over British Columbia. The
approaching ridge will also result in good drying aloft with
clear skies from Thursday through Saturday. Highs will rise into
the 70s most areas on Thursday, then well into the 70s to lower
80s on Friday. Clear skies and a drying air mass will allow
temperatures to fall into the upper 40s to mid 50s at night.

Saturday will begin a 2 or 3 day hot spell as thermally induced
troughing at the surface develops northward from California along
the Oregon coast. The thermal trough and northeasterly winds at
2000-5000 feet will give adiabatic compression for good warming at
the lower levels. Highs on Saturday will be in the 80s to near 90
with the warmest temperatures along the west slopes of the
Cascades and Olympics. The coastal zones from about Hoquiam
northward (away from the immediate beaches) will typically have
their warmest day when there is the strongest offshore flow (like
on Saturday). Albrecht

.LONG TERM...The 12Z and 18Z models continue in good agreement
through the middle of next week.

Sunday will likely be the hottest day of this brief warm spell.
Temperatures from about Seattle southward will likely get into the
lower to mid 90s. Highs in the 80s are expected elsewhere as
thermal troughing sets up somewhere between the coast and the
interior. The immediate coast will likely be 2-3 degrees cooler on
Sunday than on Saturday. Anticipated high temperatures will likely
exceed daily records in many locations. A special weather
statement was issued for the anticipated hot weather.

Latest model solutions agree now that a weak push will begin on
Sunday night and will evolve into strong onshore flow later Monday
or Monday night. It will likely be much cooler along the coast on
Monday, and the interior, while still quite warm, will likely see
about 5-8 degrees of cooling.

Temperatures will return to near or slightly below normal
beginning Monday night and continuing into the middle of next week
as marine air floods inland ahead of an upper level trough
offshore. Albrecht

&&

.AVIATION...Moderate and dry W flow aloft will slowly become WNW
by 00Z as an offshore upper level ridge begins building over the
region. The air mass continues to dry with areas of afternoon
cumulus SCT030-050 dissipating 00Z-03Z.

Moderate NW flow aloft will prevail on Thursday while the strong
upper level ridge still centered offshore slowly approaches W WA.
Good VFR conditions will continue with just some patchy morning
stratus possible.

KSEA...Good VFR conditions will continue through tonight. The are
expected today. The SCT035 layer should dissipate by 02Z.
Surface winds will remain N-NW 5-10 KT. Kam


&&

.MARINE...A surface ridge centered about 500 NM offshore will
maintain weak to moderate NW onshore flow through Friday. This
will produce SCA NW winds 15-25 KT over the coastal waters through
Thursday. W flow through the strait will also be in the SCA range
through this evening. NW flow will increase through the Strait of
Georgia late this afternoon, spilling over to the waters north of
the San Juan Islands for SCA winds 15-25 kt.

The offshore surface ridge will begin building inland over B.C.
on Friday which will shift the low level flow to a N to NE
offshore direction. This dry offshore flow will increase the
warming over W WA, eventually allowing a thermally induced surface
trough over Oregon on Saturday to expand northward along the WA
coast. With onshore flow weakening and becoming offshore, winds
Friday through Sunday should be weak. A shift back to westerly
onshore flow may occur Sunday night or Monday night. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM 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 for rough bar from 11 PM this evening to 6
     AM PDT Thursday for Grays Harbor Bar.

     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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Thursday for Northern Inland
     Waters Including The San Juan Islands.

&&

$$

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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