Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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815
FXUS66 KSEW 240959
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
259 AM PDT Sat Jun 24 2017

.SYNOPSIS...High pressure aloft and low level offshore flow will
bring record breaking high temperatures to the area this weekend.
The upper level ridge will move east on Sunday with the low level
flow turning onshore late in the day cooling the coast down a few
degrees. Sunday will be the warmest day in the interior. Onshore
flow will strengthen Monday with temperatures cooling back to near
normal with some night and morning clouds. Temperatures will warm
a bit later next week as high pressure aloft rebuilds over the
area.

&&

.SHORT TERM...The heat is on. Satellite imagery shows there is not
a cloud in the sky over Western Washington early this morning.
With the clear skies a wide range of temperatures at 2 am, near 50
in the San Juan Islands to the mid 60s in the Seattle area.

Record breaking high temperatures this weekend. The heat advisory
from the Seattle area southward including the foothills will
remain in effect. Upper level ridge over Western Washington
today. Thermally induced surface trough expanding north along the
Oregon coast today reaching the Washington coast later this
afternoon into this evening. Surface gradients are already
offshore for the most part. Only the cross Cascades gradients are
slightly positive at 09z/2am this morning but they are also
trending toward offshore. Temperatures aloft warming with model
850 mb temperatures ending up near plus 20C by 00z Sunday. Winds
at 850 mb and 925 mb also turning easterly today. End result
record breaking high temperatures along the Coast today with a few
record highs over the interior. High temperatures in the 90s will
be common from about Seattle southward and along the coast. North
of Seattle highs will mostly be in the 80s with some 70s right
near the water.

Little change in the pattern tonight. Lows will be warm in the
Seattle metro area with a record high minimum temperature possible
Sunday morning ( record 62 in 2006 ) as temperatures only drop
into the lower to mid 60s. For the remainder of the area 50s will
be common.

Small changes to the pattern on Sunday. The upper level ridge
over Western Washington shifts into Eastern Washington. The
thermally induced surface trough moves inland during the afternoon
with the low level flow turning onshore along the Coast. High
temperatures will be tricky for the Coast with a couple hour
difference in the transition to onshore flow meaning a 5 to 10
degree difference in the high temperature. With the thermally
induced surface trough still near the Coast in the morning will
only go for 5 to 10 degrees of cooling leaving highs in the 80s.
For the interior Sunday will be the warmest day with model 850 mb
temperatures a couple of degrees warmer versus Saturday. With the
thermally induced trough over the area in the afternoon surface
gradients/winds will be light. Expect up to 5 degrees of warming
versus Saturday`s highs for the interior with the hottest
locations in the Southwest Interior push 100 degrees. Highs
elsewhere will range from the mid 80s to mid 90s. Record highs are
forecast for all of the climate stations in Western Washington on
Sunday. The forecast high of 95 degrees in Seattle is just one
degree short of the all-time warmest June day ( see climate
section below ).

Onshore flow increasing Sunday night and continuing into Monday.
Upper level ridge well to the east. The resulting push of marine
air Sunday night into Monday morning will bring clouds to the
Coast with the low clouds making as far as the Seattle area. The
clouds will burn off over the interior in the afternoon. Even with
the sunshine the transition to low level onshore flow will cool
high temperatures on Monday for the interior as much as 25
degrees with 70s common across the area.

.LONG TERM...Extended models in good agreement with low level
onshore flow continuing through the period. Weak trofiness over
the area into Thursday before 500 mb heights start to increase as
an upper level ridge begins to build offshore. High temperature
near normal into Thursday with highs in the 60s to mid 70s.
Temperatures warming a few degrees on Friday with highs in the
upper 60s to near 80. Felton

&&

.AVIATION...A strong upper level ridge axis is off the coast early
this morning, and it will move east across Western Washington
late today. Moderate northwest flow aloft will become light this
afternoon, and weak low-level northerly offshore flow will prevail.
The air mass will be dry and stable through today, then some mid-
level instability will develop tonight. Clear skies throughout
Western Washington through Saturday night.

KSEA...Clear skies and northerly flow through the next 30 hours.
Haner

&&

.MARINE...A weak thermal pressure trough is located along the
Washington coast this morning. It will remain in place until
Sunday morning, maintaining weak northerly and offshore winds. A
diurnal pickup in northerly wind is expected this afternoon, with
localized winds in excess of 21 knots over the coastal waters this
PM.

A turn to onshore flow will occur on Sunday afternoon. By Monday
evening, onshore flow will be strong, with westerly Gales possible
in the Strait. Moderate onshore flow will then continue for much
of next week. Haner

&&

.CLIMATE...The last time Seattle had back to back 90 degree plus
days in June was June 28th and 29th, 2008. Since records started
at Seattle-Tacoma airport in 1945 there have only been 2 days with
highs 95 degrees or more in June, June 30th 1995 and June 9th,
1955. Both days the high was 96 degrees.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory from noon today to 9 PM PDT Sunday for Bellevue
     and Vicinity-Bremerton and Vicinity-East Puget Sound
     Lowlands-Hood Canal Area-Seattle and Vicinity-Southwest
     Interior-Tacoma Area.

PZ...Small Craft Advisory from 3 PM this afternoon to 10 PM PDT this
     evening 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.

&&

$$

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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