Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241609
AFDSEW

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

.SYNOPSIS...High pressure aloft and low level offshore flow will
produce record breaking high temperatures for much of Western
Washington this weekend. The high pressure aloft will move inland
Sunday with low level flow turning onshore late in the day and
cooler air reaching the coast while the interior will have its
warmest day. Onshore flow will strengthen Monday with temperatures
cooling back to near normal with night and morning marine clouds.
The pattern of morning clouds and afternoon sunshine should prevail
next week.

&&

.SHORT TERM...Low level pressure gradients finished the transition
to offshore this morning and from the movie Beverly Hills Cop
soundtrack - the Heat Is On. Seattle-Wenatchee and Seattle-Hoquiam
gradients turned offshore earlier this morning. Thermally induced
lower pressure at the surface is off the coast supporting the low
level offshore flow and substantial warming across the region today.

The upper ridge has moved inside 130W this morning and satellite
imagery shows no clouds under this strong subsidence aloft. The UIL
raob this morning shows a lot of warm air aloft in the lower levels
and if it totally mixes down to the surface, lower to mid 90s along
the coast are in the cards...and in the forecast. Other daily high
temperature records likely to fall today and Sunday. The heat
advisory remains in effect and is on track.

The thermal surface trough is forecast to linger near the coastline
tonight into Sunday morning. This translates into a warm night
tonight with mild lows in the 50s. The Seattle record high minimum
of 62 in 2006 may fall.

The next question is when and how will this hot weather pattern
break. The 12z progs in thus far agree on moving the offshore upper
ridge onshore today and inland Sunday. The thermal trough follows,
moving to the western interior Sunday PM and east of the Cascades
Sunday night. Low level flow should swing onshore for the coast
during the day Sunday for cooler high temperatures while the
interior has its warmest day. Anticipate the low level onshore flow
to then surge inland to the interior Sunday night with nature`s air
conditioning taking hold.

This pattern change will bring marine clouds into much of the
western interior by Monday morning and along with much cooler
daytime temperatures. Some locations may drop 20 degrees or so from
Sunday`s highs. The marine clouds should give way to afternoon
sunshine in many areas though. Buehner

.LONG TERM...from the previous discussion.
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...An upper level ridge will remain over the region for
NW flow aloft. The low level flow will be offshore. VFR conditions
will persist.

KSEA...VFR. Winds will be northerly 6-14 knots, strongest this
afternoon through early this evening.

&&

.MARINE...
A thermally induced trough of low pres along or just off the coast
will result in offshore flow through Sunday morning. The trough will
shift E of the Cascades Sunday afternoon, causing the flow to become
onshore and strengthen.

The combination of strengthening onshore flow Sunday afternoon along
with a sharp contrast between rapidly cooling temperatures on the
coast and much warmer temperatures across the interior will result
in the potential for gale force winds over parts of the Strait of
Juan de Fuca Sunday night.

Onshore flow will strengthen further on Monday for solid gales over
parts of the Strait of Juan de Fuca.

&&

.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. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory until 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 5 PM this afternoon to 10 PM PDT this
     evening for 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.

&&

$$

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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