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000
FXUS66 KSEW 231049
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PDT Fri Jun 23 2017

.SYNOPSIS...An upper ridge will bring dry weather with a warming
trend through the weekend. High pressure aloft will shift inland
on Monday with onshore flow developing. Temperatures will moderate
closer to average with highs mainly in the upper 60s to to low 70s.
Dry weather should continue next week but with areas of low clouds
giving way to afternoon sunshine over the interior.

&&

.SHORT TERM...Skies are clear across the area this morning with dry
low level northerly flow and an upper ridge building offshore. The
ridge will strengthen and shift eastward over the Pacific Northwest
this weekend. 500 mb heights build close to 5900M resulting in
compressional heating and increasingly offshore flow. Dry weather
will prevail.

Gradients today will become more northerly which might hinder high
temperatures slightly at locations right along the water including
Seattle. Areas further inland east of the Sound and also
across the southwest interior will see the biggest jump in
temperatures, but all areas will be much warmer. Highs in the low
80s will be common across Western Washington, except reaching
the upper 80s or near 90 over the warmest interior areas.

The hottest day at the coast is expected Saturday, including places
like forks and Hoquiam as offshore winds develop.  Areas around the
southwest interior and greater Puget Sound to the south of Everett
will also reach near 90 as the flow become more offshore. Models
indicate the thermally induced low will shift inland over Puget
Sound on Sunday with easterly cross Cascade gradients peaking. This
should bring the hottest temperatures of the year to the I-5 metro
areas including Seattle and Bellevue. ECMWF and GFS MOS give a
max temperature range from the low to upper 90s. Decided to boost the
forecast high for Sea-Tac to 94 well above the record of 88 set in
2006. With a couple days expected to reach near or above 90 in the
metro area fairly early in the season, decided to issue a heat
advisory for the Seattle-Tacoma-Bellevue metro areas southward to
the southwest interior and east toward the foothills for midday
Saturday through Sunday evening.

.LONG TERM...A transition to onshore flow still looks to occur on
Monday. The latest 00z model runs still show a weak push Sunday
night which may bring some low clouds to the coast partially into
the lower Chehalis Gap. It looks like a more gradual push scenario
with any low clouds clearing quickly inland and still abundant
afternoon sunshine but with gradually increasing onshore flow. Upper
heights will also be falling. Foothills may hold out with one more
hot day in the mid to upper 80s but current timing of the push would
halt temperatures from warming much above 80 by early to mid
afternoon over most the area.

A weak trough brushes mainly to the north of the area Tuesday.
Models are mostly dry although a stray shower could occur over the
northern Cascades. The odds of anything measurable are very low and
will keep the forecast dry. Lower heights and stronger onshore flow
will bring much cooler temperatures. Highs will be more seasonable
in the upper 60s to low 70s through Thursday. Clouds will be
stubborn along the coast which will also intrude inland during the
night and morning hours. However, partly to mostly sunny skies are
expected inland during the afternoon and early evening hours.

&&

.AVIATION...A strong upper ridge axis will slowly move from its
current position near 135W to east of 130W on Friday night.
Moderate northwest flow aloft will weaken a bit later today. Weak
low-level offshore flow is expected on Saturday morning. The air
mass will be dry and stable, with clear skies prevailing.

KSEA...Clear skies and north flow for the next 30 hours. Haner

&&

.MARINE...A surface ridge will remain over the northeast Pacific
through Saturday. Meanwhile, a strong heat low over California and
southern Oregon will maintain northerly gradients over the
waters. Winds over the coastal waters are following a diurnal
cycle, with northerlies peaking during the late afternoon and
evening, then falling off during the morning. This cycle will
repeat itself today and Saturday. Gradients will weaken on
Sunday morning. Onshore flow will develop Sunday evening, then
become strong on Monday.  Haner

&&

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

PZ...None.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html