412
FXUS66 KSEW 151618
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
918 AM PDT Tue Jul 15 2025

.UPDATE...No changes to the overall forecast, very warm
conditions today into Thursday with some moderation Friday.
Please see the aviation and marine sections below for updates to
those forecasts.

&&

.SYNOPSIS...Upper level ridge over the area today will weaken
later Wednesday. Low level offshore flow developing today
continuing into Wednesday. Low level flow turning back onshore
Wednesday afternoon into Thursday as a weak upper level trough
moves by to the northeast. Low level onshore flow pattern
continuing into the first part of next week keeping temperatures
a little below normal with varying degrees of late night and
morning low clouds.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Satellite imagery shows
stratus over the southern portion of Grays Harbor county right
up to the Thurston county border and a few clouds over the
Cascades. Clear skies for the remainder of the area.
Temperatures at 3 am/109z were in the 50s to lower 60s. Open
those windows up and let some cool air inside. It`s going to be
in short supply the next couple of days.

Upper level ridge building over Western Washington today with a
thermally induced surface trough moving up from Oregon in the
afternoon with the low level flow turning offshore. High
temperatures will be about 10 degrees warmer than Monday, in the
mid 70s to mid 80s along the coast and 80s to lower 90s inland.

Low level offshore flow continuing overnight. Cross Cascade
gradient going negative in the evening and remaining negative
into Wednesday morning. The resulting light/easterly surface winds
will keep lows in the 60s in locations exposed to the easterlies
like the Cascade foothills. Locations without any easterly wind
dropping into the upper 50s.

Wednesday will be the warmest day of the next week. Temperatures
aloft peaking with model 850 temps in the plus 17C to plus 20C
range. Thermally induced surface trough over the area in the
morning will move east of the Cascades in the afternoon. This
transition to onshore flow will cool the coast down in the
afternoon but over the interior just create light winds which will
allow high temperatures to warm into the 80s to mid 90s. Highs on
the coast in the 70s will occur in the middle of the day. Not
expecting any records Wednesday. There have been a couple of very
hot July 16ths in Western Washington. On July 16th 1979 Seattle
had a high of 98 degrees tied for the 9th warmest day on record at
Seattle Tacoma airport. There was a hotter July 16th before
Seattle-Tacoma airport was built. In 1941 the high at the Federal
Building in downtown Seattle was 100 degrees. The only time
between 1894 and 1944 when the Federal Building was the official
Seattle weather station that the mercury reached 100 degrees. It
was hot all across Western Washington. Olympia reached 103 degrees
which was the warmest day on record in Olympia until the 104
degree day on August 9th, 1981. The 103 degrees on July 16th, 1941
is still tied for the 5th warmest day in Olympia.

Upper level ridge weakening Wednesday night but the low level
onshore flow is not very strong. Southwesterly surface winds with
a shallow layer of marine air will provide some relief in the
early morning hours with lows dropping into the 50s.

Zonal flow aloft combined with weak low level onshore flow
creating a weak marine push scenario Thursday. Low clouds along
the coast will move inland early but likely remain west of Puget
Sound. Highs 5 to 10 degrees cooler over the interior, in the
upper 70s to mid 80s. With onshore flow all day highs on the coast
will have trouble getting out of the 60s.

No change to the HeatRisk across Western Washington early this
morning with moderate HeatRisk over the interior today and
tomorrow. The heat advisory will go into effect at 10 am this
morning and will remain in effect through 10 pm Wednesday.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...The main theme for the
extended period is a few days of slightly below normal
temperatures after Friday. Models in general agreement with the
low level onshore flow pattern continuing through the period in
varying degrees. Zonal flow aloft giving way to some weak
trofiness Sunday into Monday. This will deepen the marine layer
over the interior potentially making Sunday the coolest day. This
feature could also produce a late day shower over the North
Cascades. Highs in the 70s and lower 80s Friday lowering to the
upper 60s to mid 70s Sunday and Monday. Felton

&&

.AVIATION...Northerly flow aloft as upper level ridging remains
offshore. VFR conditions for all terminals this morning will
continue throughout the TAF period, with the exception of KHQM that
is currently IFR. Latest satellite trend shows the low-level clouds
starting to retreat this morning, and expecting improvement into VFR
in the next 1-2 hours. Winds across the area will increase this
afternoon to 8 to 12 knots, with the potential of a few gusts
reaching 20 kts at times.

KSEA...VFR conditions throughout the TAF period. Northerly winds
this morning 4 to 8 knots will increase after 18z to 9 to 14 knots
with a few gusts potentially reaching 20 kts throughout the
afternoon. Winds look to remain elevated throughout the evening
before gradually decreasing overnight.

Mazurkiewicz/29

.MARINE...High pressure remains offshore this morning. A small
shift this morning will bring offshore winds throughout the area
waters. Onshore flow will return late Wednesday evening with the
potential of Small Craft Advisory winds through the Strait.
Northerlies will increase periodically over the coastal waters
through midweek as a thermal trough builds over the coast - which
may bring SCA conditions at times which will need to be monitored.

Combined seas this morning 7 to 9 feet will lower slowly to 6 to 8
feet by Wednesday and continue to lower to 4 to 6 feet by the
weekend.

Mazurkiewicz

&&

.FIRE WEATHER...High pressure returns to the area today.
Afternoon RH values will lower to critical thresholds with a
thermal trough building north. While some uncertainty remains to
how far north and west the trough builds, expect to see at
least some light east component to the surface winds near the
Cascade gaps in zone 659 and portions of 657 and 658 beginning
this afternoon. A Fire Weather Watch for these areas remains in
effect. Confidence is lower in seeing more widespread easterly
wind, but this may serve to further reduce RHs. Depending on the
placement of the thermal trough, expect to see a deeper mixed
layer and increased instability over the Cascades with mixing
heights potentially rising into Wednesday. Today and Wednesday
will have the worst fire weather conditions for existing
incidents and the potential for any new ones if they were to
begin. Cooling trend beginning Thursday with temperatures
dropping below normal Saturday. Minimum RH values increasing
Thursday into the weekend with the return of low level onshore
flow. Felton

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Fire Weather Watch through Wednesday evening for Black Hills
     and Southwest Interior Lowlands-West Slopes of the
     Central Cascades Generally above 1500 Feet-West Slopes of
     the North Cascades Generally above 1500 Feet.

     Heat Advisory from noon today to 10 PM PDT Wednesday for City
     of Seattle-Downtown Everett / Marysville Area-Eastern
     Kitsap County-Eastside-Foothills and Valleys of Central
     King County-Foothills and Valleys of Pierce and Southern
     King Counties-Foothills and Valleys of Snohomish and
     Northern King Counties-Foothills and Valleys of Thurston
     and Lewis Counties-Foothills and Valleys of the North
     Cascades-Lowlands of Lewis and Southern Thurston Counties-
     Lowlands of Pierce and Southern King Counties-Lowlands of
     Western Skagit and Northwestern Snohomish Counties-
     Lowlands of Western Whatcom County-Middle Chehalis River
     Valley-Northern Hood Canal-Olympia and Southern Puget
     Sound-Shoreline / Lynnwood / South Everett Area-Southern
     Hood Canal.

PZ...None.

&&

$$