478
FXUS66 KSEW 012147
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
247 PM PDT Mon Sep 1 2025

.SYNOPSIS...Upper level ridging will build back into western
Washington this afternoon and will continue through much of this
week. A warming and drying trend will continue through at least
midweek, with daytime temperatures well above normal. Some
slight cooling and potentially more active weather is may be
possible by the end of the week.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Latest satellite imagery
shows the widespread marine stratus from this morning slowly
starting to burn back. Stratus will continue to dissipate leading
way to generally sunny skies later this evening. Along with sunny
skies, likely will see some haze from existing wildfires across
the area. Smoke will mostly be aloft, limiting any air quality
concerns for the time being. High temperatures this afternoon will
remain in the mid to upper 70s across much of the area.

Upper level ridging will start to build more influence through
the area on Tuesday, with temperatures warming into the upper 70s
into the lower 80s throughout much of the interior. Areas in the
Cascade valleys will warm even further.. with temperatures in the
upper 80s into the mid 90s expected, especially areas like
Packwood, Marblemount, and Darrington. Moderate (Orange) HeatRisk
are expected throughout much of area, excluding the coast, where
Yellow (Minor) HeatRisk values will exist. Warm and dry conditions
will further into Wednesday, with temperatures warming into the
mid 80s for interior areas and mid 90s for the valleys. The
Cascade valleys will not get much relief as slight offshore flow
keeps those areas warm and dry, with localized pockets of Major
(Red) HeatRisk. Therefore, a Heat Advisory has been issued for the
Cascade Valleys until Thursday evening for prolonged warm and dry
conditions. Again, the emphasis is on the Cascade valleys for the
hottest temperatures.

An upper low offshore will start to influence western Washington
slightly, with temperatures being a touch cooler across the area
on Thursday with the arrival of onshore flow. Some of this onshore
flow may not reach the Cascades, so temperatures in those areas
will still remain warm (in the upper 80s.)

.LONG TERM /FRIDAY THROUGH MONDAY/...Ensembles are continuing to
struggle with the path of the aforementioned low. For now, it
looks like troughiness and this low will have influence over the
pattern, resulting in a gradual cool down into the weekend. Some
ensembles have introduced some monsoonal moisture coming up into
the region on Friday/Saturday, which could help churn out some
isolated showers and or thunderstorms, particularly over the
mountains, during those time frames. Confidence is low at this
time but have mentioned the slight chance precipitation during
those times. Temperatures look to still be above seasonal normals,
but stay in the mid to upper 70s.

Mazurkiewicz

&&

.AVIATION...South to southeasterly flow aloft will continue into
Tuesday as an upper low slides further offshore and ridging builds
into Western Washington. Stratus continues to slowly clear and lift
this afternoon around Puget Sound, with with MVFR cigs also
lingering along the coast. VFR conditions with high clouds at times
this evening into tonight for interior sites. Stratus will spread
inland again early Tuesday morning into Puget Sound, resulting in
IFR cigs for the interior, with areas of LIFR, especially along the
coast. Stratus will likely clear a bit quicker on Tuesday as an
upper ridge builds into the area, with VFR for Tuesday afternoon. In
addition, haze may increase aloft through tonight due to wildfires
in WA/OR. Light W/SW winds will transition more NW on Tuesday.

KSEA...VFR conditions will continue into early tonight. Stratus will
spread inland towards Puget Sound early Tuesday morning. At this
time there is approximately a 25 to 30% chance of IFR cigs from 11z
to 18z. Cigs will likely clear a bit quicker on Tuesday, with VFR
conditions generally by 20z. Light west winds will transition more
northerly around 02z and remain light N into Tuesday morning. JD

&&

.MARINE...A deep low pressure system will remain stationed offshore
through the week with high pressure building inland. Onshore flow
will continue with this pattern. This will result in westerly pushes
through the Strait of Juan de Fuca. At this time, SCA winds are not
expected through Thursday for the Strait of Juan de Fuca, with a
stronger westerly push possible late Friday. Otherwise northerly
winds will increase along the Coastal Waters late Wednesday into
Thursday, and will need to monitor for SCA wind gusts during this
period. Otherwise, northerly winds will increase each afternoon for
Puget Sound.

Seas of 3 to 5 feet will build to 4 to 6 feet Wednesday and continue
through late week. JD

&&

.FIRE WEATHER...Gradually hotter temperatures and very dry
conditions will continue to develop each of the next several days.
These conditions, coupled with light offshore flow during the day,
will likely result in elevated fire conditions around Wednesday in
the Cascades. These factors will also yield poor to moderate RH
recovery in the mid-slopes and ridgetops as the inversion
strengthens overnight, followed by good mixing in the afternoon
hours. Marine influence may limit how low daytime RHs drop for areas
near Puget Sound and through the I-5 corridor, but conditions remain
elevated to the east in the Cascades. Isolated thunderstorm chances
increase late in the week as monsoonal moisture increases across
parts of the region, though confidence remains low in the specifics.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory from noon Tuesday to 5 PM PDT Thursday for
     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.

PZ...None.
&&

$$