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000
FXUS66 KSEW 040346
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
846 PM PDT Tue Oct 3 2023

.UPDATE...No changes to the overall forecast. Please see the
aviation and marine sections below for updates to those forecasts.

&&

.PREV DISCUSSION... /issued 219 PM PDT Tue Oct 3 2023/

&&

.SYNOPSIS...An upper ridge located offshore will build over
Western Washington into Wednesday. Light low level flow will keep
moisture near the surface through Wednesday, however, resulting
in times of drizzle or light showers. Increasing northerly and
offshore flow into Thursday. A warmer, drier pattern Thursday into
the weekend. A more active pattern early next week.

.SHORT TERM /TONIGHT THROUGH FRIDAY/...Mostly cloudy conditions
this afternoon for most of Western Washington with generally clear
skies across the northern Olympic Peninsula and towards the San
Juan Islands. Some clearing into this evening with increasing
clouds tonight into Wednesday morning as a trough moves across
southern Canada, bringing some slight enhancement to precipitation
potential Wednesday morning. Generally light rain and drizzle will
exist during this period. Conditions will become drier later
Wednesday into Wednesday night as upper ridging builds more into
the Pacific Northwest.

The upper ridge will bring more northerly flow on Thursday with
low level offshore flow developing on Friday. Given the abundant
moisture at the surface and clear skies, fog development will be
likely Thursday morning, as well as Friday morning. However,
clearing is expected each afternoon with increasing winds.
Temperatures will also warm Thursday into the mid 60s to low 70s,
and further Friday into the 70s for most areas, and even a few 80s
are possible in the Cascade valleys.

.LONG TERM /SATURDAY THROUGH TUESDAY/...The upper ridge situated
across the Pacific Northwest late week will slowly progress
eastward over the weekend. Saturday will likely be the warmest day
for the interior, with widespread 70s. There is some spread in
guidance, however, with NBM suggesting a 20% chance of KSEA
reaching 80 degrees Saturday afternoon. Onshore flow will then
begin to increase on Sunday as the ridge shifts east, and
troughing developing across the NE Pacific. This will allow for
increasing clouds, as well as some cooling. In addition, ensemble
guidance suggests the potential for increasing precipitation
beginning Sunday afternoon for the Olympic Peninsula. A more
active period of weather is likely beginning late Sunday into
Monday with a frontal system tapping into sub-tropical moisture
for increasing rain and perhaps breezy winds. Troughing will
likely dominate the pattern into Tuesday for cooler weather and
potential showers. JD

.AVIATION...Northwest flow aloft will continue as broad ridging
offshore starts to nudge closer to W WA. A few scattered clouds
remain over the Puget Sound lowlands, while marine stratus is
beginning to make its way onshore. Low MVFR ceilings make their way
into the Puget Sound lowlands around 06z-09z and continue into
Wednesday morning. Conditions will likely deteriorate to IFR as we
head into the morning, with a 40% chance of IFR ceilings through the
interior heading into 15z. With light low level flow, and a more
stable airmass filtering into the region, patchy fog will be
possible tomorrow morning through interior terminals, and can
locally deteriorate conditions to even LIFR, especially around
12z-16z.

KSEA...VFR conditions with a few mid-level clouds present.
West/southwest winds have gone to calm conditions. Low MVFR clouds
return tonight around 10z-12z, with at least a 25 to 35 percent
chance of LIFR ceilings heading into 13z-16z. Winds will remain
lightly out of the south, then will switch to light northerly winds
after 19Z.

Mazurkiewicz/LH

&&

.MARINE...Surface ridging will continue to amplify offshore, with
northwesterlies over the coastal waters. Low clouds and significant
moisture near the surface will likely generate fog, which may become
dense into tomorrow morning. Ensembles indicate around a 40% chance
of visibilities less than 1 NM for the coastal waters north of James
Island and into the West Entrance of the Strait of Juan de Fuca.
Patchy dense fog will remain possible for the aforementioned areas
through tomorrow night. A thermally induced trough will start to
develop near the coastline on Thursday,shifting inland on Friday,
with offshore flow through the interior waters. Small Craft Advisory
easterlies look to be possible Thursday night into Friday for the
Central and Eastern Strait of Juan De Fuca.

Coastal seas around 6 to 8 feet this afternoon will gradually
subside to around 4 to 6 feet through midweek.

Mazurkiewicz/LH

&&

.HYDROLOGY...No river flooding is expected through the next seven
days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$