642
FXUS66 KSEW 050339
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
839 PM PDT Thu Sep 4 2025

.SYNOPSIS...An upper level ridge will continue to keep conditions
warm and dry through tomorrow. Hazy conditions will continue with
the present pattern through at least this weekend due to smoke
being put out by fires in the region. The ridge will shift further
inland Friday night, which will give way to a couple disturbances
from an upper level low this weekend into next week. Besides
cooler temperatures, there is a chance of showers and a slight
chance of thunderstorms with these disturbances, with the thunder
threat including the entire coverage area on Saturday.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...The Red Flag Warning for
the west slopes of the Cascades will expire at 9 PM this evening
as planned. Still lots of wildfire smoke over the region with
Moderate air quality in the Seattle metro and Whatcom County. 33

Previous discussion...Current synoptic pattern depicts an upper
level ridge over the Pacific Northwest region, with a couple of
lows on both sides of the ridge helping to amplify it somewhat. A
surface trough also sits to the east in eastern WA. Satellite
shows some high clouds and quite a bit of smoke aloft due to fires
in the area, giving the sky a hazy appearance. Only areas near
fires will see the smoke/haze mix down to near the surface during
the day. A red flag warning continues for the Cascades through
tonight due to hot, dry and unstable conditions (see fire
discussion below). Temperatures remain on the climb this
afternoon, and despite some of the haze, there`s already been some
areas reaching the 80s (particularly the Cascade Foothills).
Otherwise, due to increasing smoke aloft, it may be difficult for
additional interior areas to break 80 today (except for southern
interior and Puget Sound areas).

On Friday the upper level ridge will move inland, and an upper
level low will dig southward off the coast of the Pacific. It will
remain offshore through early next week, with a few shortwave
troughs swinging around the low affecting much of the northwestern
CONUS. The first disturbance Friday evening will bring a 20%
chance of thunder in the south Cascades, and a 20-30% chance of
showers extending into the lowlands.

Saturday and Sunday will increased chances of showers across the
region with up to a 50-60% of PoPs. There is also a more
widespread area included in the threat of thunder on Saturday,
with the lowlands and Olympics/coast seeing a 20% chance of
thunder, and the Cascades seeing a 30-35% chance. The threat
Sunday of thunder is more confined to the Cascades (30%), and the
north interior and Olympics (15-20% chance). Saturday will have
the most instability across the region with mean NBM CAPE up to
1500 J/kg in the mountains, and lapse rates up to 7.5 deg/km, so
will need to watch for the potential of a couple stronger storms
producing localized small hail, gusty winds, and downpours
(possibly affecting burn scars, though it will not be possible to
predict exact areas affected this far out).

Temperatures will drop considerably this weekend, with highs only
reaching the mid/upper 70s (Saturday), and low 70s (Sunday). Lows
however will stay in the upper 50s to low 60s (due to cloud
coverage). Winds will remain light, and switch from northwest
tonight/Friday, to southwest this weekend. Dew points will still
remain slightly muggy (upper 50s to low 60s in the
interior/coast).

.LONG TERM /MONDAY THROUGH THURSDAY/...Ensembles are in good
agreement with the low moving inland early next week. Monday will
see the last chance of thunderstorms into next week (Cascades and
most of the interior lowlands), with showers lingering regionwide
through midweek. Highs will remain cool with temperatures in the
60s to low 70s, and lows cooling into the 50s.

HPR

&&

.AVIATION...Southerly flow aloft will continue an upper ridge axis
east of the Cascades gradually weakens and an upper trough remains
offshore. The low level flow is weakly onshore. Shallow marine
stratus producing areas of LIFR/IFR ceilings and reduced surface
visibility will spread partially inland once again on Friday
morning.

KSEA...Hazy conditions and variable amounts of high and mid clouds
will continue tonight. A shallow marine layer producing LIFR
ceilings is expected to reach the terminal for a short period once
again on Friday morning. Surface winds west to northwest 4 to 7
knots this afternoon becoming light and variable again tonight.

27/McMillian

&&

.MARINE...A weak surface ridge will remain over the coastal and
offshore waters into Friday with lower pressure over the interior
producing light onshore flow. A weak trough will gradually move into
the coastal waters over the weekend into early next week, but will
produce little in the way of impacts.  27

&&

.FIRE WEATHER...Warm, dry, and unstable conditions continuing
into Friday. The very dry conditions and increasing instability in
the afternoon hours today will keep the Red Flag Warning for fire
weather zones 658 and 659 in effect through tonight but the RH
improvements for Friday will lower the fire weather conditions
from critical to elevated Friday. Shallow marine layer in the late
night and morning will continue to give excellent RH recoveries
in the lowlands. Even with more onshore flow, the layer will not
be deep enough tonight to improve RH recoveries in the mid-slopes
and ridges in the Cascades and Olympics. Cooler conditions over
the weekend with a deeper marine layer will bring about an end to
the threat. There is even a chance of showers over the weekend but
also with possible widespread thunderstorms (20-35+%) in the
afternoons and evenings. there is a slight chance for
thunderstorms as early as Friday afternoon for the very southern
end of the west slopes of the WA Cascades. Confidence remains low
with exact coverage and intensity of the convective activity,
though fire weather conditions will remain elevated through the
weekend due to the threat of lightning. Felton/JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 9 PM PDT this evening for West Slopes of
     the Central Cascades Generally above 1500 Feet-West Slopes
     of the North Cascades Generally above 1500 Feet.

PZ...None.
&&

$$