389
FXUS66 KSEW 052211
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
311 PM PDT Fri Sep 5 2025

.SYNOPSIS...A low pressure system will remain offshore over the
weekend and will move inland early next week, maintaining chances
for showers and thunderstorms across western Washington. Weak
ridging may allow for brief drying towards the end of next week,
but an active pattern looks to continue heading into next weekend.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...As a high amplitude ridge
slowly shifts east of the Cascades and an upper level trough
deepens offshore, ripples in the southerly flow aloft will bring
several rounds of showers and thunderstorms over the next several
days. Latest radar imagery shows a band of showers slowly moving
north over southwestern Washington as a weak shortwave lifts
across the region. A thick layer of wildfire smoke aloft has
inhibited convection with this round of showers, and will likely
limit high temperatures today to the upper 70s and low 80s across
the lowlands and in the mid 60s along the coast.

Another weak shortwave will move north across western Washington
tonight into Saturday morning, generating another round of showers
mainly near the Puget Sound southward. As the day progresses, a
stronger shortwave will ripple northward in the southerly flow
aloft, bringing another round of showers and thunderstorms
northward in the afternoon and evening hours. Thunderstorm
activity is favored to initiate over the Cascades near Mt.
Rainier, though some strikes cannot be ruled out over the lowlands
Saturday evening as the precipitation fills in over the lowlands
and tracks northward. Increased onshore flow and cloudier skies
combined with thick wildfire smoke will limit temperatures
Saturday to the 60s and 70s.

The upper level low offshore will creep closer to the coast on
Sunday, bringing another shortwave northward with continued
chances for showers and thunderstorms. A cooling trend will
continue under cloudy skies, with highs peaking near the 70 degree
mark across much of the lowlands on Sunday. The trough axis will
shift inland on Monday, bringing more of the same with chances of
additional showers and isolated thunderstorm activity.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Forecast models continue to
show good agreement over cooler and unsettled conditions
continuing into mid-week as the trough stalls overhead on Tuesday.
Forecast confidence decreases into the middle of next week, with
ensembles maintaining a chance of showers through the week while
deterministic models still show a weak ridge building overhead.
This could allow for a day of dry conditions across western
Washington before more incoming precipitation. Temperatures are
favored to remain below normal through the forecast period,
peaking in the 60s to mid 70s for most areas.

15

&&

.AVIATION...An upper ridge east of the Cascades and an upper trough
offshore will continue to produce south to southeast flow aloft over
Western Washington. An upper level disturbance will produce elevated
instability for isolated showers and thunderstorms across southern
portions of the area late today into Saturday morning. Weak low
level onshore flow will pull low stratus and patchy fog inland
across the interior lowlands again Saturday morning.

KSEA...Hazy conditions will persist with increasing high and mid
level clouds. Convective activity is expected to remain south of the
terminal through 12Z tonight. Another period of LIFR/IFR stratus is
likely 12Z-18Z Saturday morning. Surface winds W/NW 4 to 7 knots
this afternoon will shift to S/SW 6 knots or less tonight.

27

&&

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

27

&&

.FIRE WEATHER...Increasing onshore flow will increase the RH
values across the area including the foothills today. There is a
chance of showers over the weekend with a chance (20-35%) of
widespread thunderstorms in the late afternoon and evening
Saturday, mainly focused in the Cascades. There is also a slight
chance for thunderstorms starting as early as later this
afternoon for the very southern end of the west slopes of the
Cascades. The main hazards with these thunderstorms are erratic
winds, brief downpours, and lightning. Confidence continues to
remains low with exact coverage and intensity of the convective
activity. The pattern looks to turn cool and showery through the
middle of next week and will keep the fire danger suppressed.

Felton/Mazurkiewicz

&&

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

$$