National, Washington Weather Forecast Discussion

149
FXUS66 KSEW 081737
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
1037 AM PDT Mon Sep 8 2025
.SYNOPSIS...An upper level low offshore will push inland over
the region late tonight into Tuesday, maintaining cooler
conditions and chances for showers and thunderstorms at times
through the first half of the week. A brief period of drier
conditions will be possible Thursday into Friday, before another
system brings increasing rain chances to western Washington
over the weekend.
&&
.SHORT TERM /TODAY THROUGH WEDNESDAY/...Unsettled conditions
will continue across western Washington through the first half
of the week as an upper level low offshore gradually moves
inland late tonight into Tuesday and meanders across the Pacific
Northwest.
Temperatures today will remain close to seasonal norms, with highs
along the coast generally expected to be in the upper 60s to near
70 and highs across the interior expected to be in the low to mid
70s. Some showers and isolated thunderstorms will be possible across
the Cascades and areas south of the Sound this afternoon and evening
as moisture moves into the area from the approaching low.
Convection potential looks greater on Tuesday as the low slowly
pushes inland, with latest hi-res guidance currently indicating a
25-35 percent chance of thunderstorm development across the
Cascades. The highest likelihood for development will be across
the North Cascades, with guidance highlighting a few spots with a
35-40 percent chance of development. While uncertainty still
exists in regards to the extent of coverage and intensity expected
with these storms, the main threats will be gusty outflow winds,
small hail, brief heavy downpours, and frequent lightning. Afternoon
high temperatures on Tuesday will be the coolest within the short
term period, with highs generally expected to be in the mid 60s
along the coast and mid 60s to near 70 across the interior.
Cool and cloudy conditions will persist into Wednesday as the low
resides across the Pacific Northwest. Afternoon high temperatures
will be in the mid 60s to low 70s areawide. Showers and isolated
thunderstorms will be possible across the Cascades again during
the afternoon and evening hours, but expect coverage to be less
than that of Tuesday.
.LONG TERM /THURSDAY THROUGH SUNDAY/...The upper low will slowly
continue to push eastward Thursday and Friday while weak high
pressure starts to build in its wake offshore. At this time,
Thursday and Friday look to be the driest days for the lowlands.
Showers will still be possible across the Cascades, however, as
wrap-around moisture from the low continues to move into the
region. Temperatures will be close to seasonal norms.
The drier period looks to remain short-lived, however, as
guidance still hints at a return to more widespread shower
activity by the weekend as another system moves into the area from
the Pacific. At this time, the latest NBM 50th percentile
indicates roughly a quarter of an inch to half an inch of rain
falling across the interior lowlands and around an inch of rain
falling for areas along the coast and in the mountains.
14
&&
.AVIATION...A broad upper trough remains centered offshore with
light southerly flow aloft over the region before shifting
southeasterly as the upper level low moves inland late tonight
or early Tuesday. Light winds for most this morning with
increasing breezes this afternoon as the trough slides inland.
Low and mid level clouds have scattered out of the southern
portions of the area, but will a bit more stubborn with MVFR to
low VFR ceilings persisting through the morning. Some lingering
mid-level clouds but primarily VFR this afternoon. Low (10-15%)
chance of a thunderstorm near the Cascades, but otherwise just a
few showers near the mountains and no impacts at the TAF sites
expected. A return to lower IFR stratus for the coastal
terminals will occur overnight, with low MVFR to IFR ceilings
for portions of the interior Tuesday morning (especially
KOLM/KPWT).
KSEA...Lingering ceilings this morning through late morning,
lifting to VFR around 20z onward. Scattered clouds likely into
the afternoon. Light westerly winds this morning becoming
northwesterly and increasing to 7-9 kt this afternoon and
evening. Expect returning stratus deck overnight/Tuesday morning
with around a 40% chance period of low MVFR to IFR ceilings for
a few hours 12-16z.
&&
.MARINE...Weak surface low pressure over the coastal and offshore
waters will remain in pace through the first half of this week
with minimal impacts to area waters. A stronger surface ridge
rebuilds offshore around mid-week.
Seas 2 to 4 ft through at least Tuesday night, increasing to 4 to 6
ft by Wednesday.
18
&&
.FIRE WEATHER...Thunderstorm chances will continue across the
Cascades through Wednesday as an upper level low slowly moves
across the Pacific Northwest. The day with the highest
probability of storm development will be Tuesday, with guidance
indicating roughly a 25-40 percent chance of development, with
the most likely area being across the North Cascades. The
primary threats from any thunderstorms will be gusty outflow
winds, brief heavy downpours, small hail, and frequent
lightning.
Outside of thunderstorm activity, increased moisture, cloud cover,
and more seasonal temperatures across the area will help to alleviate
significant fire weather impacts through much of the week.
62/14
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&
$$

NWS SEW Office Area Forecast Discussion