590
FXUS66 KSEW 122128
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
228 PM PDT Sat Sep 12 2026

.SYNOPSIS...
A low pressure system will traverse western Washington tonight
into Sunday, bringing rain and chances for isolated
thunderstorms this afternoon. Drier conditions return Monday
with temperatures gradually warming back up throughout the course of
the week.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/...
Satellite shows a low pressure system centered off the Oregon
coast this afternoon beginning to work its way eastward.
Scattered showers are occuring along the coast. Chances for
showers will begin to work their way inland this afternoon, as
well as a 10-20% chance of some isolated thunderstorms, mainly
along the coast, Olympics, and portions of southwestern
Washington west of I-5. A few lightning strikes have been
observed over coastal waters, and soundings do show some
marginal elevated instability.

The main bulk of the rain will move across the area overnight
into Sunday morning. Areas north of the Strait of Juan de Fuca
will likely remain mostly dry. The areas expected to see the
most rainfall will be across the south Cascades where localized
amounts of up to 1.5-2.0" of rain will be possible. There is
around a 40-60% chance of seeing 2" of rain over the Cascades of
King County and southward. Even portions of the lowlands may see
significant rain, with a 40-80% chance of seeing 1" of rain
from Seattle southward and east of I-5.

Rain will slowly taper off from west to east, first along the
coast in the morning then through the lowlands by the
afternoon. Winds will also increase Sunday as the system
departs, but remain mostly benign with peak gusts in the 20-25
mph range.

High pressure will begin to build in the wake of the system on
Monday, with the supporting upper level trough well to the east
and northwesterly flow aloft.

Temperatures through Monday will remain below normal, with highs
in the mid to upper 60s and lows in upper 40s to mid 50s.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
A shortwave trough will move through the flow Monday night into
Tuesday, bringing a reinforcing push of clouds, through little
to no precipitation is expected with this system. Following this
system, the upper level ridge building over the northeast
Pacific will move over the Pacific Northwest through the end of
the week, bringing a drier, sunnier, and warmer pattern. A
surface thermal trough will develop, allowing winds to turn
lightly offshore. Highs look to rise by Thursday into the upper
70s to low 80s.

62

&&

.AVIATION...
An upper level low will shift inland tonight with
increasing clouds, rain and MVFR to IFR conditions expected. The
flow aloft is southerly, becoming northerly by 12z Sunday. Rain
will taper off west to east Sunday afternoon but the low level
air mass will remain moist with low level clouds lingering. 33

KSEA...Increasing clouds, MVFR conditions and rain by 06-09z.
Rain decreasing after 21z but low clouds and MVFR conditions
linger through 00z Monday. NW winds backing to SW and increasing
to 10 kt by 12z Sunday. 33

&&

.MARINE...
Surface low pressure will shift into OR tonight with
highest winds in western Washington through the Strait of Juan
de Fuca - a Small Craft Advisory is in effect. Onshore flow will
increase on Sunday as the low moves farther inland. SCA winds
are possible over Puget Sound/Hood Canal and the Northern Inland
Waters (as well as the strait). Onshore flow will ease moving
into Monday. The flow will turn offshore Tuesday and Wednesday
as a thermal trough forms along the coast. 33

&&

.FIRE WEATHER...
Fire weather concerns remain minimal through the next week.
Showers and a chance for isolated thunderstorms this evening will
be followed by wetting rains across most of the southern half
of the region overnight into Sunday. The pattern turns drier and
warmer mid to late week, with the surface winds going light
onshore. Daytime minimum relative humidity look to drop below
30% which will allow more reactive fuels to start to dry out.

62

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 6 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$