880
FXUS66 KSEW 180324
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
824 PM PDT Tue Sep 17 2024

.SYNOPSIS...Clouds and rain chances increase across the region
tonight ahead of an approaching frontal system. Lingering rain
showers into Wednesday, especially in a potential Puget Sound
Convergence Zone, as the upper trough pushes eastward. A break in
the rain and clouds for Thursday, but this will be brief as
additional rounds of unsettled conditions will continue through
the weekend and into early next week.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...No significant changes to
the forecast this evening. The previous discussion can be found
below along with updated aviation and marine sections:

A rather cool and cloudy evening is underway across Western
Washington. NWS radar depicts increasing rain offshore and
extending eastward into the Olympic Peninsula already. This will
continue to advance eastward through the night, with the highest
rainfall accumulations (around half an inch) expected west of
Puget Sound and in the Cascades. The bulk of the actions pushes
through the region overnight, but as the upper trough pushes south
and east, expect strengthening onshore flow to bring some light
drizzle or rain through the morning. Meanwhile, some lingering
showers are likely to develop in a Puget Sound Convergence Zone
early Wednesday. Expect a lull in the clouds and rain showers late
Wednesday through Thursday with weak high pressure somewhat
building over the region. The next disturbance is likely to arrive
(or at least approach) by Friday, bringing another increase in
cloud cover and cooler temperatures.

.LONG TERM /SATURDAY THROUGH TUESDAY/...The weekend will continue
with a decidedly fall-like pattern as zonal flow aloft sets up
and keeps the door open for a series of disturbances through the
first half of next week. This will maintain abundant cloud cover
and rain at times, while temperatures hold below seasonal normals.
Will need to monitor the evolution of these features as subtle
shifts in the storms can yield significant shifts in where the
moisture sets up and the resulting impacts in clouds and
temperatures. This is reflected in the probabilistic NBM guidance
exhibiting some quite large spreads in values as we get into the
early part of next week. 12

&&

.AVIATION...Light flow aloft will become northerly as an upper low
continues to slide southward along the coast and out of the area.
Widespread MVFR cigs this evening generally King CO southward, with
areas of IFR cigs along the coast. Rain continues to slowly
dissipate along the Olympic Peninsula, with patchy drizzle into
southern Puget Sound. A few showers or areas of drizzle may linger
Wednesday morning, but mostly dry conditions expected. Cigs will
lower further again tonight into Wednesday morning, with MVFR/IFR
for the interior, and LIFR/IFR along the coast. Cigs will slowly
improve to SCT/BKN VFR cigs late Wednesday afternoon into Wednesday
evening, although lower cigs may continue along the coast. Light
winds tonight will increase from N/NW Wednesday afternoon.

KSEA...MVFR cigs this evening with periods of drizzle. Cigs will
remain MVFR through the majority of tonight, mainly ranging 1000 to
2000 feet. IFR cigs are then expected Wednesday morning (mostly
between 12 to 18z), before slow improvement to MVFR late Wednesday
morning, and SCT/BKN VFR by late Wednesday afternoon. Light winds
tonight will increase to N/NW between 4 to 7 kts for Wednesday
afternoon. JD

&&

.MARINE...A weak surface low passing to the southwest this afternoon
and evening will result in gusty easterly winds over the coastal
waters into this evening before easing overnight. Offshore surface
high pressure returns Thursday into Friday helping boost
northwesterlies and possible small craft advisory levels with
combined seas rising to 7-9 feet late in the week, particularly
across the outer coastal waters.

&&

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

$$