FXUS66 KSEW 110333
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
833 PM PDT Tue Sep 10 2024

.UPDATE...Showers are moving into Puget Sound this evening with
activity continuing through tonight. An isolated thunderstorm or
two is still possible later tonight into Wednesday morning,
especially for southern areas and along the coast, but the threat
is generally limited. No major forecast updates this evening.

&&

.SYNOPSIS...A more fall-like pattern is setting up for the rest of
the week as a series of low pressure systems move through the
area. A passing disturbance brings widespread rain tonight through
Wednesday, with the potential for an isolated thunderstorm as it
moves eastward through Wednesday. Expect cooler temperatures and
abundant cloud cover to continue through much of the week ahead
with occasional shower chances. A stronger front may arrive by
late Monday or Tuesday.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Clearing skies this
afternoon with temperatures around 70 through much of the interior
at this hour. Meanwhile, increasing clouds are already moving onto
the coast and across the Olympic Peninsula ahead of the next
approaching frontal system. Expect this to bring some leading
light rain to the coast by sunset tonight and into the Puget Sound
region later overnight. Rainfall totals will generally be around a
half inch for the coastal areas and mountains, with closer to a
quarter inch for the remainder of the lowlands from around Seattle
southward. With the cooler air mass moving in, expect that the
upper elevations of Mount Rainier (but not expected down at
Sunrise or lower elevations). Temperatures will be quite cool as a
result of the rain and clouds, with temperatures Wednesday only
topping out in the 60s for many locations. As the low tracks
inland, expect to see steepening lapse rates that could support a
few isolated thunderstorms. Any storms that develop are likely to
pulse up and down quickly with gusty winds and lightning the
primary hazards, and the chances will be highest near the coastal
waters and coastline and south of Puget Sound.

A brief break in the showers and less cloud cover is expected
Thursday as the heights rise a bit in between the weather systems.
This will allow temperatures to nudge a couple of degrees warmer,
or closer to 70 degrees for daytime highs. Not much change to the
overall pattern into Friday.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...The region is likely to
remain in a cool and cloudy pattern with a return of precipitation
chances over the weekend and into the start of next week. The
weekend doesn't appear overly wet though, without a strong signal
for a focused round of heavier rain. The next front that
approaches late Monday into Tuesday will then likely bring the
next round of widespread precipitation to the region with the
ensemble guidance supporting another deeper trough moving across
the Pacific Northwest. Cullen

&&

.AVIATION...Southwesterly flow aloft continues over western
Washington as a trough digs into the region. Showers are beginning
to work their way onshore as a surface low pressure system moves in
from the west. Showers will continue to fill in across the region
tonight then decreasing in coverage Wednesday morning. Ceilings
remain VFR across the region this evening, but will lower into MVFR
to IFR at times tonight into Wednesday morning as the front moves
through the area. Visibility will be temporarily reduced in heavier
rain. There is a low chance for some thunderstorms tonight across
the southwest portions of the region, mainly offshore, though
potentially affecting KHQM. As shower activity tapers off, expecting
ceilings to be able to raise to low-end VFR late Wednesday afternoon
into Wednesday evening. Winds will be out of the southwest through
tomorrow morning at 4 to 8 kt, becoming northwest less than 6 kt
late Wednesday afternoon/evening.

KSEA...VFR conditions this evening as showers begin to approach the
terminal. Ceilings are expected to drop to MVFR early Wednesday
morning. Occasional reductions in visibility are possible in heavier
showers. Ceilings then lift to low-end VFR Wednesday afternoon.
Southwest winds 5 to 8 kt Tuesday night/Wednesday morning will
become northwest less than 6 kt.

LH

&&

.MARINE...A low pressure system centered just offshore will move
through the region tonight into Wednesday. Rain showers will
continue across the coastal waters through Wednesday, with a few
isolated thunderstorms possible late tonight into early tomorrow
morning. These may produce gusty winds up to 30 kt within a 2 to 4
hour window (best chance 9 PM to 2 AM over the coastal waters), for
which a Marine Weather Statement has been issued.

Once the trough moves through, high pressure will rebuild offshore
for the second half of the week with weak onshore/northwest flow
over the waters. The next possible disturbance will move through the
region this weekend, but the threat of any impactful winds is low.

Seas tonight will increase to 3 to 5 feet, further increasing to 5
to 7 feet by Wednesday night/Thursday before dropping back to 3 to 5
feet through rest of the week.

HPR/LH

&&

.FIRE WEATHER...An upper low pressure system moving in just south
of the area will spread rain showers into this area this evening
through Wednesday evening. Instability associated with the low
pressure trough, and the diffluence aloft, will generate a slight
chance of a few isolated lightning strikes for the southern half
of western Washington. There is a 50 to 75 percent chance of a
wetting rain for the same area. JBB

&&

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

$$