789
FXUS66 KSEW 152101
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
201 PM PDT Sun Sep 15 2024

.SYNOPSIS...Upper level trough moving southward tonight with
gradually clearing skies tonight. Monday will be dry as well with
the trough well to the south and an approaching front still north
of Vancouver Island. Front moving through Tuesday night with rain
out ahead of the front spreading over the area Tuesday. Upper
level ridge trying to build in later Wednesday into Thursday but
the ridge is not strong enough to eliminate the possibility of
systems brushing the area into next weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Satellite imagery shows
stratus beginning to break up across the western Washington
interior, while mostly clear skies prevail along the Pacific Coast
and the Strait of Juan de Fuca. In the upper levels, a trough will
push southward into California. A brief period of southerly winds
aloft may promote some limited elevated instability over the
Cascades, which will allow for convective showers to develop this
afternoon into this evening. There is a low (10%) chance for a few
isolated lightning strikes, but should be much fewer in coverage
than what occurred yesterday. Elsewhere, clouds should continue to
break up and scatter this afternoon, trending toward mostly clear
skies tonight and the trough departs. Lows tonight will be in the
mid 40s to low 50s.

Some patchy fog may develop Monday morning with mostly clear
skies, but should evaporate in the morning. Split flow aloft with
a weak ridge trying to nudge into the region will keep Monday
dry. Partly cloudy skies will allow temperatures to warm into the
upper 60s to low 70s. This is still a bit below normal, but it
will be the warmest day of the week. High clouds will filter in
Monday night as the next upper level trough and associated
frontal system approaches the region on Tuesday.

Tuesday will be much cooler with highs in the 60s with widespread
rain through much of the day. Rainfall amounts should remain
light, a few tenths of an inch possible across the interior, with
0.5 to near 1 inch possible along the Pacific Coast and the
windward Olympic mountains. Rain will devolve into showers Tuesday
night into Wednesday before tapering off Wednesday evening.

.LONG TERM /THURSDAY THROUGH SUNDAY/...The evolution of the long-
term forecast remains uncertain as models struggle to converge on
a solution. The trough will dip south in to California on
Wednesday with a ridge centered over the east-central Pacific
trying to build over the area. The ridge moves south, resulting in
more zonal flow by the end of the week. For this weekend, ensemble
cluster analysis shows about a 60% lean towards ridging for the
weekend, with warmer and drier conditions, with 40% of model runs
indicating additional troughs moving through for cooler and wetter
conditions. The mean solution shows temperatures slowly climbing
into the mid to upper 60s and a broad-brushed chance for showers
from Friday into the weekend across western Washington.

LH

&&

.AVIATION...Mainly VFR with areas of MVFR in marine stratus
lingering into the early afternoon. Clouds are favored to scour out
this afternoon with mostly clear skies overnight. A weak marine push
will bring in another round of low stratus and areas of patchy fog
early Monday morning. Winds generally from the north 5-15 kt with
onshore flow.

KSEA...MVFR with continued improvement to VFR this afternoon.
Another marine push early Monday morning may bring in MVFR
ceilings (30% chance) with a low (15%) chance of IFR ceilings
developing at the terminal. Surface winds generally light out of
the north 5-10 kt easing overnight.

15

&&

.MARINE...Generally benign conditions today as high pressure builds
over the coastal waters. North to northwest winds across the waters
today with gusts just below SCA over the far outer coastal waters
and through the Strait of Georgia across the San Juan Islands
northwestward. Building high pressure will generate a westerly push
through the Strait of Juan de Fuca on Monday, though confidence is
not high enough that it will reach SCA criteria. Low pressure will
move southward across area waters on Tuesday with high pressure
rebounding through the remainder of the week.

Seas 5 to 7 feet today through Tuesday, dropping to 4 to 5 feet
Tuesday before rising again around 6 to 8 feet as a trough moves
through mid-week. Seas will return to 4 to 5 feet by the end of the
week as conditions calm.

15

&&

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

$$