666
FXUS66 KSEW 151043
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
343 AM PDT Sun Sep 15 2024

.SYNOPSIS...Upper level trough over the area moving south later
today and tonight. Western Washington between features Monday 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 isn`t strong enough to eliminate the possibility of systems
brushing the area into next weekend.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows
clearing offshore and to the north with cloud cover associated
with an upper level trough still over the area. Doppler radar has
a few showers around mainly near the Hood Canal and the South
Puget Sound. Temperatures at 3 am/10z were in the upper 40s to mid
50s.

Upper level trough over the area this morning will sink south this
afternoon. Southerly flow aloft along the front side of the trough
will keep the threat of showers in the forecast for the Cascades
through the afternoon hours. For the remainder of the area shower
chances coming to an end this morning with some clearing this
afternoon. Air mass still slightly unstable and with some daytime
heating will see some cumulus bubble up but at this point it does
not look like these clouds will develop enough to create showers.
Sunshine this afternoon for the lowlands but the air mass aloft
is still a little cool. This will keep highs below normal, in the
mid 60s.

Upper level trough continue to move south tonight pulling the
cloud cover and shower chances away from the area. With the
clearing skies, light flow in the lower levels and plenty of low
level moisture areas of fog developing especially in the fog prone
areas like the Southwest Interior and interior river valleys. The
clearing skies will also allow temperatures to drop into the 40s
in most locations overnight.

Break in the weather Monday with Western Washington between the
upper level trough well to the south and a cold front moving down
the British Columbia coast. Fog will dissipate by midday with
sunny skies. Monday the warmest day in the next week but highs
still a little below normal, mid 60s to lower 70s.

Increasing clouds late Monday night as the front continues to
move southeast down the British Columbia coast. Possible G3
magnetic storm Monday evening which could produce an aurora over
Western Washington especially the northern portion. The front
will still be offshore Tuesday afternoon with rain out ahead of
the front reaching the coast in the morning, spreading to the
interior in the afternoon. Highs will be on the cool side with the
increasing rain. in the upper 50s to mid 60s.

Front moving through Western Washington Tuesday night with rain
changing to showers.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Confidence in the
extended forecast not very high this morning with the
inconsistent model solutions. Post frontal trough over the area
for most of Wednesday keeping at least a chance of showers in the
forecast. Weak upper level ridge trying to build Wednesday night
and Thursday but the building ridge is being suppressed by a
trough to the north. This is a change from previous model runs
where the trough was much weaker. Shortwave moving through
Thursday night or Friday. Previous runs had a much stronger ridge
building next weekend with the low level flow turning offshore.
This scenario has been pushed back into Sunday night and Monday
with another upper level trough moving through Western Washington
Saturday. Given the lack of consistency in the model runs the
forecast for Thursday night through Saturday is a broad brush
mostly cloudy/partly sunny slight chance of showers. High
temperatures remaining below normal through the period. Felton

&&

.AVIATION...Southwest flow aloft now then more variable by the
evening as an upper-low centers overhead. Low stratus becoming
widespread this morning as another round of MVFR/IFR conditions
develop. Can`t rule out patchy fog (LIFR) as well but should be
limited under the aforementioned stratus deck. Cigs are favored to
improve to VFR by 20-22z as clouds scatter. Light winds this
morning increasing out of the north as the hours progress. Low
stratus and patchy fog again possible early Monday but stratus
should be of lesser coverage.

KSEA...MVFR cigs this morning with a 20% chance of IFR cigs between
13-17z. Cigs will slowly lift and scatter around 20-22z for VFR.
Light easterly winds this morning increasing out of the north this
afternoon between 5-10 kt before again lessening tonight into early
Monday. Low stratus appears possible again Monday morning but should
burn off readily by the afternoon.

McMillian

&&

.MARINE...The previous SCA for the Central/Eastern Strait has been
allowed to expire. Now, benign conditions expected for Sunday with
winds remaining out of the northwest over the coastal waters. Gusts
up to 20 kt are possible in the far outer coastal waters. The next
best chance for headlines exists Monday evening for the
Central/Eastern strait as onshore gradients increase. The forecast
is not a slam dunk as of now but something to monitor going forward.
High pressure over the coastal waters for much of the upcoming week
aside from a weak low entering late Tuesday.


Seas 5 to 7 feet today through Tuesday, dropping to 3 to 5 feet
before rising again around 6 to 8 feet as a disturbance moves
through around midweek.

McMillian

&&

.CLIMATE...The chances for an 80 degree plus day in Seattle
decrease significantly after the first two weeks of September.
Out of the 272 80 degree plus days in September at Seattle-Tacoma
airport only 77 ( 28 percent ) of them have occurred after
September 14th. There is a small maximum around the equinox with
September 21st through 23rd accounting for 30 out of the 77 days
( 39 percent ). 80 degree plus days are rare in October with only
9 in 79 years of records at the airport. Felton

&&

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

$$