Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 011123 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
423 AM PDT Thu Oct 1 2020

.SYNOPSIS...Upper level ridge over the area through Friday will
shift east Saturday. Dry upper level trough moving through Sunday
with another upper level ridge building next week.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Satellite imagery shows
upper level ridge over Western Washington with a weak system
riding over the top of the ridge in British Columbia. Low clouds
along the coast with patchy fog inland. Temperatures at 3 am were
mostly in the 50s.

Upper level ridge remaining over the area today with the ridge
axis along the coast. Light surface gradients continuing which
combined with the smoke aloft will make it hard for the stratus
along the coast to dissipate. Fog already forming in a few places
with Shelton and Friday Harbor reporting a quarter of a mile at 3
am. A little more fog forming before sunrise but the fog will be
shallow and dissipate by mid morning. For the remainder of the
area smoke aloft will be hanging around today maintaining hazy
skies. Highs mostly in the lower to mid 70s over the interior
with mid 60s along the coast.

Upper level ridge amplifies a little tonight changing the flow
aloft to southerly over Western Washington. This could pull in
some more smoke aloft in the evening hours into Western
Washington. Little change in the lower levels with light flow
continuing into Friday morning. Stratus along the coast will
continue with patchy fog developing late over the interior. Lows
tonight in the upper 40s and 50s.

Upper level ridge axis shifting inland on Friday. Increasing
onshore flow in the afternoon developing too late to bring any of
the coastal stratus inland and cool daytime high temperatures
significantly. Upper level ridge centered over California with
light flow aloft there should help shut off the flow of upper
level smoke over Western Washington. Highs in the lower to mid 70s
inland with lower to mid 60s along the coast.

Upper level ridge shifting east of the Cascades Friday night into
Saturday with low level onshore flow increasing Friday night. This
will push the coastal stratus deck inland Saturday morning. The
onshore flow will be weak keeping the marine layer shallow enough
to allow for afternoon sunshine over the interior. Lows Saturday
morning in the 50s. Highs Saturday a little cooler over the
interior with mid 60s to lower 70s. Highs along the coast
remaining in the lower to mid 60s.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Extended models in good
agreement with a dry upper level trough moving through the area
early Sunday. This will enhance the marine layer over Western
Washington Sunday morning. The deeper marine layer will mean more
morning clouds over the interior and a later break out into the
sunshine in the afternoon. Sunday will be the coolest day in the
next week with highs mostly in the 60s.

Another upper level ridge building behind the trough Sunday night
into Monday with 500 mb heights climbing back up into the mid 580
dms. Low level flow trying to turn offshore Monday. Upper level
ridge with light offshore flow Tuesday. Models continue to show
good consistency with the ridge beginning to break down Wednesday.
Another round of above normal high temperatures Monday and
Tuesday with mid 60s to mid 70s for the interior and 60s along the
coast. Slightly cooler Wednesday with the ridge weakening. Felton

&&

.AVIATION...A return to onshore flow has led to the advection of a
shallow marine stratus layer this morning, while a weak disturbance
higher in the atmosphere is carrying some mid level clouds across
the northern half of W WA. The marine stratus layer will continue to
advect eastward through daybreak, encompassing most lowland
locations and dragging ceilings down into the LIFR-IFR categories at
all TAF sites. Fog will also accompany portions of the stratus,
which will lead to vsby reductions down around LIFR or below. Will
then see gradual improvement towards the afternoon, with clear skies
south (some high level smoke still possible) and scattered mid level
clouds north. Thursday night/Friday morning will be a repeat of
stratus advection into the interior lowlands through daybreak. Winds
will be light thru the period, calm through much of the morning with
a light northerly component this afternoon around 5 kts (more
westerly at HQM and CLM).

KSEA...VFR conditions giving way to LIFR-IFR ceilings around 12z.
Some fog will develop in the vicinity but confidence too low to
mention in current TAF. Skies clearing this afternoon. Winds calm
overnight, becoming N around 5 kts this afternoon.

&&

.MARINE...Latest surface analysis this morning shows high pressure
over most of the local area. This has allowed for generally calm
winds and the return to light onshore flow. IR satellite imagery
shows marine stratus encompassing the offshore waters and building
eastward into the interior. Expect all waters to be mostly cloudy to
overcast by daybreak this morning, with some fog accompanying the
low clouds. Conditions will gradually improve through the day with
stratus retreating back to the coast and continuing across portions
of the Strait. As skies clear this afternoon, some smoke aloft is
possible but will not pose any marine related impacts.
Tonight/Friday morning will essentially be a repeat of this morning
with marine stratus redeveloping and pushing inland.

Onshore flow will prevail through the weekend and into much of next
week, with varying amounts of morning cloud with clearing during the
afternoon. Evening pushes down the Strait of Juan de Fuca can be
expected, with small craft advisory level winds likely Saturday
night into Sunday. The next frontal system looks to hold off until
the latter portion of next week.

Seas remain tranquil through much of next week, generally between 5
and 7 feet over the offshore waters.

Kovacik

&&

.HYDROLOGY...No flooding is expected on area rivers through the
next 7 days.

&&

.CLIMATE...Record high for Seattle today 89 degrees set in 1987.
This is by far the warmest October day on record in Seattle with
the second warmest day 83 degrees on October 11, 1991. 80 degree
plus days in Seattle in October are rare with only 7 since
records started in 1945 at Sea-Tac. The last 80 degree plus day in
October in Seattle was October 3, 1993 ( 80 degrees ). Felton

&&

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

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion