065
FXUS66 KSEW 181641
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
941 AM PDT Tue Aug 18 2026

.SYNOPSIS...Weak upper level ridge over the area through
Thursday with varying degrees of low level onshore flow. Upper
level low well offshore slowly moving northeast to off the coast
Friday. Weak system spinning out of the low could produce some
showers over the area Friday. Low moving through Saturday
keeping a chance of showers in the forecast for some locations.
Weak upper level ridge with low level onshore flow returns
Sunday into the first part of next week.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...The current forecast
remains on track with no planned updates this morning. 33

Previous discussion...Satellite imagery shows high clouds
moving out of the area. Stratus moving inland as far east as the
Southern Hood Canal to the south and just west of Whidbey
Island to the north. Temperatures at 3 am/10z were in the mid
50s to mid 60s.

Stratus will continue to slowly move inland getting east of
Puget Sound a little after sunrise. The base of the layer is
near 500 feet so higher terrain like around Paine Field could
get some visibility restrictions for a few hours this morning.
The layer is shallow and will retreat back to the coastline by
midday/early afternoon. Highs in the 70s and lower 80s inland,
mid and upper 60s coast.

Little change in the pattern tonight through Thursday with weak
upper level ridging aloft and low level onshore flow. Marine
layer moving inland tonight into Wednesday morning getting east
of Puget Sound after sunrise. The layer will be a little
shallower Thursday morning with a good chance it doesn`t make
it east of the Sound. If it does it will not last long. Highs
Wednesday similar to today, 70s and lower 80s inland and mid and
upper 60s coast. Highs a couple of degrees warmer Thursday with
mid 70s to mid 80s inland and mid 60s to lower 70s coast. Lows
through the period in the 50s. Felton

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...Not a lot of
confidence in the extended forecast for Friday and Saturday.
There is good agreement that a weak upper level low will move
into the offshore waters Friday then kick out to the northeast
into Southern British Columbia and weaken Friday night into
Saturday. There is not good agreement on how much shower
activity will occur over the area while the low is in the
vicinity. Flow aloft becoming southerly Friday into Friday night
which brings up the possibility of convection over the Cascades
and Olympics Friday afternoon and evening. Ensembles aren`t
much help with a wide variety of solutions for Friday night and
Saturday for the lowlands. Current blend solution has the shower
activity confined to the coast and the northern half of the
area. Given the wide variety of ensemble solutions look for this
forecast to change in future forecast packages.

Models in better agreement Sunday and Monday with a return to
the pattern we are currently in with weak upper level ridging
aloft and low level onshore flow giving the area normal latter
half of August weather.

The record high for Seattle today is 88 degrees. This is the
first record high below 90 degrees since June 21st. The chances
for a 90 degree plus day in Seattle drop significantly beginning
today. There have been 91 90 degree plus days in August in
Seattle since records started at Seattle Tacoma airport. Only
15 have occurred August 18th or later. There have been 19 days
in September with highs in the 90s. The total of 34 90 degree
plus days August 18th or later is only 12% of 90 degree plus
days on record ( 34 out of 290 ). Almost half ( 15 out of 34 )
of the late season 90 degree days have occurred in the last 10
years. Felton

&&

.AVIATION...IFR/LIFR cigs and reduced visibility continue
across the coast and Puget Sound terminals. Slow improvement
expected this morning, with VFR conditions expected for interior
terminals by the afternoon. As for the coast, minimal
improvement this afternoon to IFR. SW this morning will
transition to W/SW this afternoon. An additional round of marine
stratus is likely overnight Tuesday into Wednesday morning,
with more LIFR to low-end MVFR coverage.

KSEA...LIFR cigs with reduced visibility this morning. Already
seeing some improvement in visibility and ceilings, and will
continue to trend this way into the afternoon. Ceilings will scatter
out this afternoon and VFR conditions will prevail into tonight.
IFR/LIFR cigs possible Wednesday morning with another round of
marine stratus. SW winds today 4-6 kt.

29

&&

.MARINE...High pressure will remain over the far offshore
waters and slowly build into area waters through the middle of
the week, which will maintain northwesterly winds over the
coastal waters. A weak system will approach area waters today,
with little to no impact to the waters. Northwesterly winds will
increase into Tuesday over the outermost waters, which may
develop steep seas around 6-7 ft with a dominant period of 7
seconds. The threat for steep seas may continue through mid-
week. High pressure will weaken late in the week as a frontal
system passes over the coastal waters. Weak westerly pushes down
the Strait of Juan de Fuca will continue this week. The next
round of westerlies that may exceed 21 kt is Friday, with the
latest probabilities around 60-80%.

&&

.FIRE WEATHER...The current weather pattern with low level
onshore flow creating good humidity recoveries overnight into
the early morning hours will keep fire weather concerns at a
minimum through Thursday.

Pattern change Friday will elevate fire weather concerns with
the potential for convection in the Cascades and to a lesser
extent the Olympics Friday and Saturday. While precipitation
amounts may bring localized wetting rains, any storms that
develop will also be capable of generating gusty and erratic
outflow winds and abundant lightning.

Fire weather concerns easing Sunday into the first part of next
week with the return of weak upper level ridging aloft and low
level onshore flow. Felton

&&

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

&&

$$