629
FXUS66 KSEW 181012
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
312 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/...
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...LIFR-IFR cigs are currently stretching along the
coast towards the southwest interior so far this morning. Stratus
will continue to move eastward throughout the morning with at least
MVFR-IFR conditions expected at most inland terminals although IFR-
LIFR conditions are possible for CLM and PWT. Some reductions in vis
will also be possible during this time as well. Cigs expected to
lift with widespread VFR conditions returning by 18-20z. SW winds
this morning transitioning more W-SW this afternoon. An additional
round of marine stratus is likely overnight Tuesday into Wednesday
with more LIFR to low-end MVFR coverage.

KSEA...VFR conditions so far this morning. Another push of marine
stratus is expected to bring IFR-MVFR cigs to the terminal around 11-
12z. Clouds expected to lift and scatter in the afternoon around 19-
20z. Light winds will pick up out of the SW around 3-5 kt. Afternoon
wind speeds expected to increase to 5-10 kt before turning lighter
again overnight. More IFR to low-end MVFR cigs possible again into
Wednesday morning.

41

&&

.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.

&&

$$