679
FXUS66 KSEW 021016
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PDT Tue Sep 2 2025

.SYNOPSIS...Upper level ridging will remain over western
Washington through much of this week, resulting in a warming and
drying trend through at least midweek. Daytime temperatures during
this time frame will be well above normal. Some slight cooling and
potentially more active weather may be possible by the end of the
week.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Latest satellite imagery
shows almost the reverse of 24 hours ago, with marine clouds
effectively making down the Strait and impacting northern portions
of the CWA...although not so far north as Skagit and Whatcom
counties...while mid-level clouds are more the story for locations
south of Snohomish county and Forks. Much of the area continues to
see temp/dewpoint spreads of only one or two degrees, so will likely
see stratus fill in many of the areas yet to experience it...it will
just occur an hour or two later than initially forecast, perhaps as
late as just before sunrise.

As stated in the Synopsis, the upper level ridge will be the
dominant weather feature for the entire short term period, resulting
in warming temperatures and drying conditions. The confounding
aspect of the forecast continues to be daytime high temperatures.
NBM continues to advertise highs both today and Wednesday that would
have an impact to HeatRisk, thus warranting the inherited Heat
Advisory. However, observed highs the past couple of days have been
considerably lower than what the NBM has been advertising, shaking
confidence in the aforementioned headline. As such, have undercut
the model to take into account the seeming gap between reality and
NBM output. Will retain headline at this time, as even though with
cooler forecast highs, daytime temps will still be unseasonably warm
and headline does an excellent job of pointing that out. Fire
Weather impacts will be addressed below.

Models remain consistent on showing a shortwave embedded in the
ridge that will bring some relief for Thursday. While not
necessarily reflected by numbers in the forecast, as again there is
some reliance on the likely still too warm NBM, temperatures will
start to trend downward with this feature, if only a little at
first, as temps Thursday will slip just a few degrees from those of
Wednesday, still ranging in the interior from the mid 70s to around
80.

18

.LONG TERM /FRIDAY THROUGH MONDAY/...Consensus on the upper level
low over the Pacific and its track remains lacking, however there is
enough agreement pointing toward the ridge shifting eastward enough
to allow the associated upper level trough to influence W WA
weather. Not only does this mean the cooling trend will continue
throughout the long term, with daytime highs returning to the more
seasonable yet still a touch warmer than average mid 70s. Models are
also advertising some slight chance PoPs, although at this time
these chances are relegated to the terrain of the Cascades. Given
the degree of uncertainty of the actual positioning of the low, it
is difficult to have any degree of confidence in solutions present
within the forecast itself, simply the transition to a slightly more
active weather pattern is enough for now until a proper consensus
can emerge.

18

&&

.AVIATION...Upper level low just off the coast this morning will
move west northwest through Wednesday. Upper level ridge over
Idaho building into Western Washington tonight. The ridge will
remain over the area Wednesday. Southerly flow aloft today
becoming southeast tonight and light Wednesday. Light flow in the
lower levels becoming onshore tonight into Wednesday.

High clouds over Western Washington early this morning obscuring
where the low stratus is at 10z/3 am. Surface observations
reporting IFR ceilings at Port Angeles and ceilings near 1000 feet
at times at KPAE. Along the coast MVFR ceilings in the north with
just scattered clouds below 500 feet over the south coast.

Tough call this morning on stratus coverage. Most model solutions
have IFR/low MVFR ceilings covering most of the area by 12z. High
cloud coverage has slowed down the development of the stratus.
Surface gradients are flat so do not think the stratus will
advect inland but instead form in place with the moist low level
of the air mass ( temperature dew point spreads are two degrees
or less ). High cloud coverage should prevent fog from forming so
will go with IFR ceilings for the most part, just delay the
development into the 13z-15z window with the exception of Port
Angeles and KPAE. Layer not looking too deep making for a
midday/early afternoon breakout with just high clouds by 21z. Weak
onshore gradients developing tonight with IFR ceilings along the
coast beginning around 06z spreading inland 10z-13z.

KSEA...High clouds all day. IFR stratus developing 13z-15z
dissipating 19z-20z. Light northerly winds becoming 4 to 8 knots
after 21z. Felton

&&

.MARINE...Weak high pressure will remain over the coastal waters
through Thursday. The high will weaken Friday and Saturday with a
low pressure system moving into the offshore waters. Lower pressure
will remain east of the Cascades through the weekend.

Light onshore flow developing tonight and continuing into the
weekend. Diurnal westerly pushes through the Strait of Juan de
Fuca each day with the Wednesday night push having the best chance
of reaching small craft advisory speeds. Felton

&&

.FIRE WEATHER...Gradually hotter temperatures and very dry
conditions will continue to develop each of the next couple of days.
These conditions, coupled with light offshore flow during the day,
will likely result in elevated fire conditions around Wednesday in
the Cascades. These factors will also yield poor to moderate RH
recovery in the mid-slopes and ridgetops as the inversion
strengthens overnight, followed by good mixing in the afternoon
hours. All of this in mind, collaboration with surrounding offices
and an eye to the unstable conditions this would create and the
already dire state of fuels, have opted to go with a Fire Weather
Watch for the Cascade zones of 658 and 659 for Dry and Unstable
conditions Wednesday. This will merit further watching though. If,
as stated above, temperatures do not get as warm as the forecast
suggests, this headline may no longer be needed. Continue to monitor
forecasts for the latest temperature and RH expectations. Showers
with possible isolated thunderstorms are forecast late in the week
as monsoonal moisture increases across parts of the region, though
confidence remains low in the specifics.

Cullen/18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for West Slopes of the Central Cascades Generally
     above 1500 Feet-West Slopes of the North Cascades Generally
     above 1500 Feet.

     Heat Advisory from noon today to 5 PM PDT Thursday for Foothills
     and Valleys of Central King County-Foothills and Valleys of
     Pierce and Southern King Counties-Foothills and Valleys of
     Snohomish and Northern King Counties-Foothills and Valleys
     of Thurston and Lewis Counties-Foothills and Valleys of the
     North Cascades.

PZ...None.
&&

$$