205
FXUS66 KSEW 272233
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
333 PM PDT Wed Aug 27 2025

.SYNOPSIS...An upper level disturbance will produce areas of
showers and isolated thunderstorms across the region, with the
best chance of convection over the Cascades tonight through
Thursday. Lightning is the main concern with convection, and red
flag warnings continue for the Cascades and east Olympics. Friday
will be the warmest day of the week with mostly clear skies. A
cool down is in store for the weekend with another chance of
convection.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...A small upper level low
remains present over the WA/OR area, as well as a trough just east
of the Cascades. Quite a bit of high cloud coverage remains over
the region this afternoon, and temperatures have struggled to get
past 70 in some areas (particularly the south interior due to
cloud coverage/convection). It`s clearer to the north, and
temperatures have had no problem reaching the 80s in some of the
valley areas. Uncharacteristically for this region, it`s also a
bit muggy with dew points right around 60. HeatRisk remains minor
for most areas, but some localized moderate areas are present in
areas that are clear (lot of it is driven due to humidity).

Now to the convection: The radar remains quite active with showers
continuing in the south interior/south Cascade areas. While the
flow has been northwesterly for all activity, much of the
precipitation has remained over the elevations. The expectation is
for the convection to primarily focus towards the Cascades tonight
into Thursday, with probability of thunder at 20-30%. There
remains a lower 10-20% chance of convection in the east Olympics
this afternoon/evening, and again Thursday afternoon. Activity in
the south Cascades will shift north of I-90 tonight into Thursday.
Some of the convection may drift down into lowland areas of
Snohomish/Skagit/Whatcom Counties (but the threat of this is
lower). The main concern is lightning (due to dry fuels from
recent warm weather - see fire weather discussion for more
details). However, a couple storms (if they are able to elevate
with the cloud coverage in place), may be able to produce small
hail, heavy downpours, and gusty winds. The threat for flash
flooding from downpours over burn scars is lower, but higher PWATs
from monsoonal moisture will be enough to increase rain output
from storms that develop (though predictability into exact amounts
remains low due to the convective/isolated nature of this
system).

Conditions dry out on Friday with increased sunshine and warmer
conditions as the upper level low/vort max moves out. This will
likely be the warmest day of the week with highs returning into
the upper 80s to low 90s in much of the interior. It will still
likely remain a bit muggy with monsoonal air still in the region.
Moderate (orange) HeatRisk will expand to include much of the
interior. Overnight lows will also remain a bit warm (low to mid
60s). Given the shorter duration/lower confidence in highs (with
some cloud coverage hanging through Thursday and it being a short
heat stretch), heat headlines will likely not be needed.

Another low is expected to pass through on Saturday with a front.
PoPs will be more widespread to include much of western WA
(especially the northern areas). There is a slight (15-25%) chance
of thunder across much of the region (including lowland areas from
King County northwards). Precipitation amounts appear light at
this time with this activity, so lightning will be a concern.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...The low/system will
retrograde back/northward out of the region Sunday (though some
shower/thunder chances remain for the north Cascades and lowland
areas). Uncertainty remains with the forecast going into next
week, but the NBM deterministic favors warmer temperatures, and
dry conditions.

HPR

&&

.AVIATION...VFR conditions today across the interior terminals with
mid to high clouds. IFR/LIFR conditions will persist along the
immediate coast into Thursday. In addition to low ceilings, low
visibility is likely within the stratus. Showers remain possible
today into Thursday. Guidance hints that the marine stratus may
filter inland overnight, with current probabilities of MVFR ceilings
at 20-40%. If the low stratus makes it into the interior terminals,
improvement is expected late in the morning. Otherwise, VFR with mid
to high clouds on Thursday for interior terminals.

Another chance (15-25%) of thunderstorms today for the Cascades and
Olympics through tonight. Thunderstorm activity is expected to
remain confined over the mountains. However, will have to monitor if
any thunderstorm activity drifts into Puget Sound. Surface winds
southwesterly today will become light and variable overnight.

KSEA...VFR conditions will persist today into Thursday. There is
small a chance (around 20%) of MVFR ceilings overnight with the low
stratus pushing inland. Chance of showers will continue into
Thursday, with thunderstorms over the Cascades. Thunderstorm
activity is expected to remain over the mountains but will have to
monitor any storms that drift west towards the terminal. Current
probabilities of thunderstorms at the terminal are less than 10%.
Southwesterly winds this afternoon will become light and variable
after 03z. Northwesterly winds Thursday morning at 4-7 kt.

29

&&

.MARINE...Southwesterly surface winds over the coastal waters will
continue through late in the week. Onshore flow will bring westerly
pushes across the Strait of Juan de Fuca each evening. Today`s push
may bring brief Small Craft Advisory gusts to the Strait. However
with probabilities of winds exceeding 21 kt being 20-40%, opted to
not issue a Small Craft Advisory. Guidance hints at a stronger
westerly push on Saturday.

A Dense Fog Advisory has been issued for the coastal waters and
Grays Harbor Bar through 1100 AM. There is a possibility that the
western portion of the Strait may need to be included in the
advisory tonight due to onshore flow.

Combined seas  3-5 ft today. Seas will continue to decrease
throughout the week to around 2-4 ft.

29

&&

.FIRE WEATHER...Red Flag Warnings for thunderstorms have been
extended for the Cascades and eastern Olympic mountains out until 9
PM Thursday night. Model soundings over the Cascades and Olympics
are suggestive of higher instability (over 500 J/kg of CAPE and very
little cap) as well as enough moisture to support thunderstorm
development due to an influx of monsoonal moisture.

Showers and a couple of isolated thunderstorms have developed over
the Cascades this afternoon. This will continue to increase through
the afternoon and evening. Another round of thunderstorms,  possibly
stronger, is on tap for Thursday. Some westward drift of storms into
the lowlands is possible, and would be most likely in Snohomish,
Skagit and Whatcom counties. This multiple days of thunderstorms
will be followed by hotter and drier conditions on Friday.

Area fuels have met and exceeded the 95th percentile ERC
values, increasing concerns for lightning strikes capable of
producing new fire starts. Outflow winds of 25 to 35 mph are
possible both ahead of and behind any storms that develop and can
create erratic wind situations that are exacerbated by complex
terrain.

Atmospheric moisture and PWAT suggest some storms will be capable of
a wetting rain. That said, given the prolonged stretch of hot and
dry weather and ongoing drought, it may not be enough to temper
ignitions in the dry nature of the fuels.


Kristell/JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 8 PM PDT this evening for East Portion of
     the Olympic Mountains-West Slopes of the Central Cascades
     Generally above 1500 Feet-West Slopes of the North Cascades
     Generally above 1500 Feet.

PZ...Dense Fog Advisory until 11 AM PDT Thursday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     Grays Harbor Bar.

     Dense Fog Advisory from 5 PM this afternoon to 11 AM PDT
     Thursday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm.

&&

$$