547
FXUS66 KSEW 242143
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
243 PM PDT Sun Aug 24 2025

.SYNOPSIS...A prolonged period of hot and dry conditions will
continue well into the middle of next week as high pressure
remains over the region. Widespread areas of Moderate (Orange) to
Major (Red) HeatRisk will continue through Tuesday across much of
western Washington. In addition, critical fire weather conditions
continue through this evening for the eastern slopes of the
Olympics and through Monday for the Cascades.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...A high pressure ridge
will continue to slowly break down overhead through Tuesday,
keeping temperatures elevated across western Washington. Although
temperatures will gradually fall a degree or two each day through
Tuesday, conditions will remain well above normal for late
August. Highs today are on track to peak in the upper 80s to low
90s across much of the lowlands, with temperatures nearing 100
degrees along the Cascade Foothills and Valleys. This is resulting
in continued widespread areas of Moderate (Orange) to Major (Red)
HeatRisk across western Washington, with little change from
yesterday. Additionally, low temperatures tonight will remain
elevated in the low to mid 60s for most inland lowland areas,
providing little relief to the hot daytime conditions. An Extreme
Heat Warning remains in effect for the Seattle-Tacoma- Bellevue
metro areas and Cascade Foothills and Valleys, and a Heat Advisory
remains in effect for all other lowland areas away from the
coast.

Temperatures Monday will remain elevated, once again only a
degree or two cooler than today for most inland areas as onshore
flow develops. Widespread Moderate to Major HeatRisk will continue
across western Washington, and the prolonged heat will prove
impactful to those without access to adequate cooling and those in
demographics vulnerable to heat stress. Forecast models have
increased temperatures on Tuesday, maintaining very similar
conditions to Monday. The current Extreme Heat Warnings and Heat
Advisories have been extended through Tuesday night for most areas
away from the coast.

As the upper ridge over the region slowly breaks down, monsoon
moisture from the Desert Southwest will be advected northward into
western Washington. This will increase the chances for isolated
thunderstorms to develop over the Cascades on Monday and Tuesday.
Elevated instability over the mountains (1000-2000 J/kg of CAPE)
and ample incoming moisture (precipitable water values over an
inch) provide a favorable environment for storms to develop along
the Cascade Crest, but Tuesday looks to provide a slightly better
environment.

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...As an upper level trough
deepens offshore, moisture will continue to be injected into
western Washington. This will result in cloudy skies, bringing
temperatures down several degrees across the region. Highs
Wednesday are on track to peak in the 70s and low 80s for most
areas, and HeatRisk will lower to Minor (Yellow) to Moderate
across western Washington. A weak shortwave is on track to move
northward across the region on Wednesday, bringing the best
forcing of the week for thunderstorm development over the
Cascades.

Towards the end of the week, ensemble forecasts continue to show
potential for continued chances for isolated showers and
thunderstorms. However, a fair amount of uncertainty exists
between models over how conditions will evolve. Temperatures are
expected to return to near-normal by the weekend.

15

&&

.AVIATION...South to southwesterly flow aloft as upper level
ridging continues over western Washington. VFR conditions at all
terminals this afternoon with a few high clouds passing by. VFR
conditions are expected to continue into Monday, with the
exception of KHQM, where marine stratus will filter into the
terminal, bringing IFR/LIFR conditions throughout Monday morning.
N/NW winds 5 to 10 knots this afternoon decreasing overnight.

KSEA...VFR conditions throughout the TAF period with some high
clouds passing by overhead. N/NW winds this afternoon around 8 to
12 knots will decrease generally to 4 to 8 knots overnight.

Mazurkiewicz

&&

.MARINE...Surface high pressure continues over the coastal waters
with continued northerly flow in the lower levels. North to
northwesterly winds over the coastal waters may be elevated at
times throughout the evening, with occasional gusts up to 25 kts
possible at times. Winds will primarily stay under small craft
advisory threshold. North winds will continue over the waters
through Tuesday, with the pattern shifting to more onshore flow by
midweek.

Combined seas 4 to 6 feet, with seas building to 7 feet primarily
in the outer coastal water zones. With a dominant period of 7 to 8
seconds, seas may be steep at times in the outer coastal waters.
Seas will subside generally 3 to 5 feet by the middle of the week.

Mazurkiewicz

&&

.FIRE WEATHER...Red Flag Warnings for the west slopes of the
Cascades and east slopes of the Olympics remains in effect through
9PM today for hot, dry and unstable conditions. Went ahead and
extended the Red Flag Warning for the Cascades through Monday
evening, as confidence is low in minimum relative humidity values
getting back above 25% tomorrow afternoon, especially as you get
closer to the crest. Maximum RH values this morning will likely once
again be below 40% for the ridgetops and mid-slopes with offshore
flow continuing though tonight. While temperatures remain warm and
the atmosphere remains unstable, increasing humidity each day will
allow for critical fire weather conditions to cease after Monday.

The upper level pattern gets messy going into Tuesday and Wednesday
as several upper level disturbances look to develop and move across
the Pacific Northwest. This will also help to draw in monsoonal
moisture from the southwest, but also provide as a mechanism to
allow for thunderstorm development. A few thunderstorms could
develop as early as Monday, but the best chances will be Tuesday and
Wednesday afternoon (15-25% probability). Localized wetting rains
will be possible with PWATs go above 1" and storm motions will be
slow. Confidence in scattered or higher thunderstorm coverage is
low at this point, so there is no Fire Weather Watch in effect.
Isolated lightning and locally gusty winds are the primary threats.

Going into the end of the week, the thunderstorm threat looks to
remain possible along the North Cascades Thursday afternoon.
Elsewhere, a larger upper level trough looks to approach the area
late week into next weekend, which could provide cooler and more
moist conditions, as well as chances for scattered showers across
the entire area. There is considerable model uncertainty, however,
regarding the evolution of this pattern going into the beginning of
September.

62

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 9 PM PDT Monday for West Slopes of the
     Central Cascades Generally above 1500 Feet-West Slopes of
     the North Cascades Generally above 1500 Feet.

     Extreme Heat Warning until 5 AM PDT Wednesday for City of
     Seattle-Eastside-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-
     Lowlands of Pierce and Southern King Counties.

     Heat Advisory until 5 AM PDT Wednesday for Downtown Everett /
     Marysville Area-Eastern Kitsap County-Foothills of the
     Western and Southern Olympic Peninsula-Lake Crescent Area
     Including US 101-Lowlands of Lewis and Southern Thurston
     Counties-Lowlands of Western Skagit and Northwestern
     Snohomish Counties-Lowlands of Western Whatcom County-
     Middle Chehalis River Valley-Northern Hood Canal-Olympia
     and Southern Puget Sound-Shoreline / Lynnwood / South
     Everett Area-Southern Hood Canal-Willapa and Black Hills.

     Heat Advisory until 5 AM PDT Tuesday for Eastern Strait of Juan
     de Fuca-Western Strait of Juan de Fuca.

     Red Flag Warning until 9 PM PDT this evening for East Portion of
     the Olympic Mountains.

PZ...None.
&&

$$