286
FXUS66 KSEW 231548
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
848 AM PDT Sat Aug 23 2025

.SYNOPSIS...A longer period of hot and dry weather will continue
into next week as high pressure amplifies over the broader Pacific
Northwest. Widespread areas of Moderate to Major HeatRisk are
expected through Monday. Critical fire weather conditions are
expected to accompany the hot weather in the Cascades and eastern
Olympic mountains this weekend.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...The latest forecast remains
on track with no updates this morning. High pressure continues to
amplify into the northwest this weekend, leading to hot
temperatures and low relative humidity. A Heat Advisory remains in
effect until Tuesday morning for most lowland locations and an
Extreme Heat Warning will remain in effect through Tuesday morning
for Cascade foothill locations and valleys. Temperatures today
will be in the upper 80s to 90s throughout Puget Sound, increasing
into the upper 90s to even 100 degrees in the Cascade valleys and
Southwest Interior. Even the coast will warm up, into the 70s
along the immediate coastlines. HeatRisk will be Moderate (Orange)
to Major (Red) today through Monday. This will mean that those
who do not have access to adequate cooling and sensitive
demographics will be most at risk. Additionally, the overnight
temperatures are forecast to climb into the mid to upper 60s
tonight, remaining in the 60s through the weekend which won`t
offer much in the way of relief overnight. Very isolated pockets
of Extreme (Purple) HeatRisk are possible near Randle in Lewis
County, as well as in the valleys of the north Cascades.

The upper level heights fall a little bit by Monday, weakening the
high pressure overhead but not outright eliminating it.
Temperatures will be slightly cooler, though relative, in the low
90s for most, but maintaining a upper 90s in the valleys.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Warm temperatures won`t end
outright as soon as the heat headlines end, and are expected to
persist into the middle of next week. Widespread Moderate HeatRisk
is expected for much of western Washington. Heat stress is
cumulative, and with multiple days in a row of hot temperatures
expected, it is important to remember that impacts to health can
still occur even at "lower" temperatures when exposure has been
extended by nature of a longer heat wave.

By the middle of next week, the ridge appears to break down,
allowing for troughing to develop along the coast. There is still
considerable uncertainty in this extended timeframe and will
continue to be monitored as it pertains to thunderstorm or
precipitation chances are concerned.

Kristell

&&

.AVIATION...South to Southwest flow aloft will continue through
Saturday with an upper ridge across the Pacific Northwest. Surface
low level flow remains northerly. VFR conditions with clear skies
early this morning. VFR conditions will continue through today, with
high clouds at times this afternoon. Light N/NE winds early this
morning will increase from the N/NW this afternoon.

KSEA...VFR conditions through the TAF period. High clouds at times
this afternoon. N/NE surface winds of 8 to 13 kts this morning will
become more N/NW this afternoon. JD

&&

.MARINE...High pressure will continue over the Coastal Waters as a
thermally induced surface low expands northwards along the coast.
Elevated north to northwesterly winds are expected along the Coastal
Waters, particularly the outer Coastal Waters, through early next
week. A few brief gusts of 20 to 25 kts are possible, but winds are
primarily expected to remain below SCA threshold. Elevated north
winds are expected for Puget Sound as well this afternoon, with
gusts of 15 to 20 kts possible. A transition to an onshore flow
pattern may arrive by the middle of next week.

Seas of 4 to 6 feet through this weekend with a period around 7
seconds. Seas will build to 6 to 7 feet for the Outer Coastal Waters
late Sunday into Monday. JD

&&

.FIRE WEATHER...Strong upper level ridging with an accompanying
thermal trough along the coast will continue building this
weekend. Offshore flow will be present throughout the weekend,
though not at significant speeds, enough to further dry the
airmass and fuels. Most zones have met or exceeded the 90th
percentile ERC (energy release component) having recovered from
recent rainfall. The only exception to that are the western slopes
of the Olympics, as that location received more of the brunt of
said previous rainfall.

Today, widespread temperatures in the 90s and 100 degrees in the
valley locations are expected. Early this morning, relative
humidity recovery has been poor, especially in the Cascades and
portions of the Olympics. In the vicinity of the Bear Gulch fire,
lower elevation RH has been between 55 and 60%, with a ridgetop RH
of 30%. As the ridge amplifies through the weekend, the
phenomenon to watch near the Bear Gulch fire will be the
progression of the thermal trough. Active fire behavior has
occurred with pyrocumulus development associated with the thermal
trough passage.

Elsewhere, relative humidity will be very low this afternoon
(25-30% in the lowlands, 15-25% in the mountains) keeping those
Red Flag Warnings going through Sunday night. The Fire Weather
Watch beginning on Monday for hot, dry and unstable conditions
will remain in place until the models have more of a semblance of
the kind of instability that may come to the region, though even
now at this lead time, there are indications of a definitive
unstable period early next week as the ridge breaks down.

Kristell

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 9 PM PDT Sunday 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.

     Fire Weather Watch from Monday afternoon through Tuesday evening
     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 Tuesday 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 Tuesday 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.

PZ...None.
&&

$$