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FXUS66 KSEW 220255
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
755 PM PDT Fri Aug 21 2026

.SYNOPSIS...
Upper level low approaching from the southwest Friday will move
into the coastal waters Saturday and into Southern British
Columbia early Sunday morning. This system will bring cooler
weather with a chance of showers and thunderstorms. Upper level
ridge rebuilding Sunday will move east Monday for dry and mild
weather. Upper low shifting inland on Wednesday with showers
returning.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
The latest forecast remains on track this evening with no
updates. Satellite does show some wildfire smoke over western
Washington due to light east winds over the crest. Sensors are
showing degraded air quality in portions of Whatcom County and
King County (due to the Three Queens fire). The air quality
should improve on Saturday as the wind flow changes directions.

Our next system will move in late tonight and into Saturday and
Sunday. This system will bring cooler weather along with
increasing chances of thunderstorms. The highest chances of
thunderstorms will be in the mountains, especially across the
northern Cascades. The highest chances of wetting rains will be
near the Canadian border. With the colder air mass in place,
Saturday`s highs will be cooler and in the 60s to lower 70s
across most areas. Showers will linger on Sunday (mainly north
part) as the low tracks N/NE into B.C.

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&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
High pressure returns early next week for more dry and mild
weather. The flow remains onshore with temperatures close to
average. The ridge will stall an upper low offshore that will
eventually shift inland by midweek for another round of showers.
The low and cooler air mass will result in below average
temperatures in the 60s and lower/mid 70s. We`re under weak
troughing through the end of the week with isolated mountain
showers.

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&&

.AVIATION...
Southerly flow aloft will increase tonight into Saturday as an
upper low offshore approaches Western Washington. VFR cigs for
the majority of terminals this afternoon with increasing mid and
high clouds. MVFR/IFR stratus continues along portions of the
coast. Stratus will move inland into Puget Sound into Saturday
morning, resulting in more widespread MVFR/IFR cigs. LIFR
stratus will exist along portions of the coast tonight into
Saturday as well. Showers and a few thunderstorms are expected
to develop late tonight into Saturday. Probabilities for
thunderstorms range to 10 to 20 percent for all terminals, with
the higher probabilities from KPAE northwards. Cigs will improve
to VFR for interior locations by the afternoon. W/NW winds this
afternoon will transition more SW tonight and increase into
Saturday.

KSEA...VFR with mid and high clouds into tonight. Stratus will
spread inland early Saturday, reaching the terminal around 12z.
Predominantly low-end MVFR cigs are expected. There is a 20%
probability of IFR cigs between 13 to 17z as well. Cigs will slowly
improve to VFR into the afternoon, between 19 to 21z. Showers and a
few thunderstorms are expected to develop early Saturday morning.
Confidence remains lower in the exact location for precipitation,
but there is a 10 to 15% probability for thunderstorms around the
terminal Saturday, peaking between 12 to 20z. W/NW surface winds
ranging 5 to 8 kts will transition SW after 00-01z.

JD

&&

.MARINE...
Onshore flow will increase this evening resulting Small Craft
westerly winds through the Strait of Juan de Fuca tonight. A
weak surface low then moves into the coastal waters Saturday
before the low dissipates Saturday night. Showers will develop
late tonight into Saturday associated with the system. There is
a also a slight chance (10-20%) for thunderstorms over both the
coastal and interior waters on Saturday. South to southwest
winds will also increase on Saturday, with gusts ranging 15 to
20 kts for portions of the interior waters and inner coastal
waters. High pressure rebuilds Sunday and remains over the area
into Tuesday. A cold front moves into the waters by midweek.

Seas will mostly be in the 3 to 5 foot range into early next
week.

JD

&&

.FIRE WEATHER...
Dry and warm conditions continuing today. Critical RHs have
been observed for locations above 2500 ft so far this afternoon.
Increasing cloud coverage and continued low- level onshore flow
this evening. Then, an upper-low offshore will track into the
region late tonight into early Saturday with the potential for
scattered showers and thunderstorms. As a result, a Red Flag
Warning has been issued for the Cascade and Olympic Mountains
late tonight into Saturday evening. The North Cascades are
currently the most favorable for thunderstorms at this time with
a 30-40% chance. Lightning may lead to new fire starts, as well
as result in gusty and erratic winds.

Low level onshore flow increases Saturday night into Sunday with
varying degrees of onshore flow through the middle of next week.
Good relative humidity recoveries each night will keep fire weather
concerns at a minimum. Chance of showers and possible thunderstorms
next Wednesday with an upper level trough moving into the area.

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&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 5 AM to 8 PM PDT Saturday for West
     Slopes of the North Cascades Generally above 1500 Feet.

     Red Flag Warning from 1 AM to 5 PM PDT Saturday for East
     Portion of the Olympic Mountains-West Portion of the
     Olympic Mountains-West Slopes of the Central Cascades
     Generally above 1500 Feet.

PZ...Small Craft Advisory until 6 AM PDT Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$