572
FXUS66 KSEW 202226
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
326 PM PDT Thu Aug 20 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 SATURDAY NIGHT/...Weak high
pressure is over the area for dry and mild conditions. The air
mass will be a little warmer on Friday with pockets of Moderate
HeatRisk in the interior and Cascade valleys (highs in the mid
to upper 80s).

The next weather system rolls in late Friday and into Saturday.
This system will bring cooler conditions, showers, and a threat
of thunderstorms to western Washington. The highest instability
will be over the Olympics and northern Cascades where scattered
thunderstorms may develop - leading to elevated to critical
fire weather conditions. The highest probability of seeing
wetting rains will be near the Canadian border. There`s a 10-20
percent chance (isolated coverage) of seeing thunderstorms in
the lowlands from Seattle northward. With the cooler air mass in
place, temperatures will track cooler than average and in the
60s to lower 70s. 33

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...Showers (mainly north
part) will linger into Sunday as the low tracks N/NE into B.C.
Dry and mild weather will return early next week with low
pressure offshore and weak high pressure inland. Temperature
ranges will be close to average each day. Low pressure shifts
inland Wednesday and Thursday for a chance of showers. 33

&&

.AVIATION...VFR this afternoon with mostly clear skies. A weak
push of marine stratus will bring IFR/LIFR conditions to the
coast this evening into Friday morning. The stratus will attempt
to push into OLM, but don`t expect reach Puget Sound terminals.
Reductions in visibility highly likely with the stratus.
Additionally, patchy fog possible in the valleys of the southern
interior and northern interior. Mid-to- high clouds will
increase Friday morning with conditions remaining VFR across
interior terminals. Minimal improvement along the coast on
Friday, with conditions likely only rebounding to MVFR.
Generally NW winds this afternoon, easing tonight.

KSEA...VFR with mostly clear skies today and increasing mid-to-high
clouds Friday morning. NW winds this afternoon 6-8 kt will
transition to NE this afternoon, easing tonight to 3-5 kt.

29

&&

.MARINE...Broad high pressure over the waters will weaken
Friday. A weak surface low will move into the coastal waters
Saturday with the associated system moving through the inland
waters Saturday morning into early afternoon. The low will
dissipate Saturday night. Additionally, there is a slight chance
(10-20%) for thunderstorms on Saturday for both the coastal
waters and interior waters. High pressure will rebuild Sunday
and remain over the area through Monday. A cold front will move
over the waters on Tuesday.

Winds will pick up behind the system on Saturday. Guidance suggests
there is a 40-60% wind gusts may approach near 21 kt through the
Strait of Juan de Fuca, with winds above 21 kt staying on the
Canadian side.

Seas remaining mostly in the 3 to 5 foot range for the next few
days. Felton/29

&&

.FIRE WEATHER...Warm, dry conditions continue into Friday. Moderate
RH recoveries are expected tonight for the Cascades. An upper low
will move northwards towards Western Washington late Friday into
Saturday. This feature will result in the development of convection
during this period. Probabilities for convection are highest over
the Olympics and North Cascades, peaking between 20 to 30 percent.
Due to the potential of scattered thunderstorms for these areas, a
Fire Weather Watch has been issued for zones 652, 658, and 661.
Convection may exist elsewhere, including the southern Washington
Cascades, however, uncertainty exists in the coverage for these
regions. Showers and thunderstorms may begin as early as 11PM-2AM
Friday night over the Olympic Peninsula, with the threat continuing
through late Saturday. Lightning may lead to new fire starts, as
well as result in gusty and erratic winds.

Onshore flow increases Sunday into early next week, resulting in
cooler conditions and increased RH values. Rain may linger into
Sunday for the North Cascades, with another slight chance of showers
later Tuesday into Wednesday. JD

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Fire Weather Watch from late Friday night through Saturday
     evening for West Slopes of the North Cascades Generally
     above 1500 Feet.

     Fire Weather Watch from late Friday night through Saturday
     evening for East Portion of the Olympic Mountains-West
     Portion of the Olympic Mountains.

PZ...None.

&&

$$