The Summit at Snoqualmie

000
FXUS66 KSEW 221621
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
921 AM PDT Sat Aug 22 2026

.SYNOPSIS...Weak upper level low off the northern Oregon coast 
this morning will continue to weaken and move northeast today. 
The low will be off the Washington coast tonight and move inland
as an upper level trough Sunday morning. Upper level ridge 
building in behind trough later Sunday will move east Monday. 
Western Washington on the backside of the ridge Tuesday. Another
upper level trough arriving Wednesday or Thursday hanging 
around into Friday.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...The upper low is slowly
approaching the western Washington coast this morning.
Convection developed early this morning, and continues across 
the Cascades, and primarily north of King County. Widespread 
thunderstorm activity has been observed over the last few hours 
over the Cascades, with minimal rainfall due to fast storm 
motions. There may be a brief break in the convection around 
midday before surface heating leads to destabilization and
another round of afternoon and evening convection, primarily 
focused in the North Cascades. Will also need to monitor the 
low, but non-zero, threat for severe wind gusts in any stronger
t-storm this afternoon into the Northern Cascades. Lightning 
may lead to new fire starts as well. 

The low weakens to an upper level trough tonight. Trough moving
inland near the Canadian border after midnight keeping at least
a slight chance of showers over most of the area. Marine layer 
moving inland again approaching the Puget Sound by 12z Sunday. 
Lows in the 50s. 

Trough moving off to the east with an upper level ridge building
into Western Washington Sunday afternoon. Shower activity
confined to the North Cascades Sunday afternoon. Low level
onshore flow with the marine layer returning to the coast in the
afternoon. Highs once again in the mid 60s to mid 70s. 

Upper level ridge continuing to build Sunday night before moving
east Monday. Onshore flow weakening bringing up the possibility
that the marine layer does not get east of Puget Sound Monday
morning. Even if it does it will be shallow and be gone by mid
to late morning. More sunshine and warmer temperatures aloft
adding up to a warmer day Monday. Highs in the 70s to lower 80s.
Lows Monday morning in the 50s. JD/Felton 

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...Extended models in 
good agreement this morning. Western Washington on the backside 
of the upper level ridge Monday night into Tuesday for one more 
warm day. Highs in the 70s to mid 80s. 

Upper level trough moving into the area Wednesday hanging 
around into Friday. Best chance for showers later Wednesday 
into Thursday. Much cooler with highs in the mid 60s to mid
70s all three days. 

Through the 21st Seattle is only 0.5 degrees above normal for 
the month of August. This cooler end to the month will make it a
close call if August ends up being above normal for  
temperatures. July was slightly below normal. The last time July
and August had below normal temperatures in Seattle was 2011.
Felton

&&

.AVIATION...Fast moving scattered showers and thunderstorms 
have developed this morning. Expecting this activity to continue
into the afternoon and then gradually subside this evening into
tonight. Any thunderstorm that impacts the terminals has the 
potential for frequent lightning, gusty/erratic winds, heavy 
downpours, and reduced visibility. MVFR to IFR cigs this morning
will continue to improve by this afternoon to VFR, with the 
exception of the coast. Another round of low cigs Sunday 
morning. 

KSEA...IFR cigs will gradually improve this morning, with VFR 
expected by the afternoon. Scattered showers and thunderstorms may 
impact the terminal through this afternoon. However, confidence is 
not high enough at the moment to include TSRA in the TAF. 
Thunderstorms will have the potential for frequent lightning, 
gusty/erratic winds, heavy downpours, and reduced visibility. 
Outside of thunderstorms, southwesterly winds 6-8 kt. 

29

&&

.MARINE...A weak surface low will move into the coastal waters 
today. The low will dissipate tonight. High pressure will 
rebuild Sunday and remain over the area through Tuesday. A weak 
frontal system will move across the waters midweek.

Winds picking up later this morning but staying 20 knots or less
into the afternoon hours. Diurnal westerlies in the Strait of
Juan de Fuca each evening for the next few days but at this
point peak winds look to be around 20 knots. 

Seas 3 to 5 feet for the next few days. Felton

&&

.FIRE WEATHER...Scattered showers and thunderstorms continue 
across western Washington this morning. As a result, a Red Flag 
Warning remains in place until the early evening. A brief lull 
is possible towards midday but RAP analysis is picking up on 
1000-1500 j/kg of surface based CAPE in the central to northern 
Cascades this afternoon. As an upper-low offshore continues to 
track inland, diffluent southerly flow aloft will help to fire 
off an additional round of showers and thunderstorms - mainly 
over the North Cascades. Upper level ridge building in later 
Sunday. Low level onshore flow pattern later Sunday through 
Tuesday will result in good relative humidity recoveries in the 
overnight hours easing fire weather concerns. Another upper 
level trough arriving in the middle of next week increasing the 
chances for convection. Felton

41/Felton

&&

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

     Red Flag Warning until 5 PM PDT this afternoon for West Slopes 
     of the Central Cascades Generally above 1500 Feet.

PZ...None.

&&

$$