153
FXUS66 KSEW 161640
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
940 AM PDT Thu Jul 16 2026
.UPDATE...
12z sounding from KUIL indicates a sharp inversion in the
lower-levels but just above, mid-level lapse rates sit around
8.6 C/km. This instability has been tapped into to say the least
as an upper-low offshore has ushered moist (PWATs 1.1-1.3 in.),
diffluent southerly aloft across the area. These factors in
concert with present instability has lead to scattered
thunderstorms off the coast and over W WA. Main threats include
lightning, small hail, erratic winds and brief heavy rainfall.
The Red Flag Warning also remains in place through the day for
the Olympic and Cascade mountains due to lightning activity.
Shower coverage is slated to decrease into the afternoon and
the aforementioned upper-low begins to lift into BC. Cloud
coverage will help to stifle high temperatures with forecast
highs in the 60s to lower 70s. The previous discussion remains
below along with an marine/aviation section.
&&
.SYNOPSIS...
Widespread showers and the threat for thunderstorms today as a low
pressure system passes through, exiting the region this weekend.
High pressure builds over the area next week, bringing a return to
warm and dry weather.
&&
.SHORT TERM /TODAY THROUGH SATURDAY/...
Current satellite imagery showing a tale of two cloud types, the
first, marine stratus along the coast and the second being mid to
high level clouds over the Cascades...mainly slop-over from
persistent showers and thunderstorms east of the crest. Latest radar
confirms this, with echoes on the east side of the Cascades. Looking
to the south though, one can already see shower development in
western OR that will make its way up into W WA later on this morning.
While the forecast by and large remains consistent with messaging
from previous days, the focus for precip and thunderstorms has
shifted somewhat. Chances for both seem to favor the western half of
the CWA. This does not mean that no activity is expected east of the
Sound, but with lingering moisture from the first wave of activity
last night, this may be enough to ease some of the instability and
make any activity today a little sluggish to start. Of course, this
inhibition would be most prevalent over the western slopes of the
Cascades and the nearby foothills and valleys. For the East Puget
Sound Lowlands, the I-5 Corridor and surrounding areas, the better
chances start around noon today and, depending on how far north you
are, will increase accordingly...the Seattle metro leaning more
toward a prime time of early to mid afternoon while the best chances
may not reach Bellingham until late afternoon or early evening.
Forecast remains consistent on precip exiting quickly with the event
wrapping up in all but the northernmost portions of the CWA by
midnight tonight. Some isolated showers may be possible Friday, but
for the most part, dry conditions return promptly Friday morning and
remain that way for the remainder of the short term period.
Cooler daytime highs areawide today with interior lowlands generally
in the lower to mid 70s while coastal areas seeing temps about 10
degrees cooler in the lower to mid 60s. The warming trend kicks in
quickly starting on Friday, although current forecast shying away
from previous solutions of a steep increase in temperatures.
Instead, Friday sees daytime highs in the mid to upper 70s /mid 60s
along the coast/ and Saturday is only incrementally warmer with
temps in the mid 70s to around 80 /mid to upper 60s along the coast/.
18
&&
.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Although upper level troughing will remain in place over W WA for
the remainder of the weekend, weather will be more determined by a
pair of upper level ridges...one over the eastern Pacific and the
other increasing in amplitude over the desert southwest/Four Corners
region. This pair will keep any moisture out of the area and well to
the north and provide a boost to temps on Sunday as highs climb into
the lower to mid 80s in the lowlands and upper 60s to lower 70s
along the coast. The eastern Pacific ridge shifts eastward, becoming
the main influence over W WA for the start of next week. This will
push temperatures up more as highs both days will be in the mid to
upper 80s inland and lower to mid 70s along the coast. Although a
weak upper trough moves into the area Wednesday, it appears to have
zero impact on temps, as daytime highs remain static. Given these
temps, HeatRisk will remain Minor/Yellow for Sunday, but as 80s
become more widespread, so too will Moderate/Orange HeatRisk...with
Moderate/Orange conditions being widespread for the remainder of the
long term forecast period.
18
&&
.AVIATION...
A cutoff upper low tracking northward off the coast will yield
southerly flow aloft over the airspace. Ahead of this system, low
marine stratus cover along the Pacific coast will continue to
support IFR/low-end MVFR cigs through much of the period. Light and
variable winds early this morning will build out of the south to
southwest to 8-12 kt with gusts around 15 kt possible, except along
the Strait of Juan de Fuca including KCLM where west-northwest flow
will continue. Radar continues to track two areas of showers and
thunderstorms moving northward this morning - first group in the
interior west of Mt. Rainier up into the east Seattle metro, and
another area along the coast, although both groups of convection
have been trending downwards as of 16Z. Main concerns is lightning,
with perhaps a few cells producing small hail and gusty outflow
winds. Tops of the storms have reached 30,000 ft so far, with
ceilings down to MVFR. Residual convection may linger into the
afternoon once the first wave moves through as the atmosphere
remains unstable with the trough (20-30% chance). Ceiling
probabilities for Friday morning favor MVFR in the interior, and IFR
for the coast.
KSEA...Showers/vicinity thunderstorms to continue this morning
though activity has been ramping down to the south and east. Can`t
rule out redevelopment this afternoon from 21Z-00Z, but this remains
a 20-30% chance. There is a 40% chance of MVFR development Friday
morning. Light west winds early this morning will back out of the
southwest and build to 8-12 kt by 18z Thu, before easing below 5 kt
again after 06-09z Fri.
Picard/HPR
&&
.MARINE...
A weak surface low meandering over the coastal waters through Friday
will see northwesterly winds early this morning back out of the
south to southwest by this afternoon, then veering out of the
northwest through the day on Friday as the low exits. Thunderstorms
will move over the coastal waters this morning, with lightning,
small hail and gusty winds all possible. Diurnal westerly flow will
continue through the Strait of Juan de Fuca, peaking each afternoon
and early evening. A Small Craft Advisory has been hoisted for the
central Strait and eastern Strait entrance from 1-11 PM PDT today
when winds will reach 15-25 kt, before easing tonight. Surface high
pressure building offshore through this weekend will see a return to
typical summertime northerly flow. Diurnal gusts may reach 20-25 kt
in the afternoon, most likely beyond 30 NM.
Coastal seas of 5-8 ft will ease slightly to 4-6 ft by tonight, with
a continued dominant west-northwest swell at 7-8 seconds. As
northerly flow strengthens through the weekend, seas will again
build to 5-8 ft, with the highest chances for seas reaching 8-10 ft
across the outer waters beyond 30-40 NM.
Picard/HPR
&&
.FIRE WEATHER...
A Red Flag Warning remains in effect for the mountains today.
Showers and scattered thunderstorms will remain a concern through
this evening with a highlight on the potential for dry lightning
(storms resulting in less than 0.25 of an inch of rainfall) and
strong outflows between 35-50 mph. Storms that develop on the west
slopes of the Olympics are more likely to produce a wetting rain
(0.25 inches or more), while storms along the Cascades will produce
sub- wetting rain amounts of 0.10-0.20 inches. As mentioned above,
latest forecast seems to be favoring more activity in and around the
Olympics as opposed to the Cascades, however with fuels in the
Cascades being drier than their Olympic counterparts, a significant
risk remains even if chances there are a little lower. Fire weather
concerns will linger through the remainder of the forecast period as
a hotter and drier pattern returns quickly over Western Washington.
This has the potential for increasing fire activity from any new
lightning starts from today`s convection.
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 11 PM PDT this evening for East Portion
of the Olympic Mountains-West 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.
PZ...Small Craft Advisory from 1 PM this afternoon to 11 PM PDT
this evening for Central U.S. Waters Strait Of Juan De
Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
&&
$$
NWS SEW Office Area Forecast Discussion