609
FXUS66 KSEW 092224
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
324 PM PDT Wed Jul 9 2025

.SYNOPSIS...Front over Cascades late this afternoon will continue
to move east. High pressure, both at the surface and aloft,
building offshore Thursday and remaining in place into next week.
Upper level ridge strengthening Tuesday and Wednesday with
thermally induced surface trough moving up the coast. The
resulting low level offshore flow could push highs into the 90s
over the southern portion of the interior Wednesday.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...Satellite imagery shows
cloudy skies over most of the area this afternoon. A little
downslope off the Olympics has created clearing on the east
slopes of the Olympics. Doppler radar has showers mostly confined
to the Northwest Interior and Central and Northern Cascades.
Temperatures at 3 pm/22z were in the 60s and lower 70s.

Cold front over the Cascades this afternoon will continue to move
east. Onshore gradients increasing with convergence zone
developing over Snohomish and extreme Northern King County by late
this afternoon. The convergence zone will remain in place through
the evening hours before dissipating. Up to another tenth of an
inch of rain in the zone with the highest amounts in the Cascades
and Cascade foothills. For the remainder of the area some
clearing over the South Puget Sound in response to the convergence
zone otherwise mostly cloudy skies. Lows tonight in the 50s.

High pressure, both and the surface and aloft, building offshore
Thursday. Marine layer over the area in the morning will dissipate
in the afternoon with weakening onshore flow and the strong mid
July sun. The cloudy morning will keep high temperatures below
normal, in the mid 60s to mid 70s.

Not much change to the pattern Thursday night through Saturday.
Upper level ridge remaining centered offshore with light low level
onshore flow. With the weak onshore flow, marine layer along the
coast will have a hard time moving inland in the overnight hours.
Temperatures aloft with the ridge axis offshore will be fairly
steady through the period. Model 850 mb temperatures bouncing
between plus 14C and plus 17C. This combined with light onshore
flow will keep temperatures from getting too warm Friday and
Saturday. Highs over the interior in the mid 70s to mid 80s.
Afternoon seabreezes will keep highs on the coast near 70.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Extended models in good
agreement with the Friday and Saturday weather pattern remaining
intact for Sunday and Monday.

Changes start to occur Tuesday with the ridge offshore gaining
amplitude and a thermally induced thermal surface trough moving up
the Pacific Northwest coastline. By Wednesday the thermal trough
will be along the Washington coast. Temperatures aloft warming
Tuesday with model 850 mb temperatures in the plus 18C to plus
21C. More warming Wednesday as the ridge continues to amplify with
850 mb temperatures in the plus 21C to 24C range. Warmer
temperatures aloft combined with increasing low level offshore
flow will give the area a short heat wave. Highs Tuesday in the
interior upper 70s to near 90 with mid and upper 70s along the
coast. About 5 degrees warmer Wednesday with 80s to mid 90s
interior and 80s along the coast. While not a record, the current
forecast high of 90 degrees for Seattle next Wednesday would be
only the 5th time in 81 years with a high 90 degrees plus on July
16th. HeatRisk solidly in the moderate category both days for the
interior. The hot and dry weather could also produce some fire
weather headlines. Felton

&&

.AVIATION...A mix bag of ceilings this afternoon with a weak system
moving across Western Washington. Puget Sound terminals have mostly
remained rain shadowed with majority of precipitation along the
coast and northern interior. Some terminals have managed improve
into VFR this afternoon, however expecting ceilings to deteriorate
overnight into Thursday morning, with another weak system expected.
Showers should continue to taper of this evening, however guidance
hints at convergence zone forming after 23z along KPAE. Northerly
winds should remain confined to KPAE, and are not expected to make
it south into KBFI and KSEA.

Generally southwesterly surface winds 8-13 kt this afternoon along
the interior terminals, decreasing overnight to 2-5 kt. Breezy
westerlies along the Strait of Juan de Fuca this evening.

KSEA...Conditions managed to improve into VFR this afternoon. Models
have been struggling with the ceilings today, however, some do hint
at MVFR conditions may return as early as 0z Thursday (30% chance),
while others keep low-end VFR persisting through the evening.
However, there is good agreement that MVFR ceilings will return
Thursday morning after 14z-15z.

Southwesterly surface winds 8-11 kt this afternoon, easing
overnight. Convergence zone northerly winds are not expected to
impact the terminal this afternoon/evening. Guidance hints at
northerly winds returning after 23z Thursday.

29

.MARINE...A weak front continues to push across the inland waters
this afternoon. Another push of westerlies is expected for the
Strait of Juan de Fuca this evening, for which a Small Craft
Advisory is currently in effect. Westerly pushes will continue to
occur across the Strait each evening, but at this time look to
remain below small craft criteria. Another weak system will move
across the waters Thursday morning. Following this system, strong
high pressure will build over the waters, and will remain the
dominant surface feature into early next week, establishing
northwesterly flow over the coastal waters. High pressure will
weaken slightly on Saturday into early next week. Guidance hints at
steep seas Sunday into early next week, with seas building to 7-9 ft
at 8-9 seconds.

29

&&

.FIRE WEATHER...Cooler and more moist conditions will begin to cease
on Thursday as skies clear and conditions being to warm up with a
weak upper level ridge building over the area. Friday will be the
warmest/driest day this week with RH values dropping back into the
25 to 35% range over the mountains and across the interior lowlands
south of Puget Sound. More robust onshore flow will help usher in
more moisture this weekend despite high temperatures remaining in
the 80s.

While there is some model discrepancy, there is consensus that a
more robust upper level ridge will being to build and move into the
area by the middle of next week. Models are beginning show the
development of a thermal trough along the coast, which would allow
for winds to shift to offshore, promoting much drier and warmer
conditions as well as some potential breezy winds through the
Cascade gaps. This pattern will continue to be closely monitored as
we approach next week.

62

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Thursday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$