369
FXUS66 KSEW 100312
AFDSEW

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

.UPDATE...No changes made to the previous forecast. Weak
convergence currently over Snohomish county as a front is well
east of the Cascades. Lows tonight are to range between the lower
to upper 50s with mostly cloudy skies. The previous discussion
remains below along with an updated marine and aviation section:

&&


.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...Northwesterly flow aloft as an upper level trough
continues to exit W WA this evening. South to southwesterly winds at
the surface generally running 5-10 kts will gradually ease further
overnight to 3-8 kts. CLM and HQM are more westerly with HQM running
speeds similar while CLM seeing speeds more in line with 8-12 kts
with occasional gusts. PAE seeing more northerly winds, also
resulting from the push down the Strait.

Majority of cigs very borderline VFR/MVFR, some on the lower side,
some on the upper. PAE is the only reporting IFR as of 7 PM PDT.
With plenty of moisture at the lower levels, cigs expected to
degrade tonight with widespread MVFR to IFR conditions. Conditions
improving by early Thursday afternoon with widespread VFR expected
at the majority of terminals, with the exception of along the coast.

KSEA...Cigs bouncing back and forth between MVFR and VFR this
evening and that may continue through around 07-09Z before
conditions finally degrade to solidly MVFR. Further lowering will
gradually occur with cigs expected to bottom out between 1500-2000
ft just slightly after 12Z and remain there into the late
morning/early afternoon. Some light drizzle may be possible, but
confidence remains low enough to not include in TAF. VFR conditions
will return Thursday afternoon.
Thursday morning after 14z-15z.

Southwesterly surface winds 5-10 kt this evening, easing a little
bit more overnight as discussed above. Guidance hints at northerly
winds returning after 23z Thursday.

29/18

&&

.MARINE...A weak front continues to exit the inland waters
this evening. Another push of westerlies down the Strait of Juan de
Fuca allows the inherited Small Craft Advisory to remain 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.

29/18


.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.

&&

$$