Startup Washington Weather Forecast Discussion

165
FXUS66 KSEW 071602
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
902 AM PDT Tue Oct 7 2025
.UPDATE...Satellite imagery shows just some high clouds over the
northwest corner of the state this morning with clear skies
elsewhere. Temperatures are recovering quickly from some cool
overnight lows with 40s and lower 50s at 9 am. Offshore flow
giving the area one more warm day. Transition to onshore flow
tonight with cooler temps Wednesday. Current forecast on track.
Updated aviation and marine sections below. Felton
&&
.PREV DISCUSSION... /issued 400 AM PDT Tue Oct 7 2025/
.SYNOPSIS...Dry and warmer conditions will continue across
western Washington for one more day today ahead of a pattern
change. Troughing will resume over the Pacific Northwest through
the remainder of the week and into the weekend, bringing much
cooler temperatures, cloudy skies, and periods of light showers.
Chances for more widespread precipitation increase into the
weekend, with the Cascades likely seeing accumulating snowfall
over higher elevations.
.SHORT TERM /TODAY THROUGH THURSDAY/...Western Washington will
see one more day of mostly clear skies and above normal
temperatures today as an upper level ridge shifts eastward.
Offshore flow through the Cascade gaps will gradually weaken
throughout the morning as onshore flow increases along the coast.
While areas along the coast will see a few degrees of cooling
today, areas across the interior lowlands will see another day of
highs mostly in the 70s.
As an upper level trough drops southward along the Pacific
Northwest Coast later tonight, a frontal system will split apart
as it reaches western Washington to bring little more than cooler
temperatures and cloudy skies. Areas along the Canadian border may
see light shower activity develop this evening into Wednesday, but
any accumulations would be minimal. Elsewhere, increased moisture
from a return to onshore flow will bring in cloudy skies Wednesday
for more typical fall-like conditions.
A cutoff low will develop offshore through the remainder of the
week, with potential for wrap around moisture to be advected
northward as the low pressure system slowly creeps southward along
the coast. Rainfall accumulations from any showers that develop
will be light, generally less than a tenth of an inch across
western Washington. Temperatures Wednesday and Thursday will cool
considerably as the trough strengthens offshore and onshore flow
increases, with highs mostly in the upper 50s to low 60s.
.LONG TERM /FRIDAY THROUGH MONDAY/...Ensemble guidance remains in
good agreement over a more active pattern emerging into the
weekend as the offshore low moves inland and another trough drops
down from British Columbia. Temperatures over the weekend will
continue on a cooling trend, with highs peaking below 60 degrees
for most areas by Saturday and Sunday. As more widespread
precipitation enters the region, snow levels will drop to roughly
4000 feet, bringing in the first mountain snow of the season.
Ensembles maintain a moderate (35% to 45%) chance of accumulating
snow through Stevens Pass over the holiday weekend, though
Snoqualmie Pass might be low enough in elevation for limited
impacts. The latest model guidance has increased snow totals for
the Cascades over the weekend, showing potential for 3 to 6 inches
or more of snow accumulation above 5000 feet. Given the first
snow of the season will fall over a holiday weekend,
recreationists should remain weather aware and use extra caution
in the higher elevations of the Cascades Friday through Monday. At
lower elevations, rainfall amounts are on track to reach up to
half an inch of accumulation over the weekend.
15
&&
.AVIATION...VFR across most terminals this morning with mostly clear
skies. The exception is KOLM & KPWT where patchy fog/low clouds is
bringing MVFR/LIFR conditions but should burn off within the next
hour or two. VFR conditions expected for the majority of Tuesday,
but the coast will see ceilings start deteriorating in the evening
hours. Cloud cover will increase early Wednesday morning after 07z,
with guidance suggesting that Puget Sound terminals may see low
ceilings as early as 10z. There is medium to high confidence (40-70%
chance) that terminals will see cloud decks below 3000 ft. Latest
guidance has a 20-50% chance of IFR and 10-30% of LIFR, with the
highest probabilities south of PAE.
KSEA...VFR at the terminal so far this morning and expected to
persist throughout the day. Cloud cover will increase early
Wednesday morning across Puget Sound and guidance suggests that MVFR
ceilings (65% chance) may impact the terminal as early as 10z-11z.
There is a 35% chance of IFR and 20% of LIFR.
Light northeasterly winds will transition southwesterly in the
afternoon to 4-6 kt. Winds then shift more westerly to northwesterly
between 18z-00z, then shift northeasterly overnight on Wednesday.
Southwesterly flow returns after 13z-14z Wednesday.
29/McMillian
&&
.MARINE...Broad high pressure over the coastal waters will continue
to weaken this morning and the thermal trough will push inland. A
weakening frontal system is expected to push across the waters
Tuesday evening, restoring onshore flow over the waters. In
addition, this system will lead to an increase in winds and wave
heights over the coastal waters. As the wave heights increase over
the coastal waters, seas will become steep with wave heights of 7-12
ft with a dominant period of 7-8 seconds. Northwesterly winds will
increase over the coastal waters as well, with sustained winds of 20-
30 kt possible. The highest wave heights and strongest winds will be
over the outer coastal waters. Seas will begin to subside starting
Wednesday morning below 10 ft and continue throughout the week.
Additional systems are likely to move over the waters, with guidance
suggesting that a strong system will move over the waters on
Saturday, bringing increased winds and seas. Latest guidance
suggests seas may build up to 15 to 16 ft on Saturday and a chance
(30-45%) of gale gusts are possible through the Strait of Juan de
Fuca.
29
&&
.FIRE WEATHER...Offshore flow will continue throughout the
morning today as an upper level ridge shifts eastward over the
region, with occasionally gusty easterly winds through the Cascade
gaps. This will cause poor overnight relative humidity recoveries
across western Washington, particularly along the western Cascade
slopes where RH values will bottom out around 20% to 30%. While
recent rains have kept fuel conditions below critical thresholds,
dry conditions and breezy easterly winds have caused elevated fire
weather concerns through the remainder of the morning today.
Offshore flow today will weaken through the morning, shifting back
to onshore flow in the afternoon. This will bring in more moist
air with much higher humidities and good overnight humidity
recoveries for the remainder of the week. Relative humidities will
continue to improve into the weekend alongside much cooler
temperatures as chances for precipitation increase.
15
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 4 PM this afternoon to 2 PM PDT
     Wednesday for Coastal Waters From Cape Flattery To James
     Island Out 10 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater Out 10 Nm.
Small Craft Advisory until 2 PM PDT Wednesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.
&&
$$

NWS SEW Office Area Forecast Discussion