Union, Washington Weather Forecast Discussion

923
FXUS66 KSEW 161703
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
1003 AM PDT Tue Sep 16 2025
.SYNOPSIS...High pressure and offshore flow will result in much
warmer temperatures today across the region. Gusty east winds
through the Cascades and foothills will result in critical fire
weather conditions as well as the potential for isolated tree
damage. Onshore flow returns Wednesday allowing for temperatures
to return to near normal. Chance for widespread precipitation will
likely return this weekend.
&&
.SHORT TERM /TODAY THROUGH THURSDAY/...Hot, dry and windy
conditions today leading to critical fire weather conditions. Most
areas will see temps in the 80s this afternoon with Moderate
HeatRisk in the Seattle metro. Cooler on Wednesday with a flip to
onshore flow. 33
Previous discussion...As a ridge of high pressure continues to
amplify over the Pacific Northwest, a resultant thermal trough
continues to develop along the coast as an upper low strengthens
over the Gulf of Alaska. This will generate a period of easterly
Cascade gap winds today, with the strongest wind gusts up to 50
mph along the western slopes of the Cascades as well as the
Cascade foothills and valleys between roughly Enumclaw and the
US-2 corridor. Easterly winds will pick up through the morning,
peak mid to late morning, and gradually weaken throughout the
afternoon and evening hours today. While sustained winds are
expected to reach 10 to 20 mph, winds will be gusty, with many
areas seeing gusts of 35 to 45 mph in the late morning. The
highest wind gusts will peak around 50 mph, likely along ridgetops
and through mountain gaps including along the I-90 corridor. This
will result in critical fire weather conditions (see FIRE WEATHER
for more detail), and will also result in a risk of tree damage
and power outages, especially given the current prevalence of
foliage on trees. As such, a Wind Advisory remains in effect for
the central Cascade foothill zones through this evening. The
strong easterly flow will also bring in much warmer air to the
region, with most lowland areas seeing high temperatures in the
80s. The increased easterly flow will also increase the risk of
smoke transport from the east, where several large fires are
producing high amounts of smoke. Temporary reductions in
visibility and air quality are possible, but the details of where
and when will strongly depend on fire behavior over the next 24-36
hours.
By Wednesday and Thursday, the upper level trough offshore will
shift inland to bring a return of much cooler temperatures
alongside onshore flow and partly cloudy skies. Most of the
region will stay dry outside of some sprinkles along the coast
Wednesday.
.LONG TERM /FRIDAY THROUGH MONDAY/...Conditions will stay mostly
dry with near normal temperatures through Friday, outside of a
slight chance of rain over the northwest tip of the Olympic
Peninsula. The next system is on track to enter the Pacific
Northwest later on Saturday, spreading several rounds of rain
across the region through the remainder of the weekend and into
Monday. Ensembles show potential for half an inch of rain across
the lowlands during this timeframe, with around 2 to 4 inches of
liquid over the mountains. The extended outlook also favors above
average precipitation continuing into the week 2 time frame,
suggesting a continuation of cool and wet conditions across
western Washington.
15
&&
.AVIATION...Upper level ridge over Western Washington will slowly
shift east with the ridge axis over Northern Idaho by early
Wednesday morning. Light flow aloft becoming southwesterly late
this morning. In the lower levels thermally induced surface trough
along the coastline with low level offshore flow. Trough shifting
inland later today with low level flow going light tonight.
Mostly clear skies through tonight. Stratus developing along the
coast after 06z spreading inland through Chehalis Valley 09z-12z.
Stratus staying west of Puget Sound Wednesday morning. Gusty east
wind in the Cascade foothills continuing into the afternoon
hours. Winds easing tonight.
KSEA...Mostly clear skies. Northeast winds 6 to 10 knots becoming
northerly 8 to 12 knots after 21z. Winds switching to southeasterly
4 to 8 knots around 06z and to southerly around 12z. Felton
&&
.MARINE...A thermally induced trough of low pressure along the
coast will slowly move inland later today and tonight. A weak
front will dissipate over the coastal waters later tonight. High
pressure will build over the coastal waters Wednesday and remain
into Friday with lower pressure inland. A frontal system will
cross the region over the weekend.
Easterly winds over the area waters today will result in Small
Craft Advisory winds at the West Entrance of the Strait of Juan de
Fuca and the northern portion of the Northern Inner Coastal
Waters this morning, as well as the southern coastal waters
between Point Grenville and Cape Shoalwater into the afternoon.
Winds along the outermost outer waters may also reach SCA criteria
Wednesday afternoon. Winds will ease thereafter with onshore flow
resuming through the remainder of the week.
Seas will range 4 to 6 feet through early Wednesday, building to
8 to 10 feet later Wednesday into Thursday, with steeper seas
during this time with a period of 10 or 11 seconds. Seas will
subside later in the week. Felton
&&
.FIRE WEATHER...Easterly offshore flow will continue to increase
this morning, peaking in the late morning and gradually decrease
throughout the afternoon and evening. Widespread gusts up to 35 to
45 mph are likely in the Cascades and the foothills, with peak
gusts reaching 50 mph or more along ridge lines and through the
Cascade gaps. While relatively humidities will be marginal in the
25% to 35% range, gusty easterly gap winds across the central
Cascades and foothills will bring in warmer and drier air, which
may lead to a quick and large reduction in RH. Critical fire
weather conditions across the Cascades and foothills into tonight
and a Red Flag Warning remains in effect for these areas through
this evening.
Easterly winds will ease by the late evening. Onshore flow will
resume by Wednesday morning providing overnight RH recoveries.
Much cooler air will move into the region, allowing the region to
moisten and leading to much higher minimum relative humidity
values. Onshore flow will continue through the remainder of the
week, with chances for widespread wetting rains as soon as
Saturday afternoon.
15/Felton
&&
.CLIMATE...While not a record high today (record 91 degrees set
in 1967), the forecast high of 85 degrees in Seattle would be the
second warmest September 16th since records started at Seattle
Tacoma airport in 1945. 85 degree plus days are somewhat rare this
time of year with only 22 in the books in the last 80 years for
the last half of September and October (20 in the last half of
September, 2 in October). In the last 15 years there has only
been 2 days after September 15th 85 degrees or warmer in Seattle,
October 16th, 2022 88 degrees (the second warmest October day on
record in Seattle) and 86 degrees on September 28th, 2017.
Felton
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning until 11 PM PDT this evening for Black Hills
     and Southwest Interior Lowlands-Northeast Puget Sound
     Lowlands Generally Below 1500 Feet-Southeast Puget Sound
     Lowlands Generally Below 1500 Feet-West Slopes of the
     Central Cascades Generally above 1500 Feet-West Slopes of
     the North Cascades Generally above 1500 Feet.
Wind Advisory until midnight PDT tonight for Foothills and
     Valleys of Central King County-Foothills and Valleys of
     Pierce and Southern King Counties-Foothills and Valleys of
     Snohomish and Northern King Counties.
PZ...Small Craft Advisory until 11 AM PDT this morning for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-West
     Entrance U.S. Waters Strait Of Juan De Fuca.
Small Craft Advisory until 2 PM PDT this afternoon for Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm.
&&
$$

NWS sew Office Area Forecast Discussion