820
FXUS66 KSEW 152240
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
340 PM PDT Mon Sep 15 2025

.SYNOPSIS...High pressure and offshore flow will result in much
warmer temperatures Tuesday 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 /TONIGHT THROUGH WEDNESDAY NIGHT/...A ridge of high
pressure is amplifying across the region today. This is resulting in
quiet conditions today. However, between the development of a
thermal trough and an approaching trough off the coast, a period of
easterly winds is expected between this evening and Tuesday night.

The main areas of concern are the western slopes of the Cascades as
well as the Cascade foothills and valleys between roughly
Enumclaw and the US-2 corridor. The main timeframe of concern is
very early Tuesday morning through Tuesday evening. Easterly
winds are expected to increase early morning, peak mid to late
morning, then gradually diminish Tuesday afternoon and evening.
While sustained winds are expected to reach 10-20 mph, winds
will be gusty, and wind gusts of 35-45 mph are likely through
these favored central Cascade areas. Gusts of 40-50 cannot be
ruled out along ridgetops and through mountain gaps like the
I-90 corridor. This will result in critical fire weather
conditions (see FIRE WEATHER for more detail), but it 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 was issued for the central Cascade foothill zones.

This easterly flow will also increase the risk of smoke transport
from the Cascades and eastern Washington into western Washington, as
as transport from local fires. Temporary reductions in visibility
and air quality will be possible, however, the details of where
and when will strongly depend on fire behavior over the next 24-36
hours.

Finally, this pattern will also result in a very significant jump in
temperatures Tuesday, with most lowland locations seeing highs in
the 80s. Areas of moderate (orange) HeatRisk will be present but
given the short duration and limited extent, no heat products appear
to be necessary.

By Wednesday, the trough will reverse winds, resulting in the return
of onshore flow, lesser winds, cooler temperatures, and improving
air quality.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...The aforementioned trough
struggles to make it on shore through the week, generally shearing
apart offshore. This will keep onshore flow going and temperatures
mild through the remainder of the workweek. By this weekend,
another trough and associated frontal system will move towards
the region, likely bringing a good chance of widespread
precipitation starting sometime Saturday or Sunday. The extended
outlook favors above average precipitation continuing into the
week 2 time frame.
-Wolcott-

&&

.AVIATION...Upper level ridge over Western Washington tonight will
light flow aloft. Ridge shifting east Tuesday with flow aloft
becoming southwesterly. In the lower level thermally induced
surface trough moving up coastline tonight with low level flow
going offshore. Trough shifting inland Tuesday with low level
flow turning onshore late in the day.

Mostly clear skies through Tuesday with patchy valley fog in wind
sheltered areas 13z-17z. Gusty east wind in the Cascade foothills
developing late tonight and continuing into Tuesday morning.
Easterlies easing Tuesday afternoon.

KSEA...Mostly clear skies. North winds 8 to 12 knots turning
northeasterly 4 to 8 knots around 06z. Possible winds get all the
way around to easterly Tuesday morning 14z-18z. Felton

&&

.MARINE...A thermally induced trough of low pressure will expand
up the Washington coastline tonight then move inland later
Tuesday. A weak front will dissipate over the coastal waters
Tuesday night. High pressure will build over the coastal waters
Wednesday and remain into Friday with lower pressure inland. A
frontal system will reach the area this weekend.

Small craft advisory easterly winds at the West Entrance to the
Strait of Juan de Fuca and the northern portion of the Northern
Inner Coastal Waters.

Seas will range 4 to 6 feet through early Wednesday. Seas then look
to build to 8 to 10 feet later Wednesday into Thursday, with steeper
seas during this time due to a period of 10 or 11 seconds. Seas
will subside late week. Felton

&&

.FIRE WEATHER...Easterly offshore flow will increase this evening and
peak Tuesday morning. While relatively humidities will be marginal,
gusty easterly will develop, particularly across the central Cascades
and foothills late tonight through Tuesday morning leading to critical
fire weather conditions for the Cascades and the foothills through
at least late Tuesday afternoon and possibly into the evening.
Easterly winds easing Tuesday evening with the low level flow
turning onshore Wednesday giving the area cooler conditions, much
lighter winds, and much higher minimum relative humidity values.
Onshore flow continuing through Friday. The next chance for more
widespread precipitation late Saturday into Sunday. This system
has the potential to produce wetting rains especially over the
northern portion of the area. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from 5 AM to 11 PM PDT Tuesday 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 from 3 AM Tuesday to midnight PDT Tuesday night
     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 from 6 AM to 11 AM PDT Tuesday for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$