575
FXUS66 KSEW 212121
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
221 PM PDT Sun Jun 21 2026
.SYNOPSIS...
Strong high pressure will build under a ridge across western
Washington early this week. Temperatures will increase going
into Monday and Tuesday, with highs into the 80s and 90s for
many in the interior. Moderate HeatRisk will remain a concern
for those spending significant time outside. Additionally, dry
air with a weak offshore and thermal trough pattern will lead to
elevated fire concerns for grassy and shrubby areas that are
dry. The second half of the week is significantly cooler with
the first chance of precipitable moisture in a couple weeks,
with breezy conditions at times.
&&
.SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/...
Western WA remains quite dry this afternoon, with the surface
pattern remaining quite neutral in between lower pressure to the
east, and higher pressure to the west. A few high clouds on
satellite are moving east across the interior, with some clouds
rolling south from B.C., and a large area of stratus offshore.
Highs today remain in the upper 70s to low 80s in the interior
(but may come in a few degrees cooler due to the high clouds).
The coast will stay in the 60s. Lows tonight drop into the low
to mid 50s.
An upper level ridge offshore will begin to move inland tomorrow
and Tuesday, with the thermal trough setting up in the interior
(rather than the coastline). As heights build with more sinking
motion provided by the higher pressure, temperatures will warm
quite a bit going into Monday and Tuesday, with Tuesday being
the warmest day this week. There remains a heat advisory across
a majority of the lowlands (including Cascade valleys, islands,
and all but the immediate Washington/Pacific Coast and
mountains). This goes into effect 11 am Monday and expires for at
11 PM Tuesday night (except for Puget Sound which goes through
11 PM Wednesday night). Temperatures will be quite warm
(especially on Tuesday), with widespread highs into the upper
80s and low 90s. HeatRisk is moderate for much of the interior,
and a couple isolated pockets of major HeatRisk can`t be ruled
out for portions of Puget Sound/Seattle metro (although the
major likelihood remains at 20-30%). Overnight lows will
increase into the upper 50s, with low 60s in the Seattle area.
Those spending time outdoors next few days should take breaks in
the shade, or in an air conditioned room when possible, stay
hydrated, and wear sun-screen and loose fitting clothing to
minimize the potential for heat-related illness.
&&
.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Beginning Tuesday night into Wednesday, the onshore gradients
are expected to increase significantly as the ridge moves
inland, which should increase cloud coverage significantly going
into Wednesday. There remains some model disagreements into how
warm it will be exactly going into Wednesday (especially given
the timing of the onshore push), but the message of moderate
HeatRisk continuing into Wednesday for Puget Sound remains
(especially given the overnight lows are the warmest this
morning). Thus the heat advisory will continue for now into
Wednesday, but there remains a descent spread into the
temperatures for the first onshore push (and some limitations of
the models handling the first push).
As the ridge flattens and move east, ensembles bring in a cooler
pattern as the flow transitions to zonal, and eventually a
trough pattern going into Friday for the region as a low drops
down from the Gulf of Alaska. Highs will cool into the 60s and
70s for much of the interior, and an increase of moisture from
the trough/Pacific will lead to the first chance of rain/showers
in a couple of weeks. The wettest day appears to be Friday with
at least a 10-15% chance of thunder with this activity.
Winds will become breezy at times in this part of the forecast.
The southwest interior/Strait of Juan de Fuca areas will see the
winds first Wednesday, as well as the immediate ridge areas of
the Cascades. Friday and Satuday`s front may result in more
widespread winds for the interior. While winds gusting over
20-25 mph appear likely, probabilities for over 30 mph remains
low.
HPR
&&
.AVIATION...
VFR regionwide this afternoon as a few high clouds pass over the
terminals this afternoon and evening. Note that stratus remains
offshore at MVFR criteria. Winds will shift from the west to the
north this afternoon around 5-10 kt, shifting to northeast at 5
kt or less overnight. Higher pressure will build over the
terminals Monday under a ridge, with flow weakening aloft.
Due to the warmer temperatures, expect an increase in density
altitude going into the afternoon on Monday. The probability for
low ceilings remains confined to the Washington/Pacific Ocean
coastline, and the south Cascades, where a 50-70% chance of IFR
ceilings remain during the overnight and early morning hours.
Otherwise, expect VFR elsewhere into tomorrow with winds
switching back to the north/northwest 5-10 kt and a few gusts
to 18-20 kt possible in the afternoon.
KSEA...VFR expected through the TAF period. Winds Sunday/Monday
afternoon will continue out of the northwest at 5-10 kt, with a
few gusts to 18 kt possible Monday afternoon. Overnight winds
will switch to northeast at around 5 kt.
HPR
&&
.MARINE...
High pressure will increase offshore early this week as an upper
level ridge moves inland. The immediate coastline and coastal
waters will likely continue to see north/northwest winds due to
the thermal trough setting up closer into the interior. The
remaining inland areas will see a mix of light northwest to
northeast winds with the pattern through Tuesday night. Onshore
will return Tuesday night, with a trough also dropping down with
a front by late week. The next best chance of winds over 20 kt
will come late Tuesday in the outer coastal waters, with a
strong push likely in the Strait of Juan de Fuca Wednesday into
Thursday (30-50% chance of gale gusts with this push). The late
week front will have the potential to produce widespread winds
across most waters with gusts over 20 kt.
Seas 4-6 ft through the end of the week, the 7-9 ft seas with
short periods of 7-8 seconds with the frontal system late this
week.
HPR
&&
.FIRE WEATHER...
High pressure will increase inland under a ridge early this
week. This ridge combined with a thermal trough setting up
inland in the interior will result in a few days of elevated
fire conditions via warm, dry, and conditional unstable air
across the region. The offshore flow in the interior/Cascades
will result in humidity values dropping into the twenties, and
even the teens going into Monday, Tuesday, and potentially
Wednesday. Fuels are approaching critical levels, but the main
concern remains dry grasses or shrubs, or dead piles of fuels
where brush fires may be able to start (as seen the past couple
days). Onshore flow will return late Tuesday night and make it
inland on Wednesday with improved moisture, and the first chance
of wetting rain in a couple of weeks for much of the region.
HPR
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Heat Advisory from 11 AM Monday to 11 PM PDT Tuesday for
Eastern Strait of Juan de Fuca-Foothills and Valleys of
Thurston and Lewis Counties-Foothills and Valleys of the
North Cascades-Foothills of the Western and Southern
Olympic Peninsula-Island County-Lower Chehalis River
Valley-Lowlands of Lewis and Southern Thurston Counties-
Lowlands of Western Skagit and Northwestern Snohomish
Counties-Lowlands of Western Whatcom County-Middle
Chehalis River Valley-Northern Hood Canal-Olympia and
Southern Puget Sound-Port Townsend Area-San Juan County-
Southern Hood Canal-Willapa and Black Hills.
Heat Advisory from 11 AM Monday to 11 PM PDT Wednesday for
City of Seattle-Downtown Everett / Marysville Area-
Eastern Kitsap County-Eastside-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-Lowlands of Pierce and
Southern King Counties-Shoreline / Lynnwood / South
Everett Area.
PZ...None.
&&
$$
NWS SEW Office Area Forecast Discussion