663
FXUS66 KSEW 061020
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
318 AM PDT Sat Jun 6 2026
.SYNOPSIS...
Low pressure will move directly over western Washington today,
bringing widespread showers with the chance for isolated
thunderstorms this afternoon and evening. Any storms that develop
will be capable of lightning, gusty winds, and brief periods of
heavy rain. While the threat for thunder ends this evening, a
generally troughy pattern will keep weather cool and unsettled for
the first half of the upcoming week. A warming and drying trend
takes hold late next week.
&&
.SHORT TERM /TODAY THROUGH MONDAY/...
Current radar imagery continues to show scattered showers over
portions of the CWA. Rainfall rates associated with any of these
cells is generally light, although some cells over portions of
Whatcom and Skagit counties are producing brief periods of moderate
rainfall. At this time, thunder remains a non-issue.
That will not remain the case, however, as the potential begins to
increase as the upper level low begins to move over W WA beginning
late this morning. General instability associated with the leading
edge of such a feature coupled with cold air aloft will certainly
lead to another day of thunderstorm potential...and models and SPC
outlook has rightly locked on to this. However, there are some
offsetting factors that look to limit the amount of activity that
may spring up this afternoon and early evening. First, while there
is cold air aloft, the air at the surface is not terribly toasty
either as daytime highs are progged to remain largely in the mid to
upper 50s. In turn, these cooler surface conditions will keep any LI
and available CAPE pretty low. These offsetting factors actually
serve to help highlight where the better chances for thunder will
emerge...mainly around Puget Sound, the south Sound area and the SW
interior/Chehalis valley. NBM ProbThunder values echo this, but
might be a little too generous...at times indicating a 30-40 pct
chance for thunder in these areas. To take into account offsetting
factors, trimmed to allow 30 pct to be the roof and even that being
limited to the 2-6 PM PDT window. This upper low ejects out of the
area overnight tonight, taking the bulk of activity with it,
although and potential for thunder looks to wrap up by sunset or
maybe an hour after.
A shortwave ridge following in the wake of the aforementioned low
will bring a few degrees of warming as daytime highs creep into the
upper 50s to lower 60s but is not strong enough to remove much of
the moisture in the area. Clouds look to remain in place, albeit
with some breaks...and even an isolated shower or two will be
possible, mainly in the Cascades.
A frontal system reaches the coastline Sunday night but hesitates to
push inland until Monday afternoon...when the best chances for
rainfall emerge. Temps take a minor hit to reflect this, only
dropping a degree or two from Sunday`s highs, but still remaining
within the upper 50s to lower 60s. No thunderstorm threat appears to
accompany this frontal passage at this time.
18
&&
.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
A series of upper level lows will keep troughy conditions over W WA
into the first half of the upcoming week...and as such keep cooler
temperatures and the chances for precipitation in the forecast. As
these seem to follow one right after the other, it will be difficult
to differentiate from rains associated with the upper low exiting
Monday night and the fresh system entering into the area Tuesday
into Wednesday. Latest forecast and model data suggest a brief dip
in PoPs Tuesday night into early Wednesday morning, but will need to
monitor future runs to see if consistency will build confidence in
this window or if it shifts about both in areal terms and
temporally. Cooler daytime highs will remain the rule for both of
these days with Tuesday still ranging in the upper 50s to lower 60s
while Wednesday sees a slight warming with lower 60s area-wide.
As this all happens over land, upper level ridging will continue to
build over the eastern Pacific. Once this feature starts moving
eastward Wednesday night, models do show it beginning to scour out
any lingering moisture...although how quickly varies model to model.
Wide agreement that Thursday and Friday will be dry thanks to this
feature as its impacts spread eastward and the warming trend for
these two days starting to show signs of being able to leap over
tall buildings in a single bound. Daytime highs Thursday see plenty
of upper 60s to lower 70s for the interior and Friday jumps further
with mid 70s to near 80 expected...making up the distance from below
normal to above normal very quickly.
18
&&
.AVIATION...
Southwesterly flow aloft will become more westerly and then
northwesterly throughout the day as an upper level trough moves
inland across the Pacific Northwest. Scattered shower activity
continues across western Washington early this morning and will
continue throughout the day with isolated chances for thunderstorms
possible again this afternoon and evening. The greatest
probabilities of thunderstorm development remain along Puget Sound
and the Chehalis Valley, however confidence in extent of activity
remains too low for inclusion in any single TAF at this time.
Current conditions at area terminals remain largely VFR early this
morning, but could see conditions lower to more widespread MVFR
conditions across the area terminals between 12-15Z.
Conditions look to largely rebound to VFR by this afternoon, though
brief reductions to MVFR conditions will still be possible within
any heavier shower activity. Latest hi-res guidance hints at a weak
convergence zone developing across the central Sound between 03-06Z
tonight as onshore flow increases. SW winds persist between 4-8 kt
this morning and will increase to 8-12 kt by late this morning.
Winds may briefly become variable and shift to the north across the
central Sound within the vicinity of the PSCZ tonight.
KSEA...VFR conditions will lower to MVFR for a couple hours between
12-15Z. Showers will be possible at the terminal though the day,
with isolated thunderstorms possible this afternoon and evening,
generally between 19-00Z. A PSCZ may sink southward into the
terminal tonight between 03-06Z and could bring a brief reduction in
ceilings and visibilities to the terminal, as well as a quick shift
of winds to the north. Otherwise, expect southwest winds to persist
throughout much of the day and increase to 8-12 kt later this
morning, before easing again late tonight.
14
&&
.MARINE...
A trough of low pressure will move onshore today, with increasing
onshore flow expected along the central and eastern Strait of Juan
de Fuca this afternoon and evening. A few isolated thunderstorms
will be possible across the area waters this afternoon and evening
which could briefly bring down visibilities.
A weak surface ridge will then traverse the waters on Sunday, before
another frontal system moves into the waters on Monday. Southerly
winds will increase across the coastal waters Sunday night into
Monday, and will likely bring the next round of headlines to the
area waters. Onshore flow will increase in the system`s wake with
Tuesday`s push of westerly winds looking like the strongest at this
time along the Strait. Onshore flow will continue into Wednesday as
high pressure builds back into the coastal waters. A weak system may
then move across the area waters on Thursday.
Seas will generally range between 6-8 ft today, before briefly
subsiding towards 4-6 ft on Sunday. Seas then look to build back to
6-8 ft Tuesday and persist at this range through Thursday.
14
&&
.FIRE WEATHER...
Although lightning will be possible today and rainfall amounts may
not quite make it up to the 0.25 in threshold separating wet and dry
storms, Min RH values exceeding 60 pct should limit any fire weather
concerns today. As the generally troughy pattern remains in place
over W WA in the first half of the upcoming week, again, some
limiting moisture is expected to remain in place.
As upper level ridging builds into the area starting Thursday,
conditions, including fuels, will need to be monitored as
temperatures are expected to climb to above normal values entering
into the week 2 time period.
18
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&
$$
NWS SEW Office Area Forecast Discussion