699
FXUS66 KSEW 312236
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
336 PM PDT Thu Jul 31 2025

.SYNOPSIS...Chances for showers and thunderstorms will continue
across the Cascades through tonight and into early Friday. Drier
and cooler conditions in store for the weekend. Upper level
troughing will develop Sunday night into next week, bringing in a
cooler pattern with chances for showers.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...This afternoon`s satellite
imagery tells the story for today. Water vapor imagery shows two
main areas of vorticity, one over east-central Washington near
where there is currently a line of thunderstorms moving across the
upper Columbia Basin, another near the OR-NV-CA tri-point border
where thunderstorms are beginning to bubble up. Over western
Washington, most areas west of the Cascades are fairly clear with
some high level clouds overhead. The southern two-thirds of the
Cascades are stuck under a thick mid-level cloud deck with some
scattered showers while the northern third remains clear with
convection starting to initiate. Thunderstorm chances will remain
best across the northern third of the Cascades through this
afternoon, but the overall thunderstorm threat cannot be
diminished just yet as the upper level wave continues to track
northward. However, models have been backing off on the amount of
thunderstorms possible tonight into Friday morning. We will
continue to monitor and update as needed, but for now, the Flash
Flood Watch will continue to remain into effect through late
tonight as any thunderstorms that do develop will have high rain
rates that may threaten burn scars, especially our most sensitive
scars in the North Cascades. Showers may continue into the
overnight period and shift more towards the foothills/interior
Puget Sound, but this is a more low-confidence scenario as showers
should weaken as they do so.

The thunderstorm threat will shift mostly east of the Cascades on
Friday as the system begins to depart to the northwest, but
chances for additional showers and storms across the far eastern
portions of the North Cascades will again be of concern.
Elsewhere, it is likely that dense fog will redevelop across the
area waters, so some patchy dense fog may be possible along the
immediate coast Friday morning.

The upper level flow pattern will shift to be southwesterly Friday
night, allowing for cooler, more stable air to filter into the
region, allowing us to return to the more typical, summertime
pattern of morning stratus/patchy fog and afternoon sun.
Temperatures will decrease a few degrees each day, from the low
80s Friday to the mid 70s on Sunday. Lows will be in the 50s.

.LONG TERM /MONDAY THROUGH THURSDAY/...The upper level pattern
will become more dominated by troughs as one looks to move
through Sunday night into Monday, with several additional systems
possible through mid to late next week. Overall, this indicates a
period of slightly below normal temperatures (in the low to mid
70s) and lows in the 50s. Highest QPF amounts look to be in the
mountains and the Pacific Coast with lower amounts across the
interior.

62

&&

.AVIATION...VFR area-wide this afternoon and will prevail for the
rest of today for the interior terminals. Another marine stratus
push is expected Friday morning. However, this push doesn`t appear
to be very strong, so MVFR ceilings will may be limited to the
coast. The latest guidance has 25-40% of seeing MVFR ceilings for
the interior terminals. Along the coast IFR and LIFR ceilings are
possible, with probabilities around 65% and 35%, respectively.
Improvement into VFR is possible after 21z along the coast.

Showers/thunderstorms will continue to develop over the Cascades
this afternoon/evening. However, not expecting any significant
impacts to the terminal, as guidance has thunderstorm activity
primarily confined to the Cascades. May see some light rain showers
impact the interior terminals along Puget Sound, but confidence is
not high enough to include them in the prevailing line.

KSEA...VFR expected for the rest of today. Guidance hints at a 25-
30% chance of seeing MVFR ceilings Friday morning. Otherwise, VFR
ceilings will prevail Friday with mid to high clouds. Not expecting
thunderstorm activity over the Cascades to impact the terminal this
afternoon. May see some light rain showers. Winds this evening will
be NW this evening at 5-6 kt. Guidance hints that winds may become
N/NE for a couple hours between 2z-6z. Surface winds will become SW
overnight between 8z-9z, with speeds of 4-6 kt.

MGF

&&

.MARINE...High pressure over the coastal waters will interact with
lower pressure inland, maintaining onshore flow through the weekend.
Westerly pushes through the Strait of Juan de Fuca will occur each
evening. A Small Craft Advisory is currently in effect for the
Central and East entrances of the Strait through Friday morning.
Additional, small craft pushes are possible this weekend. A weather
system will move over the waters late Monday night into Tuesday.

Fog is expected to develop again over the coastal waters, with
visibilities less than 1 NM likely. Thus, a Dense Fog Advisory has
been issued for the coastal waters to go into effect tonight through
Friday morning. Localized dense fog is also possible in the Strait
of Juan de Fuca Friday morning.

MGF

&&

.FIRE WEATHER...Southerly flow aloft with unstable air mass over
the Cascades and the interior of Western Washington tonight into
Friday. So far this afternoon thunderstorms have been mostly east
of the Cascades with just a few strikes west of the crest near
the Canadian border. Thunderstorm chances continue tonight for the
Cascades with a slight chance in the Cascade foothills. Storm
motions will be slow. PW values in the 1.2 to 1.4 inch range.
While widespread wetting rains are not expected, flash flooding
could pose a threat tonight if a storm stays over a burn scar.
Western Washington has seen thunderstorms come off the Cascades on
this date. On July 31, 1984 storms made it into the Seattle area
in the evening with some well know photographs taken during the
event.

Thunderstorm chances decrease Friday. PW values decrease as well,
down to 0.7 to 0.9 inches so any storms that develop Friday will
not include heavy showers. Thunderstorms chances are low enough to
not have any headlines in the fire weather forecast.

Low level flow turning onshore Saturday with the flow aloft
becoming southwesterly. This will bring about an end to the
convection threat in the mountains. Low level onshore flow
continuing through the middle of the week with slightly below
normal temperatures with good humidity recoveries overnight. Upper
level trough could produce some showers in the middle of next
week. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flash Flood Watch through late tonight for Cascades of Pierce
     and Lewis Counties-Cascades of Snohomish and Northern King
     Counties-Foothills and Valleys of Snohomish and Northern
     King Counties.

PZ...Small Craft Advisory until 1 AM PDT Friday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$