933
FXPQ50 PGUM 160731
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
531 PM ChST Sat Aug 16 2025

.Marianas Synopsis...
Satellite imagery shows mostly cloudy skies over the region, while
radar imagery shows scattered showers and isolated thunderstorms.
Altimetry and both buoys suggest combined seas of 2 to 3 feet. Winds
are gentle.

&&

.Discussion...
A few passing trade-wind troughs are expected to pass through the
Marianas over the forecast period. This will bring periods of
showers and possible thunderstorms for tonight and Sunday. A break in
the showers is anticipated for Sunday night and Monday. Models
suggest a second slower moving trough will arrive Monday night and
linger through Wednesday. More pleasant weather may return for the
end of next week.

Time-heights show a deepening moisture layer over the next 10 days
with omegas of 5 peaking on Tuesday. A weak upper-level divergence
and passing circulation will lend some aid to these passing troughs.

.Marine...
Light to gentle winds are expected to become gentle to moderate by
Sunday. Combined seas 3 feet are expected to build to 3 to 5 feet by
Monday, then begin to fall by Tuesday. Surf will also rise along east
facing reefs. This will cause the risk of rip currents to be elevated
to moderate. The surf is anticipated to begin falling around Tuesday.
Overall, benign seas are expected through the forecast period.

&&

.Tropical Systems...
JTWC is monitoring Invest 90W, located between Majuro and Kwajalein,
near 7N168E. 90W is currently rated "sub-low" for potential
development. Model guidance keeps 90W embedded within a broad, robust
trade-wind trough, currently moving through eastern Micronesia. Much
uncertainty still exists on the evolution of 90W, specifically when
and where it might develop its own circulation.

&&

.Eastern Micronesia...
A fragmented ITCZ pattern stretches across the Date Line near 8N,
to a broad trade-wind trough that extends across western and
northern Marshalls, diving south to Kosrae and ending at a weak
trough between Pohnpei and Chuuk. The bulk of convection remains near
the aforementioned trough between Kosrae and Majuro, where upper-
level diffluence is helping produce deep convection across the
northern Marshalls, with satellite and surface observations showing
isolated to scattered thunderstorms and occasional strong to near-
gale force gusts. JTWC is monitoring Invest 90W located within this
feature, near 7N168E, and is currently rated sub-low for potential
development. Low-end scattered showers are expected for Pohnpei
tonight increasing to high-end scattered showers for Kosrae and
Majuro due to proximity of the broad trade-wind trough and 90W. As
the overall feature propagates westward, showers increase to numerous
across Pohnpei Sunday and Sunday night. Models are beginning to key
in on gust potentials as this moves into Pohnpei, so have also added
occasional gusts to 25 kt, which will mainly be near heavier showers.
Afterwards, a fragmented ITCZ pattern is expected to prevail across
the region, bringing periods of scattered showers for much of the
upcoming week.

Buoy and altimetry data show combined seas 2 to 4 feet near Pohnpei
and Kosrae, with seas temporarily increasing around 5 feet Sunday
night through Monday night due to an increase in trade swell as well
as gusty showers producing locally higher wind waves Sunday and
Sunday night near Pohnpei and Sunday night for Kosrae. Meanwhile, 4
to 6 foot seas within Majuro waters continue, decreasing to 3 to 5
feet mid to late week, due to a decrease in trade swell.

&&

.Western Micronesia...
Mostly sunny to partly cloudy skies are seen across Palau and Yap
with a few isolated showers present. Partly cloudy to mostly cloudy
skies are seen across Chuuk with scattered showers entering the
eastern coastal waters. Buoy and altimetry data show combined seas
between 2 to 4 feet across the region.

The main two focus with this forecast package are the potential
impacts of Invest 90W for Chuuk and then a monsoon like pattern
developing across Palau and Yap. JTWC designated Invest 90W this
morning and was located near 7.5N, 168.7E at 00z today. Currently
widespread showers and isolated thunderstorms are seen with this
system. Current guidance show little to no development of 90W before
moving into Chuuk State Sunday night. That said, the widespread
convection present on satellite suggests the potential for numerous
to widespread showers and wind gusts up to 30 knots. This feature
will be monitored closely as any additional development may lead to
an SPS issuance for Chuuk. Rain chances will decrease throughout next
week for Chuuk, but scattered showers will persist through Wednesday
with additional troughs behind 90W expected to move through the
region.

Quiet conditions prevail for Yap and Palau and are expected to
continue through tomorrow. By Sunday night, a surface trough looks
to amplify over Palau and Yap and bring scattered showers. The peak
for these showers looks to come from Monday night through Tuesday
night where high end (50%) scattered showers are forecasted. As
Invest 90W advances westward later next week, guidance shows a
monsoon like pattern developing over Palau and Yap. This keeps a wet
pattern over Palau and Yap through the week.

Benign marine conditions continue across the region with combined
seas between 2 to 4 feet. The marine forecast looks to remain fairly
consistent across Palau and Yap with little change from current
observations. For Chuuk, and increase in combined seas are expected
due partly to Invest 90W increasing wind waves, as well as an
increase in trade-swell. Combined seas of 4 to 6 will be possible
Sunday night through Tuesday before diminishing towards the latter
half of the week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.
&&

$$

Marianas: Williams
East Micronesia: Cruz
West Micronesia: Bowsher