313
FXPQ50 PGUM 171823
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
423 AM ChST Mon Aug 18 2025

.Marianas Update...
A weak trade-wind trough moving through the region is producing low-
end scattered showers across the Marianas this morning. Although
lightning has not been detected through the night, isolated
thunderstorms remain a possibility as well. The rest of the forecast
remains unchanged for now. An approaching tropical disturbance,
Invest 90W, will bring increased convection to the region Tuesday
into Tuesday night. For now, the forecast maintains scattered showers
with POPs of 40%. If 90W begins to strengthen earlier than expected,
POPs may need to be increased.

&&

.Tropical Systems Update...
Invest 90W is now over eastern Chuuk State. 90W remains disorganized
with a robust trough extending northeast from near 5N149E to 8N154E,
then turns north-northeast to end near 15N159E. Extensive convection
is associated with 90W as it moves toward the northwest. Latest
model guidance shows 90W remaining a sharp trough for the next
couple of days, until it moves west of the Marianas. Invest 90W will
continue to be monitored closely over the coming days for any
indications of development.

&&

.Eastern Micronesia Update...
The trough associated with Invest 90W is now over western
Micronesia. Convection continues within weak convergence to the east
of the trough. Pohnpei is currently seeing isolated showers with the
convergence, however, convection is expected to increase later this
morning. For Kosrae, the ITCZ remains active, though fragmented, over
and east of the island. This will maintain a risk of thunderstorms
through the day with scattered showers moving in near noon. Majuro
can expect mostly cloudy skies with isolated showers. The remainder
of the forecast and marine conditions remain unchanged.

&&

.Western Micronesia Update...
A robust trade-wind trough moving through Palau is generating
scattered showers and isolated thunderstorms over the Republic. This
trough looks to move west through the morning, allowing showers to
decrease to isolated later this morning. This same trough moved west
of Yap. A band of convergence trailing behind the trough will
maintain scattered showers across Yap Proper through the day.
Invest 90W remains the primary player for Chuuk. As the trough moves
through the region, Weno will see numerous showers and isolated
thunderstorms today. As 90W continues northwest, convection will
decrease over Weno this afternoon. The remainder of the forecast
remains unchanged for the rest of the forecast period.

&&

.Prev discussion... /issued 503 PM ChST Sun Aug 17 2025/

Marianas Synopsis...
Local observations shows mostly cloudy skies. Radar imagery shows
isolated showers for CNMI and scattered showers building into Guam`s
coastal waters. Winds are gentle to moderate. Combined seas are 4 to
5 feet.

Discussion...
An unsettled pattern has taken up residence in the Marianas. Tonight
showers are returning to the Marianas, starting with Guam and working
its way northward. This will be a prelude to what is anticipated for
the latter half of this week. A short lull, about 36 hours is
expected for Monday and Tuesday. Showers and thunderstorms are
expected to return for Tuesday night into Wednesday morning and
remain through the latter half of the work week. This next wave of
showers is in anticipation of Invest 90W passing near by and other
trade-wind troughs passing through the region. Drier conditions may
return this weekend.

Marine...
Combined seas have built to 4 to 5 feet and are expected to remain
that high for the first half of the week, then begin to fall by
mid week. Surf has risen along east facing reefs. This has caused
the risk of rip currents to be elevated to moderate. The surf and
risk of rip currents is expected to remain elevated through the first
half of the week, then falling below 5 feet by end of the week.
Overall, benign seas are expected through the forecast period.

Tropical Systems...
JTWC Invest 90W remains a sub-low and continues to be poorly
organized. This morning`s ASCAT showed a sharp trough, with no
indication of a developing circulation. The main indicator for this
system is the 850mb and 700mb vorticity. Current satellite depiction
shows shear-induced convection on the leaning edge of the trough,
which is producing numerous to widespread showers and isolated to
scattered thunderstorms. This current convective pattern shows sign
of a poorly organized system with little to no chance of development
in the next 24 hours. As 90W approached the Marianas in a couple of
days, model guidance shows conditions that could lead to development
of the system. This is mainly due to the TUTT cutting off further
east while an ULAC leadings to upper divergence giving the system a
boost. Nonetheless, Invest 90W is expected to bring showers and
thunderstorms to the Marianas on Tuesday and potentially Yap on
Wednesday. Those in the Marianas or northern Yap State should
continue to monitor forecasts over the next couple of days.

Eastern Micronesia...
An active, but fragmented ITCZ pattern stretches across the Date
Line near 10N, and has two main areas of convection near a couple of
embedded trade-wind troughs. One extends southwestward from the Date
Line to Majuro and another trough farther east near Pohnpei which
contains a weak vorticity feature that is being monitored by JTWC,
known as Invest 90W. This is roughly located near 7N157E, but overall
convection surrounding this feature has weakened in the short-term
and is not expected to bring significant impacts to the region. The
peak in showers and thunderstorms across Pohnpei earlier today has
gradually lessened as the trough pushes off to west this afternoon.
However, model guidance still show some occasional gusts to 25 kt for
Pohnpei and Kosrae tonight, so have kept that in the forecast even
though convectively-driven gusts are less likely. The trough moving
across Majuro is expected to maintain scattered showers through the
evening, lift northwest and weaken slightly. Then a period of
relatively quiet weather is expected Monday through Tuesday for
Majuro. Model guidance show a fragmented ITCZ pattern over the next
few days, then becoming more organized and bringing increasing
showers to all forecast locations starting around midweek.

Buoy and altimetry data show combined seas have increased to 3 to 5
feet near Pohnpei and Kosrae, due to a slight increase in trade
swell. Meanwhile, 4 to 6 foot seas near Majuro continue, temporarily
subsiding to 3 to 5 feet mid to late week.

Western Micronesia...
Isolated showers are seen across Palau while scattered showers are
seen just outside of the coastal waters of Yap and Chuuk. Altimetry
and buoy data show combined seas between 2 to 4 feet across Palau
and Yap, and 3 to 5 feet across Chuuk.

There are two distinct disturbances we are tracking across western
Micronesia. This first is un-designated, but currently has a good
bit of 850mb and 700mb vorticity and healthy upper level divergence
due to an Upper Level Anti-Cyclone (ULAC). Numerous showers and
isolated thunderstorms are seen along the leading edge of this
system with scattered showers further east. These showers and
thunderstorms are already impacting eastern Yap State this afternoon
and will move into Yap Proper this evening and through tonight. PoPs
were increased to numerous for Yap tonight to match the satellite
trends. The forecast is a little more uncertain for Palau, with the
leaning edge of showers not expected until late tonight or early
tomorrow morning. Kept Palau`s forecast scattered with 40% PoPs for
tonight.

The second disturbance is designated at JTWC Invest 90W, which is
rated sub-low. Invest 90W remains weak and disorganized and has
generally trended downward today. That said, numerous showers are
seen across eastern Chuuk State and guidance continues to show an
increase in convection to widespread showers late tonight as 90W
approaches from the east. There was consideration in issuing a plain
text headline for Chuuk for tonight and Monday, but due to
uncertainty in the evolution of 90W, the headline was not included
at this time.

Active weather looks to continue through much of the week for Palau
and Yap due to Invest 90W and a fragmented ITCZ developing to the
south of it. A relative lull will be possible for Chuuk the second
half of the week with isolated to low-end (30%) scattered showers
forecasted.

Little to no changes were made to marine conditions with this
forecast. Combined seas continues to remain benign through the week
except for some occasional choppy seas do to showers tonight and on
Monday across Chuuk.

&&

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

$$

Marianas/East and West Micronesia/Tropical Updates: Kleeschulte
Marianas: Bowsher
East Micronesia: Cruz
West Micronesia: Williams