204
FXPQ50 PGUM 111057
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
857 PM ChST Mon Aug 11 2025

.Update...
So far, showers and thunderstorms have remained isolated across
Rota, Tinian and Saipan this evening. However, The influence of the
TUTT has caused isolated thunderstorms to develop just west of Guam
and led to a slight increase in showers over the past couple of
hours. Updated the forecast for Guam to add scattered showers and
isolated thunderstorms for the rest of the evening, but hi-res model
guidance continues to suggest that shower coverage will decrease
after midnight. Will continue to monitor conditions to what how the
atmosphere evolves and if it starts to drift from what models are
predicting.

&&

.Prev discussion... /issued 451 PM ChST Mon Aug 11 2025/

Marianas Synopsis...
Partly cloudy skies with only spotty showers and light to gentle
winds prevail across the Marianas this afternoon. Buoys and
altimetry show seas between 3 and 4 feet.

Discussion...
A drier pattern is now in place across the Marianas. This is due to a
weak upper-level low developing to the west of the islands, placing
upper-level convergence over the area. This has inhibited convection
across the islands. As this upper low strengthens overnight and
drifts west, it will allow weak divergence to move over the CNMI,
with isolated thunderstorms expected to move into the coastal waters
during the evening. The divergence looks to remain over the Marianas
through midweek, maintaining the risk of thunderstorms through
Wednesday. For Guam, light and variable winds with little cloud cover
will result in island convection Tuesday and Wednesday afternoons.
If the winds remain light and variable Thursday and possibly Friday
as the models suggest, island convection could continue for a couple
more days. Other than the island convection, an overall dry pattern
is expected through the beginning of next week.

Marine...
Sea conditions remain benign. Light to gentle winds are expected
through the week. Combined seas of 3 to 4 feet will slowly subside,
dropping to between 2 and 3 feet Tuesday night. An upper-level low
will produce isolated thunderstorms through Wednesday.

Tropical Systems...
At 1:00 pm, Tropical Storm was centered southeast of Okinawa near
21N133E. Podul is moving west across the Philippine Sea at 14 mph.
Podul is expected to make a slight turn toward the west-northwest
with little change in forward speed. Max winds have increased to 65
mph. Podul is still expected to slowly strengthen, possibly becoming
a typhoon some time Tuesday. For more information on Podul, please
refer to bulletins from the Joint Typhoon Warning Center under WMO
header WTPN32 PGTW and from the National Weather Service under WMO
header WTPQ32 PGUM.

Eastern Micronesia...
A band of trade-wind convergence leading into an east-west oriented
trough is producing scattered to numerous showers and isolated
thunderstorms across the northern Marshalls and the northern FSM.
Patchier showers are found within a light and variable wind pattern
to the south, including the main forecast locations. The
aforementioned trough will continue to weaken, with light to gentle
trades returning tonight for Majuro and Tuesday for Pohnpei and
Kosrae. A fairly quiet pattern is expected through much of the week,
then ITCZ showers may start to rebuild starting near Majuro late
week.

Marine conditions look to remain benign through the week. Combined
seas are expected to remain between 3 and 5 feet near Pohnpei and
Kosrae, comprised of light trade swell and longer-period southeast
swell. Combined seas of 4 to 6 feet are expected near Majuro,
comprised of mainly trade swell. Light and variable winds across
Pohnpei and Kosrae become light to gentle easterlies Tuesday and
increase to light to moderate over the weekend. At Majuro, light to
gentle northeast to east trade winds increase to light to moderate
late week as the ITCZ begins to build.

Western Micronesia...
Satellite observations show scattered showers along the northern
coastal waters of Palau and isolated showers across Yap and Chuuk.
Altimetry and buoy data show combined seas between 2 to 4 feet across
the region.

The biggest change with this forecast package was to introduce
numerous showers to Palau`s forecast. Convergent flow has begun to
set up across Palau, which will help lead to shower and thunderstorm
development tonight. This forecast leans closer to the GFS based on
ongoing satellite trends. The current depiction of isolated to low-
end (30%) scattered showers seen over Palau this afternoon more
closely matches the 00z GFS than the 00z ECMWF, which shows ongoing
numerous showers already occurring over Palau during the day today.
The ECMWF solution would likely limit shower and thunderstorm
development due to stratification of thunderstorms and used up
atmospheric energy. The forecast for tonight is a blend 80% GFS and
20% ECMWF due to the real-time error in the ECMWF`s forecast. The
main challenge will be the timing of the numerous showers for Palau.
Both the GFS and ECMWF show showers persisting into tomorrow morning
before dissipating in the late morning to early afternoon period. The
rest of the day looks to be dry, so the forecast starts with
numerous showers and then becomes isolated in the afternoon. Looking
ahead towards later in the week for Palau, a lull in shower activity
is expected with isolated to low-end (30%) scattered showers
forecasted from Tuesday night through Thursday night. Late this week
and into the weekend looks to bring increased shower activity as a
Near Equatorial Trough (NET) forms south of Palau.

Much quieter conditions are seen across Yap and Chuuk. For Yap, there
may be a brief period of scattered showers late this evening, but due
to the brief window for these showers and the current isolated
showers observed, the forecast kept shower coverage isolated for
tonight. Shower coverage looks to increase starting Tuesday night due
to surface convergence between a surface trough and a surface ridge.
Low-end (30%) scattered showers look to continue across Yap through
the week. Meanwhile, Chuuk looks to remain isolated to low-end
scattered showers through much of the week except for Wednesday night
when a fragment of the ITCZ may briefly flare up. A more prominent
increase in ITCZ activity looks to impact Chuuk later this weekend.

Little to no changes were made to the marine forecast. Benign
combined seas of 2 to 4 feet look to continue through the week. Some
choppy waters may be possible under heavier showers near Palau
tonight.

&&

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

$$

Update: Schank
Marianas: Kleeschulte
East Micronesia: Cruz
West Micronesia: Williams