163
FXPQ50 PGUM 281754
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
354 AM ChST Sun Jun 29 2025

.Marianas Update...
The main forecast challenge was deciding to lower shower coverage to
scattered at Tinian and Saipan, or retain the current numerous
wording. The midnight model runs didn`t offer much help in deciding
this, which is of no surprise given two tropical disturbances in the
area (see tropical section below). Time heights do show deep
saturation to 50K feet or more, with episodic bursts of positive
omega across the region, so we opted to leave the current forecast as
is, with the main updates to precipitation amounts and temperatures
being made.

&&

.Tropical Update...
The Joint Typhoon Warning Center (JTWC) opened up Invest 99W, located
northeast of the Marianas at 15.6N 148.9E, which is ~215 miles ENE
(east-northeast) of Saipan. We also have Invest 98W located at 14.1N
138.7E, roughly 475 miles WSW (west-southwest) of Saipan.

After reviewing the latest model data, we strongly agree with JTWC
that both Invests (rated as "sub-low" on both, meaning no significant
organization is expected during the next 24 hours) will mainly act
to strengthen a slowly-developing WSW monsoonal flow that extend
across northern Palau near Koror, Yap, and at least the southern
portions of the Marianas over the next 24 to 48 hours, gradually
backing across Guam and the CNMI, while maintaining a similar
orientation across northern Palau and Yap. As it strengthens Monday
into Tuesday, increasing sustain winds are possible, but these winds
look to remain below 20 knots (TD - Tropical Depression) strength is
25 knots).

This pattern will be conducive for periods of locally heavy rainfall,
with gusty winds probable in the heavier showers or thunderstorms
that develop. However, temporal proximity is difficult to determine
until the systems become better organized, helping to act as a
focus/foci for enhanced surface convergence and entrainment. Stay
Tuned!

&&

.Eastern Micronesia Update...
Satellite shows fairly quiet conditions across the region. CIMSS
MIMIC TPW product showing atmospheric moisture above 2 inches,
reaching up to 2.40 inches near Pohnpei, although not much in the way
of significant surface features this morning. Convergence southeast
of a weak tropical disturbance, Invest 98W, looks to increase
scattered showers over Pohnpei starting later this morning and over
Kosrae by the afternoon. Overall, no major changes were needed for
the forecast.

Overnight scatterometry data shows light to moderate winds across
the region. Buoy and altimetry data reveal seas near 4 to 6 feet.

&&

.Western Micronesia Update...
Palau and Yap:
The main forecast updates were to lower temperatures at Palau
(especially near Koror) and Yap, given the expected cloud density
and high-end scattered shower coverage at Yap and numerous showers at
Koror. We thought about bringing Yap to numerous coverage as well,
but will defer to the day shift to give it (the forecast) another
model run or two, with day shift also getting a better idea via
visible satellite imagery.

Chuuk:
No significant changes were made, at this time, to your forecast.
However, dependent on shower coverage and satellite trends, a late
morning update to lower temperatures a degree or two is possible.

&&

.Prev discussion... /issued 531 PM ChST Sat Jun 28 2025/

Marianas Synopsis...
Gentle southwesterlies prevail across the region. Satellite imagery
and Doppler radar indicate slow-moving cells carrying light rain
across much of the Marianas. Buoys indicate seas are between 3 and 4
feet.

Discussion...
A newly-developing Invest 98W well west of the Marianas, is expected
to invigorate the wet southwesterly monsoon pattern that is
returning to the region. As 98W starts moving along the leading edge
of the monsoon trough north-northeastward, this will create a
convective eastern flank that will envelop the Marianas under
significant showers and thunderstorms once again beginning Sunday.
Elevated shower and thunderstorm conditions along with higher
southwesterly winds are expected through at least Tuesday, before a
ridge builds in from the south, allowing the winds to weaken and
showers to diminish by the middle of next week.

Marine...
Combined seas of 4 to 5 feet look to increase a foot or two by
Tuesday as a southwest monsoon swell and winds build into the
region. A moderate risk of rip currents is expected along east facing
reefs of the Marianas for at least the next several days, with rip
current risk to briefly become moderate along west reefs starting
Tuesday.

Hydrology...
While showers diminish slightly overnight, a developing monsoon
disturbance well to the north of the region will allow a wet
southwesterly pattern to develop across the region by Sunday night,
bringing a higher likelihood for flash flooding over the next
several days. As such, the Flash Flood Watch might be extended and
potentially include Tinian and Saipan, if conditions warrant. In the
meantime, 3 to 5 inches are still expected to fall over Guam and Rota
in the next 36 hours, especially late Sunday and Sunday night.

Tropical systems...
Two tropical disturbances are seen across the region. Invest 97W no
longer has an identifiable circulation center and is roughly located
northwest of the Marianas with little convection evident with the
system. The Joint Typhoon Warning Center (JTWC) has downgraded 97W to
sub-low, meaning it is not expected to develop into a significant
tropical Cyclone.

Invest 98W, centered west of the Marianas near 14N139E, shows more
organization than 97W, with moderate to deep convection to the south
of the center. This is now the circulation that the models are
favoring for development over the next few days. Models are in pretty
good agreement that 98W will slowly strengthen over the next several
days as it drifts northward. Beyond that, models diverge, with some
showing 98W moving rapidly north then northeast, while others
maintain a north to northwest drift a bit longer. Either way, it is
not likely that 98W will make any significant impact to the Marianas,
though it could impact the far northern CNMI. 98W will continue to
be monitored for any indications of strengthening or changes in
expected movement over the next few days.

Eastern Micronesia...
For Pohnpei, a trade-wind trough has become quasi-stationary to the
east of Pingelap. This trough is currently in a weakening phase and
is expected to re-invigorate over the nighttime hours as it
gradually moves westward. A wet trade-wind pattern is anticipated
through next week.

For Kosrae, will see showers clearing up over the next 12 to 36
hours before the next sharp trough moves in bringing with it the
another round of showers. Showers are expected to remain through
most of next week.

Majuro is expected to clear up as the trade-wind trough continues to
break down over the next day or so. Off again, on again showers are
expected through most of next week as weak passing trade-wind
troughs move through the area.

Altimetry shows combined seas of 4 to 6 feet near Majuro. Kalo and
north point buoys show 4 to 5 foot see heights. Winds are expected to
be gentle to moderate through the middle of next week, diminishing
to light to gentle for the latter half of next week. Overall, benign
seas are expected.

Western Micronesia...
The monsoon trough remains the main player across far western
Micronesia for the next few days. A monsoon surge is evident just
north of Kayangel, though it is weak at this time. This surge is
expected to strengthen over the next couple of days, bringing
increased convection to Palau later tonight into Monday, with Yap
seeing increased convection Sunday through Monday night. The highest
probabilities for showers looks to be Sunday and Sunday night for
Palau and Monday for Yap. During these times, locally heavy rainfall
and gusty winds with showers are possible. By Tuesday, a dry pattern
will push into both Yap and Palau as a weak ridge of high pressure
pushes in while the monsoon trough drifts farther north.

For Chuuk, a trade-wind trough to the east will continue to approach,
bringing increased showers and a few thunderstorms to Weno later this
evening. There are a couple more weak troughs seen to the east of
Chuuk, over eastern Micronesia, that will move through Chuuk over the
next day or so, maintaining scattered showers and isolated
thunderstorms through Sunday night. By Monday, a dry pattern returns
to Chuuk, though this period looks to be relatively short. By Tuesday
evening into Wednesday, a more robust trough looks to move into
Chuuk, setting up a more unsettled pattern that will last through the
end of next week.

Not much going on in the marine environment. the most notable thing
will be with the swell for Palau and Yap. Beginning Sunday night,
the primary swell will gradually turn south, then southwest for Palau
and the monsoon swell becomes the dominant swell. This will occur a
little later for Yap, around Monday. By midweek, the southeast swell
looks to again become the dominant swell. Seas of 3 to 5 feet tonight
will subside to between 2 and 3 feet by the middle of next week.
Winds look to remain gentle to moderate for both Yap and Palau
through Monday night, then subside a bit, becoming light to gentle.
Winds will remain south to southwest through the coming week.

For Chuuk, seas will remain in the 3 to 4 foot range through
Thursday, with mostly light to gentle winds. Gentle to moderate winds
are expected Monday night into Tuesday.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Flood Watch through late tonight for GUZ001.

MP...Flood Watch through late tonight for MPZ001.

Marianas Waters...None.
&&

$$

Marianas/Tropical Update: Doll
East/West Micronesia Update: Cruz
Marianas: Montvila
East Micronesia: Bowsher
West Micronesia: Kleeschulte