681
FXPQ50 PGUM 290650
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
450 PM ChST Sun Jun 29 2025

.Marianas Synopsis...
Satellite shows mostly cloudy skies with scattered showers over Guam
and Rota, with partly cloudy skies and isolated showers over Tinian
and Saipan. Light to gentle southeast to south winds prevail across
the Marianas. Buoys show seas between 2 and 4 feet while altimetry
shows seas between 3 and 5 feet.

&&

.Discussion...
Showers have decreased across the Marianas today. However, this is
expected to be short-lived. Invest 99W remains to the northeast of
the Marianas, drifting slowly north-northeast, with Invest 98W seen
well west of the Marianas. As 99W drifts northward, it will allow an
overall wet, unsettled, monsoon pattern to move across the Marianas.
Latest model guidance continues to indicate that a weak monsoon surge
will also push into the region, with a Tropical Upper Tropospheric
Trough (TUTT) remaining to the north, providing a broad area of
divergence to aid in the generation of showers and thunderstorms
across the Marianas. The islands look to remain under the influence
of the monsoon flow, TUTT and monsoon surge through at least Tuesday,
maintaining numerous showers and isolated thunderstorms, though the
heavier showers are only expected through Monday night. Monsoonal
flow looks to continue a bit longer, maintaining south to southwest
flow into the weekend. This will keep an overall unsettled pattern
over the islands for the next several days, though shower coverage is
expected to decrease to scattered Tuesday night, with PoPs
(Probability of Precipitation) dropping to around 30 percent through
Sunday.

&&

.Marine...
Not much of a change in the marine forecast. Seas of 3 to 5 feet
tonight look to increase 1 to 2 feet over the next couple of days as
a southwest monsoon swell moves into the area while the trade-wind
swell remains unchanged. Seas are expected to be around 4 to 6 feet
through midweek, then subside a bit as the monsoon swell subsides.
Light to gentle winds tonight look to become gentle to moderate
Monday and remain at those levels through the week. Winds could
become occasionally fresh as the monsoon surge moves through the
region. The rip current risk will remain moderate along east facing
reefs for the next several days. As the monsoon swell increases
Tuesday, the rip risk is expected to become moderate for a day or so
along west facing reefs.

&&

.Hydrology...
A Flash Flood Watch remains in effect for Guam and Rota. Although
showers remain limited this afternoon, they are expected to increase
through the evening, with numerous showers expected near midnight.
Locally heavy showers capable of producing Flash Flooding are
possible. The increased showers are expected to spread north through
the night, with numerous showers for Tinian and Saipan. Based on
possible rainfall amounts, this Watch could be reissued to include
Tinian and Saipan, if conditions warrant.

&&

.Tropical systems...
There are two disturbances found along the monsoon trough. The
first one is Joint Typhoon Warning Center`s (JTWC) Invest 98W, seen
as an exposed low level circulation center (LLCC), centered near
15.1N 136.6E. 98W is slowly moving west-northwest over the
Philippine Sea with little change in intensity. This is expected to
continue for the next couple of days, with very low probability of
development.

The second disturbance is JTWC`s Invest 99W, centered at 16.6N
148.5E. This feature is interacting with divergence seen southeast
of a TUTT cell centered near 20N150E. 99W is slowly drifting north-
northwest. Little development is expected with 99W as it continues to
slowly drift northward over the coming days.

The main affect that the two disturbances will have across the
region is to help increase monsoon flow across the Marianas over
the next couple of days. Both Invest 98W and 99W will be monitored
closely over the next few days for any indication of development.

&&

.Eastern Micronesia...
A broad surface trough embedded within a generally moderate to fresh
trades will be the primary weather maker across eastern Micronesia
for the next several days. Patchy showers will make their way across
Majuro Monday, followed by Kosrae Tuesday, and the Pohnpei around
midweek, with periods of a drier cycle in between at all three
forecast locations. While shower coverage is expected to remain
lower-end near Majuro, The leading edge of said trough has been
producing outflow boundaries measured to 30 knots using satellite
analysis tools, so some lines of showers may carry much more
significant gusts overnight and Monday. Otherwise, largely moderate
to fresh trades expected during the trough passage, and gentle to
moderate trades elsewhere for much of the week.

Combined seas of 3 to 5 feet near Pohnpei and Kosrae, are expected to
increase an additional 2 feet around midweek as swell and localized
wind waves increase, before tapering down once again before the next
weekend. Combined seas of 5 to 7 feet near Majuro, will momentarily
increase another foot Tuesday, before dropping back down to between 5
and 7 feet for the rest of the forecast period.

&&

.Western Micronesia...
A southwesterly monsoon pattern will continue to bring heavier
showers and thunderstorms to Palau overnight and to Yap Monday,
along with gustier winds near these heavier showers. As various
monsoon disturbances move northward and shift the monsoon trough
along with it, a ridge will form along the eastern side of said
trough, allowing conditions to improve significantly with only
isolated showers expected at Palau and Yap beginning Tuesday.
Meanwhile at Chuuk, locally diffusive flow induced by the monsoon
trough and embedded disturbances, is allowing a drier regime to ensue
through at least midweek, followed by a return of patchy showers
thereafter.

Combined seas of 4 to 6 feet are expected to taper down a foot or two
at all three forecast locations in the second half of the week, as
various swells and localized wind waves diminish.

&&

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

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

Marianas Waters...None.
&&

$$

Marianas/Hydrology/Tropical: Kleeschulte
East/West Micronesia: Montvila