794
FXPQ50 PGUM 301829
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
429 AM ChST Tue Jul 1 2025

.Marianas Update...
Satellite and radar observations show mostly cloudy skies and
scattered showers across the Marianas. Ritidian and Ipan buoys show
seas between 3 to 5.

The main change with this update was to adjust PoPs for the CNMI
today and tonight. The monsoonal pattern has become more active and
convection is increasing upstream from the CNMI. The axis of heavy
rainfall has shifted north of Guam and the Flash Flood Watch was
canceled. See hydrology update for more details. PoPs were increased
across the CNMI as both guidance and satellite support high-end
scattered showers (50%) today and tonight. By Wednesday, we`re
looking for showers to taper down across the Marianas as the monsoon
flow shifts further north. A typical trade-wind pattern looks to
develop later this week with chances of showers coming from trade-
wind troughs.

No changes to the marine forecast. The southwesterly swell has begun
to move into the region and combined seas are expected to rise to 4
to 6 feet tonight. An increase in surf is expected across west
facing reefs, which could lead to a moderate risk for rip currents
tonight as well.

&&

.Tropical Update...
There are currently 3 invest across the Western Pacific. Invest 98W
is no longer in our Area of Operations (AoR) with a 12z position of
14.2N, 126.9E. Invest 98W is currently rated low, which means TC
development is unlikely within 24 hours.

Invests 99W and 90W are located near 17.7N, 145.1E and 18.5N, 150E
respectively. Both invest are rated sub-low by JTWC. Currently, the
invest are interacting with a TUTT to produce scattered to numerous
showers and isolated thunderstorms. Invest 99W and 90W are also the
main driver in the development of the monsoon trough that is
developing across the CNMI today. Both systems are expected to move
generally northward with Invest 90W being favored for potential
development.

&&

.Hydrology Update...
The Flash Flood Watch has been canceled for Guam. Although showers
remain, the majority of showers are stratiform and are not producing
heavy rainfall. Locally heavy rainfall will be possible through this
afternoon, but the risk for flash flooding has diminished. Heavy
rainfall will be possible across the CNMI today, but a Flash Flood
Watch was not issued to the drier antecedents and overall only 1 to 2
inches of rain expected through tonight.

&&

.Eastern Micronesia Update...
No significant changes were needed as the forecast remains on track.
The main feature of interest is a trough departing Majuro this
morning, heading west, which will allow scattered showers to develop
at Kosrae by afternoon, while becoming isolated at Majuro tonight.
For Pohnpei, the approaching trough should bring scattered showers to
the region around midnight or shortly thereafter.

&&

.Western Micronesia Update...
No significant changes were made to the forecast. A monsoon
southwesterly flow will keep showers fairly active for Palau and Yap.
The trough for Chuuk did weaken as the energy got absorbed into a
developing tropical disturbance to the north. That said, enough
surface convergence with good moisture quality should produce
scattered showers today, possibly becoming isolated tonight.

&&

.Prev discussion... /issued 721 PM ChST Mon Jun 30 2025/

Marianas Synopsis...
Moderate southwesterlies prevail across the region, with satellite
imagery and Doppler radar indicating numerous showers and some
thunderstorms in Guam and Rota coastal waters. Most recent altimetry
passes show seas are 3 to 5 feet, and the buoys show sea heights of
2 to 4 feet.

Discussion...
An unsettled weather pattern continues over the Marianas as the
monsoon trough builds to the north of the region. A few embedded
disturbances within the monsoon trough are expected to further
destabilize the local atmosphere, allowing some sporadic development
of showers and thunderstorms (albeit less in coverage) the next few
days as said features continues to try to consolidate to the
north. These showers will continue to build overnight, with a peak
expected just near midnight. As these disturbances move to the north,
showers will diminish significantly, with only isolated to low-end
scattered showers and some thunderstorms expected for the rest of
the week.

Marine...
Combined seas of 3 to 5 feet are expected to build to 4 to 6 feet by
Tuesday, persisting as such in CNMI waters for the rest of the week.
However, seas in Guam coastal waters are expected to gradually
diminish as the monsoon trough pulls northward, allowing
southwesterly swell and localized wind waves to diminish. A moderate
risk of rip currents is expected along east facing reefs of the
Marianas for much of the week, with rip current risk potentially
becoming moderate along west reefs by the weekend as well, once
another pulse of southwesterly swell makes its way across the region.

Hydrology...
A lull in showers earlier today has brought only 0.05 inches of rain
at the airport, as most of the convection stayed just southwest of
Guam waters, and concentrated within and along the eastern flanks of
Invest 90W northeast of Saipan. Once these nearby disturbances
destabilized the atmosphere later in the afternoon, showers have
developed that are expected to bring wetting rains and flash
flooding to Guam overnight. As such, a Flash Flood Watch remains in
effect for Guam through Tuesday afternoon, as this wet monsoon
pattern lingers. High uncertainty remains at this time if these
convective cells will persist downstream over the CNMI, but if
conditions warrant, a Flash Flood Watch may be extended to the CNMI
as well.

Tropical systems...
There are three tropical disturbances across the region this
afternoon. Two of these disturbances are associated with the
northeastern end of the monsoon trough. Newly formed Invest 90W is
found west-northwest of Saipan, northwest of Guam, at 16.0N 149.5E.
Invest 99W is seen farther north, near 19.2N 145.6N. Neither of these
have a distinct circulation center, but are located along a more
pronounced kink in the troughing. These disturbances are expected to
drift north over the next few days, with 90W being the more favored
disturbance for any development. Latest model guidance shows a
circulation developing with 90W as moves north, staying east of the
northern CNMI. As is the norm, the GFS is more aggressive with the
development, though all models show the system developing well north
of the Marianas and east of the northern CNMI.

Invest 98W is seen well west of the Marianas, centered near 15.2N
132.6E, moving slowly west. 98W has been upgraded to low, meaning
development is expected, but not within the next 24 hours. Invest 98W
will continue to drift west, with model guidance in pretty good
agreement that it will strengthen west of 130E. Invest 98W is not
expected to have any direct impacts with the Marianas, Palau or Yap
as it moves farther away. For more information on Invest 98W, see
bulletins issued by the Joint Typhoon Warning Center under WMO header
ABPW10 PGTW.

Eastern Micronesia...
A trade-wind trough is approaching Majuro, interacting with a
developing Inter-Tropical Convergence Zone (ITCZ). Numerous showers
and isolated thunderstorms are now moving into Majuro waters. The
trough is expected to move west overnight, approaching Kosrae Tuesday
morning. Latest guidance shows the trough losing a little steam by
Tuesday, therefore, set PoPs (Probability of Precipitation) at 40
percent. If the ITCZ remains as strong and the trough maintains its
intensity as well, PoPs may need to be increased. The same is
expected for Pohnpei Tuesday night. The ITCZ looks to drop a bit
farther south, allowing a drier pattern to move into Majuro Tuesday
night. Other than a brief period of scattered showers as a trade-
wind trough moves through the Marshalls on Wednesday night, the dry
pattern is expected to continue into the weekend. The ITCZ, however,
does look to remain over Pohnpei and Kosrae through most of the week,
maintaining increased convection through Thursday for Pohnpei and
through Friday for Kosrae.

Little changes expected for the marine forecast, with combined seas
of 5 to 7 feet at Majuro slowly subsiding over the next few days,
dropping to between 3 and 5 feet for the weekend. Seas of 4 to 6 feet
for Kosrae and Pohnpei could reach 5 to 7 feet Tuesday night into
Wednesday before subsiding to between 3 and 4 feet for the weekend.
Gentle to moderate winds are expected through the week for Kosrae and
Majuro, with gentle to moderate winds becoming light to gentle for
Pohnpei around midweek.

Western Micronesia...
A southwesterly monsoon pattern will continue to bring higher-end
scattered showers to Palau and Yap tonight, followed by another
uptick in showers along with gustier winds and thunderstorms near
Palau Tuesday. 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 Yap beginning
Tuesday and at Palau Tuesday night. Meanwhile at Chuuk, locally
diffusive flow induced by the monsoon trough and embedded
disturbances mentioned previously, is allowing a drier regime to
ensue through Tuesday. After, some patchy showers will return,
followed by a leading edge of a sharp trough or vorticity feature
bringing very strong gusts and heavier showers around midweek.

Combined seas of 4 to 5 feet are expected to taper down a foot or
two at Palau and Yap in the next few days, as various swells and
localized wind waves diminish. Then, a northwest swell generated by a
distant TC well northeast of the Philippines will start to trickle in
by the next weekend. At Chuuk. combined seas of 3 to 5 feet, will
momentarily increase another foot or two around midweek as the
aforementioned tropical feature will make its way across and produce
choppier seas during passage.

&&

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

$$

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