517
FXPQ50 PGUM 291803
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
403 AM ChST Mon Jun 30 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 4.

The main change with this update was to adjust PoPs based on QPF
through Tuesday. Guidance has increased QPF for today and tonight,
but QPF decreased slightly for Tuesday. The lull in precipitation has
continued tonight across the Marianas, but recent satellite trends
show an increase in showers beginning to develop. The main player for
the Marianas appears to be Invest 99W. The positioning and strength
of 99W will dictate where the heaviest rainfall will occur over the
next couple of days. There was enough confidence to continue the
Flash Flood Watch for Guam through Tuesday afternoon. This was due in
combination with antecedent rainfall of 4 to 8 inches over the last
few days across Guam and another 2 to 4 inches of rain possible
through Tuesday night. The LREF 90th percentile is 3.50" for Guam and
with EFI showing values around 0.8 and shift of tails greater than 1
across Guam, this gives some credence towards us reaching the 90th
percentile. See hydrological update for more information.

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 today. An increase in surf is expected across west facing
reefs, which could lead to a moderate risk for rip currents by
Tuesday.

&&

.Hydrological Update...
A Flash Flood Watch remains in effect for Guam. The Flash Flood
Watch for Rota has been canceled. The lull in shower activity across
the Marianas has continued into the night, but latest satellite
observations show in uptick in convection across the region. Many
rain gauges across Guam have seen 4 to 8 inches of rain over the last
5 days. Guidance continues to hint towards an additional 2 to 4
inches of rain over the next couple of days across much of the
Marianas. The Flash Flood Watch was extended for Guam through Tuesday
due to a combination of the prior wet period and the additional
heavy rain expected. Less rain has fallen across the CNMI, which led
to the watch being cancelled for Rota this morning. A re-issuance may
be necessary if the forecast trends wetter for the CNMI.

&&

.Tropical Update...
There remain two disturbances along the monsoon trough. As of 12z,
Invest 98W is currently located near 14.6N, 134.7E. Convection
remains disorganized but guidance shows gradual development over the
next few days as it exits our Area of Operations (AoR).

The second disturbance is Invest 99W, currently located near 17.2N,
146.6E as of 12z. Invest 99W is currently interacting with a TUTT to
produce numerous to widespread showers to the south and east of the
sharp trough axis. The interaction with the TUTT should limit the
potential development for Invest 99W. Invest 99W will drift north-
northwestward and will assist in the development of a monsoon
pattern across the Marianas.

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 Update...
Kosrae:
A subtle surface trough is moving through Kosrae right now and this
will provide showers and isolated thunderstorms this morning, but
then as the trough moves of to the west a ridge axis will move in by
late afternoon, helping to clear out the clouds some with just
isolated showers anticipated by then.

Majuro:
A surface trough is moving into your area right now and will continue
through tonight, allowing showers to become numerous with locally
heavy rainfall possible at times. As such, the temperatures were
adjusted downward a couple of degrees to account for the increasing
cloudy cover/density and rain-cooled air.

Pohnpei:
A surface trough has moved off to the west towards Chuuk, but a
subtle trough departing Kosrae should keep scattered showers going
for you today. Tonight becomes a little more uncertain as the trough
may wash out, keeping you at just isolated shower coverage, but we`ll
keep an eye on this and let the day shift take another look at this
potential.

&&

.Western Micronesia Update...
Palau and Yap:
A developing monsoon southwesterly monsoon flow will keep things
fairly wet today, before potentially decreasing some tonight as the
flow edges north. This is something the day shift will keep an eye on
in terms of rainfall coverage and probabilities for tonight, as if
the flow is slower to lift we`ll potentially need to increase
rainfall probabilities tonight.

Chuuk:
A trough inbound from Pohnpei is showed to wash out in the model QFP
(Quanative Precipitation Forecast), as it gets absorbed into Invest
99W to the north. Satellite analysis offers a different view though,
and linear extrapolation would support higher precipitation amounts
that just a few hundredths of an inch of rainfall the models
portrayed. Thus, opted to go with my gut feeling and satellite trends
and maintained low-end scattered showers (30 percent probabilities)
through tonight, but if this feature holds together the day shift may
need to increase rainfall potential further. Stay tuned!

&&

.Prev discussion... /issued 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 Tuesday afternoon for GUZ001.

MP...None.
Marianas Waters...None.

&&

$$

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