Flash Flood and Strong Wind in Guam, August 25 to 28, 2025

Area Forecast Discussion
National Weather Service Tiyan GU
638 AM ChST Mon Aug 25 2025

.Marianas Update...
Satellite shows mostly mid and high clouds with just isolated showers
within the Marianas Waters. The buoys show combined seas of 2 to 3
feet which are expected to rise to between 3 and 5 feet by Tuesday.
This was all well handled in the current grids so no changes were
needed.

&&

.Eastern Micronesia...
Eastern Micronesia has a trade-wind trough near Pohnpei and a broad
zone east of it where wind is converging into the trough. This is
creating a large area of showers and thunderstorms that covers all 3
of the major forecast points. At least for today this was well
covered so no changes were needed.

&&

.Western Micronesia...
Invest Area 93W has moved out of Western Micronesia, and even the
convection associated with it is now west of Koror. There are still
plenty of scattered showers around, for both Palau and Yap, however
those are now associated with the monsoon trough. Chuuk will continue
to be under a lobe of vorticity sticking out of a weak circulation to
their west, which will maintain scattered showers there through
tonight. These were all handled well by the current forecast so no
changes were made.

&&

.Prev discussion... /issued 505 PM ChST Sun Aug 24 2025/

Marianas Synopsis...
Radar and satellite imagery shows scattered showers and isolated
thunderstorms over the Marianas. Altimetry and buoys show combined
seas of 2 to 3 feet. Winds are gentle to moderate.

Discussion...
Over the regions, a hint of a weak surface trough interacting with a
dash of trade-wind convergence, then, add in a splash of upper-
level divergence and you have a recipe for scattered showers and
isolated thunderstorms, like what we have seen for today. This
concoction of interacting features is expected to remain over the
region through tonight. A lull is expected for Monday and Monday
evening. This lull will be due to a weak ridge. After this lull,
showers are anticipated to return, for the morning and afternoon,
for the remainder of the forecast period.

Marine...
Combined seas are anticipated to gradually rise by about a foot by
Tuesday. This will cause the surf along east facing reefs to meet the
threshold of 5 feet. This will cause the risk of rip currents to
become moderate along east facing reefs. There is also a moderate
risk of lightning through at least Tuesday.

Tropical Systems...
Invest 93W is currently located right along the far western edge of
western Micronesia, near 9N130E. 93W is expected to continue to move
west-northwest, exiting the region later this evening. Significant
development is not expected east of 130E.

Eastern Micronesia...
The Inter-Tropical Convergence Zone (ITCZ) is finally becoming more
active and spreading across eastern Micronesia as the models were
predicting. The ITCZ is currently seen to the south of Majuro,
stretching westward, passing north of Kosrae and Pohnpei. A trade-
wind trough currently seen between Pohnpei and Kosrae is interacting
with the trough to generate numerous showers and isolated
thunderstorms to the east of Pohnpei. These showers are approaching
Pohnpei, with scattered showers expected to increase to numerous
through the evening. Upper-level divergence is also present over the
area, helping enhance shower coverage. For Kosrae and Majuro, weak
troughs approaching both locations look to enhance convection a bit
this evening, though coverage looks to remain scattered since the
main convergence band with the ITCZ is found south of Majuro and
north of Kosrae. For the most part, an overall unsettled and wet
pattern looks to remain over the region for the next several days,
though Kosrae and Pohnpei may see a couple short-lived dry periods.

No major changes were made to the marine forecast. Seas of 5 to 6
feet at Kosrae and Majuro, and 4 to 5 feet at Pohnpei tonight look
to subside a bit by Monday morning. Seas will drop to between 3 and
4 feet for Pohnpei, and 4 to 5 feet for Kosrae and Majuro. Mostly
light to gentle winds will continue for Pohnpei and Kosrae, with
gentle to moderate winds at Majuro continuing through midweek.

Western Micronesia...
A somewhat unsettled monsoon pattern persists across much of western
Micronesia. A monsoon trough extends eastward into the region
through a weakening Invest 93W, which sits near the western border
of Guam's Area of Responsibility (AOR) near 9.4N 130.3E. The monsoon
trough then extends further east toward a weak disturbance spinning
just west of Chuuk Lagoon around roughly 8N148E, but the west to
southwest flow south of the trough axis is discontinuous over Yap
State between the two circulations. Visible satellite this afternoon
shows scattered to numerous showers within the monsoon flow south to
southeast of Palau and south of Yap, becoming more numerous near
Eauripik in Yap State and across western Chuuk State, where the
disturbance near Chuuk is interacting with the eastern extent of the
monsoon trough. Numerous showers are also seen along 130E near the
center of 93W, with drier conditions seen to the east over Palau and
Yap Proper.

Invest 93W is expected to keep meandering northwest and exit the
region overnight, while fairly dry conditions persist for Palau and
Yap this evening. However, increasing convergence within the monsoon
trough will build scattered showers across the region overnight,
especially near Palau. Palau can expect a wetter monsoon pattern to
persist over the next few days with rounds of heavier showers
throughout the week. High-end scattered showers (50% Probability of
Precipitation (PoP)) may become numerous from time to time this week
and locally heavy showers will be possible. However, due to
uncertainty in timing and recent model runs overestimating
precipitation coverage, the current forecast reflects only up to 50%
PoPs for the next few days for Palau. This may change in subsequent
forecast packages, but for now the highest rainfall amounts look to
be focused further south of Koror. Yap will remain on the drier side
of the spectrum but can still expect periods of scattered showers
for much of the week.

Further east, Chuuk can expect increasing showers through the next
day or two as a more robust trough/disturbance crosses the area,
followed by a lull in showers around Tuesday. Around Wednesday night,
a robust ITCZ-like disturbance looks to build northwest into the
area, leading to a much wetter pattern through the latter half of the
week.

Benign marine conditions continue, with slightly higher seas near
Palau as a higher south to southwest swell has moved into the region,
associated with the ongoing monsoon pattern. Recent altimetry data
indicates combined seas of 3 to 5 feet for much of the region. A
slightly stronger pulse of trade swell may increase seas by 1 to 2
feet for Chuuk over the next day or two, before slightly relaxing in
the latter half of the week. Gentle to moderate winds will continue
across much of the region this week.

&&

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

Marianas/East and West Micronesia Updates: Stanko
Marianas: Bowsher
East Micronesia: Kleeschulte
West Micronesia: DeCou