686
AXPQ20 PGUM 032357
TWDPQ

Tropical Weather Discussion
National Weather Service Tiyan GU
957 AM ChST Thu Sep 4 2025

Tropical Weather Discussion for the Western North Pacific between
the Equator and 25N from 130E to 180. The following information is
based on recent satellite imagery/data, weather observations, radar,
and meteorological analysis.

...SPECIAL FEATURES...

None.

...OTHER WEATHER SYSTEMS...

MONSOON TROUGH...
The main axis of the monsoon trough has lifted north of the region,
following 95W, now Tropical Depression 21W (Peipah), which is
centered near 29N131E and north of the region. A line of scattered
showers and isolated thunderstorms does extend along the convergent
monsoon tail that extends southward from TD 21W (Peipah), extending
through the region from 25N132E and ending near 19N131E. The monsoon
trough is not expected to be a major feature within the region over
the next several days as the main trough axis remains west and north
of the region.

TRADE-WIND TROUGHS...
A series of trade-wind troughs extend across the region, traveling
trough the gentle to moderate flow that extends across the region.
The first trough is located northwest of Palau and extends northeast
to about 15N138E, well north-northeast of Yap, where a weak
circulation may be trying to develop. No defined circulation can
currently be found on this morning`s visible satellite due to ongoing
convection and cloud cover, and microwave imagery is inconclusive.
However, the CIMSS 850 mb vorticity analysis shows a somewhat compact
area of vorticity over this location. Model guidance keeps any
circulation that may develop fairly weak as it shifts northwest and
out of the region sometime this weekend. This trough is producing
scattered showers and isolated thunderstorms to the north of Palau
and Yap, mainly between 8N and 15N from 130E to 143E. This trough is
expected to shift west over the next couple of days, exiting the
region by the weekend, and pulling showers northwest of both Palau
and Yap.

The second trough is moving through the Mariana Islands, generating
scattered showers from Guam up to about Anatahan. Thunderstorms have
been fairly isolated, but do increase slightly in coverage east-
northeast of Saipan due to upper-level divergence associated with
the TUTT to the north. This trough will shift west, moving north of
Yap and Palau this weekend, so showers with this trough are expected
to remain mainly north of both locations. The third trough is
centered near 12N152E, followed by the fourth near 14N158E, and the
fifth and final trough near 15N180. All three of these troughs are
generating scattered showers along the trough axis, while the Global
Lightning Detection (GLD) network has shown very little lightning
within any of the troughs over the past couple of hours. As these
troughs shift westward, expect periods of scattered showers to
continue across the Marianas into the weekend as the two closes
troughs move through the Marianas. Influence from the TUTT to the
north will continue the potential for isolated thunderstorms near the
Marianas as these troughs push through.

TUTT...
Upper-level water vapor satellite imagery shows the main axis of the
TUTT entering the region near 22N180 and extending westward, passing
through a TUTT cell, centered north of Wake Island near 21N165E, and
ending northwest of the Saipan near 18N143E. Very little convection
is associated along the TUTT or near the TUTT cell as satellite
imagery shows isolated towering cumulus to the north of Wake Island,
with the GLD network reporting very infrequent lightning over the
past several hours. The TUTT does appear to be providing upper-level
divergence to the trough passing through the Marianas, leading to
isolated thunderstorms to the east of Saipan, within the area of
scattered showers that extend from near Saipan to about 150E. The
TUTT is expected to change very little over the next few days, while
the embedded TUTT cell is expected to shift westward and move across
the Marianas, to the north of Saipan, early next week which will
increase the potential for thunderstorms across the island chain.

...ITCZ...
A fragmented ITCZ extends from south of Pohnpei near 5N158E, across
Kosrae, and ends near Majuro near 6N172E. The zone of convergence
after ending near Majuro and does not redevelop until east of the
Date Line. Along the ITCZ, scattered showers and isolated
thunderstorms, extend along the ITCZ mainly between 2N and 7N, mainly
affecting Kosrae and the southern RMI south of Majuro. The fragmented
ITCZ is expected to shift westward over the next few days,
potentially morphing more into a surface trough as the zone of
convergence weakens. This is expected to increase showers at Pohnpei
overnight into Friday, and Friday night for Chuuk, while showers are
expected to decrease at Kosrae overnight or early Friday morning.

$$

Schank