458
AXPQ20 PGUM 280101
TWDPQ

Tropical Weather Discussion
National Weather Service Tiyan GU
1101 AM ChST Thu Aug 28 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...

TROPICAL AND MONSOON DISTURBANCES...
There are currently no active Invests or tropical systems located
within the region. However, there are a few areas worth monitoring.
First, there is a weak, broad elongated surface circulation centered
north-northeast of Yap at around 12N139E. This is co-located with a
maximum in 850-700 mb vorticity according to the latest analysis from
CIMSS. Scattered to numerous showers are seen along the northern
flank of the circulation where it intersects with a broad trade-wind
trough. This circulation looks to remain a transient, disorganized
feature within the monsoon trough as it meanders south over the next
day or so, but models indicate it may strengthen and organize this
weekend in response to a monsoon surge, supporting a much wetter
pattern over far western Micronesia and the Marianas. Another weak
circulation is seen to the east of Chuuk near 7N154E, evident in
visible satellite imagery as well as GFS streamlines. This
circulation is associated with a very weak 850-700 mb vorticity
maximum in the area. Numerous showers are seen south and southwest
of the circulation center, associated with strong convergence and
troughing within the southerly flow south of the feature. Lastly,
there is a more significant vorticity maximum located along 160E from
850-500 mb, from around 4N to 7N. This is associated with sharp
troughing at the surface along the Intertropical Convergence Zone
(ITCZ), but a low-level circulation is not currently evident in this
morning`s scatterometry or visible satellite. Latest deterministic
models indicate a lower probability for development for these latter
two areas over the next few days.

...OTHER WEATHER SYSTEMS...

MONSOON TROUGH...
The monsoon trough extends nearly across the entire region. It
enters the region near 14N130E and extends east-southeast through a
weak circulation located north of Yap or near 12N139E. The monsoon
trough then extends southeast to 7N143E before curving east-
southeast into a weak circulation east of Chuuk near 7N154E.
Finally, the monsoon trough extends eastward to 8N162E where it runs
into the the ITCZ and trade-wind convergence. Scattered to numerous
showers and isolated thunderstorms are seen along and south of the
monsoon trough and widespread showers and isolated thunderstorms are
seen where the monsoon trough interacts with the above circulations.
South of the monsoon trough, a monsoon surge has led to a Westerly
Wind Burst (WWB) spanning from the equator to 4N and 130E to 145E.
Guidance varies on the strength of the WWB and the monsoon surge, but
there is pretty good agreement on a northward lift of the monsoon
trough and surge over the next few days, potentially bringing
impactful weather to Palau, Yap, and the Marianas this weekend.

TRADE-WIND AND SURFACE TROUGHS...
Several trade-wind and surface troughs are seen throughout the
region. Near the Marianas, a broad trough extends south-southwest
from around 23N146E into a very weak elongated circulation to the
north of Yap centered near 12N139E. Scattered to numerous showers
extend across much of the trough axis near and north of the
circulation, becoming more discontinuous and patchy north of 16N.
Further west, another trough extends south-southwest from near Iwo To
at 25N140E to near 17N135E. This trough is interacting with an
upper-level low to produce scattered showers and isolated
thunderstorms just east of the trough axis, extending northward to an
east-west oriented band of showers just south of Iwo To. Just east
of the Marianas, a trade-wind trough extends south-southwest from
east of Alamagan near 17N148E to just east of Saipan around 15N146E.
Numerous showers and isolated thunderstorms extend along this trough
axis and southeastward along the associated area of convergence
upstream, extending northwestward to cover much of the northern
CNMI. The troughs just east and west of the Marianas look to diminish
over the next day with a decrease in nearby shower coverage
overnight, but the trough further northwest looks to stick around for
a little longer as it slides west, crossing 130E around Friday night
with little change in strength.

To the east, a sharp surface is seen embedded within the ITCZ to the
east of Pohnpei, extending south from around 9N162E to curve
southwest through 4N157E. Scattered to numerous showers are seen
along the trough axis, mainly within an area of strong convergence to
the south of Pohnpei. This morning`s scatterometry shows some areas
of strong convective gusts in the 25 to 35 knot range, mainly over
southern Pohnpei and Kosrae States. This trough looks to lift
northward as an ITCZ fragment over the next day or so, eventually
crossing the Marianas this weekend and contributing to a wetter
pattern there Friday night through Sunday.

TUTT/UPPER-LEVEL LOWS...
The TUTT extends west-southwest into the region near 24N180, curving
southwest into a weak TUTT cell just southeast of Wake Island near
18N168E. A portion of the TUTT then extends northwest to an upper-
level low centered over Minamitorishima around 24N154E, which is
interacting with subtle troughing and convergence at the surface to
produce scattered showers and isolated thunderstorms just east of the
island. This low looks to shift westward to the south of Iwo To over
the next day or two, potentially enhancing showers over the Marianas
this weekend, while the TUTT remains semi-stationary near Wake Island
over the next few days. Another upper-level low is seen to the west
of Iwo To, interacting with a surface trough to produce scattered
showers and isolated thunderstorms far southwest of the island. This
feature looks to shift west and exit the region over the next day or
two.

OTHER SYSTEMS...

...ITCZ...
The ITCZ extends westward into the region near 9N180 passing through
the northern Marshall Islands, passing west-southwest through a
sharp surface trough east of Pohnpei, then continuing to a weak
surface circulation to the east of Chuuk near 7N154E. Scattered
showers are seen along the ITCZ near the Date Line, becoming numerous
across much of eastern Micronesia along and south of the ITCZ, mainly
from Majuro westward to just south of Weno, Chuuk. Isolated
thunderstorms are seen as well, mainly within the areas of moderate
to deep convection south of Kosrae and Pohnpei. The ITCZ looks to
become more fragmented by the end of the week as it lifts north, with
the trough east of Pohnpei breaking off to approach the Marianas this
weekend, while another fragment reasserts itself over the central
RMI eastward to the Date Line. The ITCZ then diminishes through early
next week.

$$

DeCou