850
AXPQ20 PGUM 200018
TWDPQ

Tropical Weather Discussion
National Weather Service Tiyan GU
1018 AM ChST Sun Sep 20 2026

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 Monsoon Trough enters Guam`s AOR at 12N130E wnd extends east
through Guam to a col at 12N154E (north-northeast of Chuuk). It
then dives southeast through Majuro and exits the AOR at 5N180.
South of this feature, mainly southerly winds persist between 130E
and Chuuk, becoming southwesterly at Pohnpei and westerly from
Kosrae eastward. Most of the convection in the form of scattered
showers and thunderstorms is found from just north of Koror,
through Yap, to southeast of Guam. Along the remainder of the
trough, convection is more isolated in coverage.

The trough has settled south as expected and looks to be stalling
its southern drift. Overall, a favorable pattern exists for
convection to train over the same areas over the next 24 hours,
which could give Yap and Guam some locally heavy rainfall. South
of there, unusually dry moisture values are seen in the 6 layer
ALP (All-Layer Precipitable water) loop between the equator and 6N
from 130E to south of Pohnpei at 158E. This will continue to
suppress convective development there. Otherwise, look for the
trough to remain convectively active through the middle of next
week while maintaining it`s current alignment/position.

SURFACE TROUGHS...
A weak surface trough is starting to form just east of the
Marianas, with Doppler radar and visible satellite imagery
suggesting initial attempts at a broad/weak circulation may be
trying to form near 13N149E. Scattered showers and a few
thunderstorms are starting to develop along this feature as well.
The pattern would favor a quasi-stationary to very slow movement
west with this feature over the next 48 hours, which could enhance
rainfall potential from Guam to Saipan during this time.

A surface trough extends north-northeast from a broad/weak
circulation near Bikini Atoll to just east of Wake Island. This
feature is dragging moisture northward with MIMIC PWATs
(Precipitable Water Values) over 2.5 inches, allowing for
scattered showers and isolated thunderstorms to develop along and
behind (just east of) the trough due to good surface convergence.
This feature looks to continue its westward journey the next few
days with a similar convective behavior expected.

$$

Doll