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AXPQ20 PGUM 180054
TWDPQ

Tropical Weather Discussion
National Weather Service Tiyan GU
1054 AM ChST Sat Jul 18 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...

TROPICAL AND MONSOON DISTURBANCES...
Invest 90W continues to meander down across the Chuuk/Pohnpei
States boarder region, with the latest updates from JTWC placing
the center near 8N155E, and this looks good based on visible
imagery and streamline analysis. The system continues to struggle
as drier air is pushing against it`s northern fringes of the
circulation aloft, as the dry air is associated with the tail end
of a mid-latitude trough. Additionally, 200mb analysis shows
about 20 to 25 knots of wind shear aloft, which certainly is not
allowing for further organization/intensification. We expect this
disturbance to meander down there for another 48 to 84 hours.
This is a change from previous thinking, as we had to wait for the
mid to upper-level vorticity centers to decide what they wanted
to do. The ALPW (All Layer Precipitable Water) shows one center in
the 700-500mb and 500-300mb layer has ejected westward to near
5N143E. Meanwhile, the SFC-850mb and 850-700mb layer vorticity
center remains with 90W, as it also does 500mb and above. So
essentially, a "piece of energy" pinched off and headed west.
Could this be tied to "fringe effects" of the incoming MJO
(Madden-Juliean Oscillation)?

Looking ahead (past 84 hours), the general theme seen in the
models and how we expect the pattern to evolve would favor any
northeast ejection of 90W to turn more westward (with a fairly
sharp turn) towards the Marianas, bringing at least some gusty
winds and locally heavy rainfall, albeit delayed potentially to
the latter half of next week. Until then, it`ll (90W) continue to
bring increased rainfall potential to locations from extreme
eastern Yap State eastward to Pohnpei and Kosrae. Periods of
locally heavy rainfall and isolated wind gusts to 25 knots remain
possible. Special Weather Statements for eastern Yap State
through Pohnpei and Kosrae remain in effect to cover these
concerns.

...OTHER WEATHER SYSTEMS...

SURFACE TROUGH...
A surface trough extends east from 90W near Chuuk, through a col
just southwest of Kwajalein, then continues east through Majuro
to another little, but well-defined circulation near 8N178E. The
trough then exits Guam`s AOR near 8N180. Aside from good
convective coverage and strength near 90W, convection is also
fairly widespread from Pohnpei east-southeast through Kosrae, to
just south to southwest of Majuro. Here, good convergence along
the trough is helping focus convective development, and this
should continue the next few days.

OTHER SYSTEMS...
We have a few features of interest this morning. The first
involves the tail end of a mid-latitude trough that extends
southwest, entering Guam`s AOR near 16N180, It then turns west-
northwest at first near 15N170E, becoming more northwesterly after
it crosses 160E, before ending near Minamitorishima. Ahead of
this trough, a fairly large ribbon of drier air is suppressing
convective development, which is hindering 90W`s development as
the effects of this drier air are felt on the northern and
northeastern flanks of the system, while also suppressing
convection across the northern half of the RMI and Majuro. As this
trough peels east by the end of the weekend, 90W should have
somewhat better potential for more convective development to
attempt to envelop the circulation. Rainfall potential may
increase near Majuro as well if any easterly waves interact with
this.

Next, we have a fairly dry west to southwest flow that crosses
130E as it enters Guam`s AOR, passing over the southern half of
the Republic of Palau, especially south of Koror/near Helen`s
Reef. This is suppressing what would normally be a more moist,
monsoon-like flow. Unfortunately, this looks to continue through
the weekend and probably longer.

Then, we have persistent southeasterly to southwesterly cross-
equatorial flow, which is helping to complicate tropical
cyclogenesis to an extent, while also sending a southerly to
southwesterly swell to many locations in the southern FSM and RMI.
Although swell heights are only 2 to 3 feet (posing little
hazard/effect to the atolls), it`s unusual and note-worthy, as is
the surface southwest to west winds observed. This pattern has
been around for over a week now, and looks to continue for the
foreseeable future.

$$

Doll