103
FXPQ50 PGUM 180744
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
544 PM ChST Mon Aug 18 2025

.Marianas Synopsis...
A tropical disturbance, known as Invest 90W, is located near 10N147E,
between Guam and Chuuk. Showers and thunderstorms are gradually
building into the region from the east-southeast. Some of these
outflow boundaries east of coastal waters are moving at 20 to 30 kt.
Buoys show seas around 4 feet with altimetry indicating an increase
in seas to the east, between 5 to 7 feet.

&&

.Discussion...
A tropical disturbance is expected to move south of Guam overnight,
producing a period of locally heavy showers and occasional strong to
gale-force gusts across the region, especially for Guam and Rota.
Saw better support from the latest model guidance to include a period
of numerous to widespread locally heavy showers for Guam and Rota
from late evening to Tuesday morning. Tinian and Saipan will also
see a similar peak in showery weather around that time, with locally
heavy showers possible around midnight. As this feature pushes off
to the west, trade-wind convergence in its wake is expected to
prolong scattered showers through at least Tuesday night. Late week
and over the weekend, periods of scattered showers are continue due
to weak troughs and convergence in the trade-wind pattern.

&&

.Marine/Surf...
A Small Craft Advisory will be in effect for Guam and Rota from late
evening through Tuesday morning. Combined seas are also expected to
peak to around 6 feet Tuesday. Afterwards, conditions are expected to
subside to light to gentle winds and 3 to 5 foot seas by midweek.
There will be a moderate risk of rip currents along east facing reefs
for the next few days and remain low along all other reefs. Swell
and surf will gradually subside starting Tuesday night, allowing the
rip current risk to become low along all reefs by the end of the
week.

&&

.Tropical Systems...
JTWC has upgraded Invest 90W to medium, meaning that TC development
is likely, but expected to occur beyond 24 hours. The 06Z position
for Invest 90W was 9.7N and 147.6E, or roughly 270 nm southeast of
Guam. Invest 90W`s convective structure has improved over the last
12 hours, but the overall storm remains disorganized. Invest 90W is
generally moving west-northwest with the circulation expected to pass
south of Guam tonight. The greatest impacts are currently seen well
north of the center with winds up to 25 knots possible tonight
across Guam and Rota`s coastal waters. Invest 90W is expected to
continue to the west-northwest, likely staying north of Yap as it
consolidates and potentially becomes a tropical cyclone. For more
information please see the JTWC bulletin under header ABPW10 PGTW.

&&

.Eastern Micronesia...
Pohnpei:
Inflow into a developing circulation, Invest 90W near Chuuk, and an
area of strong convergence over Pohnpei and to its east has lead to
widespread showers and scattered thunderstorms this afternoon, which
should continue into the evening, becoming scattered showers with
isolated thunderstorms by midnight as the atmosphere gets worked over
and a mid-level cap potential develops. Sufficient convergence will
remain in place for low-end scattered showers (30 percent
probabilities) to persist through Wednesday, before a drier pattern
moves in.

Kosrae:
You`re at the tail end of the Pohnpei convergence, but, sufficient
lift and moisture warrants keeping low-end scattered showers (30
percent probabilities) in the forecast for tonight (mainly to around
midnight, but it could continue into the first half of the morning).
Then, showers become isolated for Tuesday and the first half of
Tuesday night, before increasing lift via surface convergence, and
similar or slightly better moisture values allow for showers to
become scattered after midnight Tuesday night, which will continue
through the end of the week.

Majuro:
A dearth of convergence and seasonably-dry moisture values
(precipitable water values around 1.5 inches) will relegate any
shower coverage to isolated. There convergence will continue to be
meager until the leading edge of a trough, currently at 175W, moves
in. That said, a "tongue" of moisture on the latest MIMIC water vapor
imagery suggests extends across the Date Line between 4N and 7N, with
the increase more broad just east of the Date Line. Thus, isolated
showers should increase to scattered coverage Tuesday night and
continue through the week.

Light to gentle winds will generally prevail, but they could briefly
push the gentle to moderate range from time to time. Seas of 4 to 6
feet will continue through the end of the week.

&&

.Western Micronesia...
Numerous to widespread showers are seen across eastern Yap State and
western Chuuk State. Isolated to scattered showers are seen across
Palau, Yap Proper, and Weno. Altimetry data shows combined seas
between 2 to 4 feet across Palau and Yap and 5 to 7 feet across
Chuuk.

JTWC Invest 90W continues to be the main feature across western
Micronesia. The bulk of the convection has begun to exit Chuuk State
and is entering Yap State. The current thinking is that the majority
of the impacts from 90W will remain north and then northwest of Yap,
however, numerous showers are expected as the system moves west-
northwestward over the coming days. Towards the second half of the
week, an active pattern is expected to continue for Palau and Yap as
a monsoon like pattern looks to develop. A relative lull in activity
is expected for Chuuk Wednesday onward as low end (30%) scattered
showers are forecasted through Friday night. These showers will
mainly be from trade-wind troughs.


Little to no changes were made to the marine forecast. Seas remain
benign across Palau and Yap, while some choppy seas are present
across Chuuk where showers are occurring. Swells will generally
decrease across Chuuk the next few days, bringing combined seas down
to 3 to 5 feet.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...Small Craft Advisory from 10 PM this evening to 10 AM ChST Tuesday
     for PMZ151-152.

&&

$$

Marianas: Cruz
East Micronesia: Doll
West Micronesia/Tropical: Williams