182
FXPQ50 PGUM 240903
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
Issued by National Weather Service Honolulu HI
703 PM ChST Tue Jun 24 2025

.Marianas Synopsis...
The Marianas sit sandwiched between a meridional oriented (SSW-to-
NNE) monsoon trough to the west and a steady trade-wind regime to the
east. Radar shows clusters of showers sweeping through Guam, with
lesser amounts across the main islands of the CNMI. Ritidian and Ipan
peg seas under 4 feet with a combo of trade swell and wind waves.
Tropical Storm Sepat (02W) remains far to the north near 30N.

A wetter pattern is still in store later this week as the monsoon
trough resets and protrudes into the Marianas. Seas will remain
fairly benign this week, dominated by trade swell. East swell and
surf is slated to spike briefly the second half of Wednesday into
Thursday morning as a slight E-SE trade surge moves through.

&&

.Discussion...
Transition to wet season continues with moisture-laden trades (2.22"
PWAT) inciting a few rounds of thunderstorms just west of Guam this
afternoon. A similar theme may be in store the next day or so as a
couple of weak troughs slide through, resulting in an uptick in both
winds and showers tonight, and perhaps again ~Wednesday night.

Winds then begin to shift southerly or even southwesterly on
Friday/Saturday as the meridionally-oriented monsoon trough
reorganizes and extends eastward into the Marianas. Model guidance
is still mixed on exactly what this reorganization ultimately looks
like. While development of a tropical cyclone in the vicinity of the
Marianas is possible, the ongoing differences in guidance between
various deterministic global models, and substantial model spread in
ensemble member data leads to continued low confidence at this time.
Heavy rain is the biggest clear concern at this time for Guam up to
Saipan.

&&

.Marine...
E to SE trades build a notch tomorrow and Wednesday night, allowing
seas (and short-period surf) to rise enough to trigger a moderate rip
risk for east facing reefs. Winds and seas then relax a notch on
Thursday as the pattern eases and the trough moves in. Normally calm
and smooth west facing coasts may see a bump in chop this weekend as
the monsoon tries to edge in from the west.

&&

.Hydrology...
There is a potential for Guam and the CNMI to see rainfall amounts
varying from 5 to 10 inches by Sunday, though locally heavier amounts
are possible. This rainfall is expected to produce minor flooding
across the islands, with flash flooding possible in the areas of
heavier rainfall.

Residents in the Marianas, especially those in flood-prone areas,
should monitor this developing weather pattern closely as watches,
advisories and warnings could be issued late this week.

&&

.Tropical systems...
With Tropical Storm Sepat far to the north, our attention turns to
the reorganization of the monsoon trough in the vicinity of the
Marianas.

The GFS, in addition to being the most aggressive in rate
of development, has strongly favored the eastern side of the islands
for genesis. However, recent runs of the GEFS have shown some
westward spread in the potential development area. While its still
too soon to state any certainty, one thing to watch for in the next
24-36 hours will be for a push of westerly winds from the Yap area to
the southeast of Guam by tonight or early Wed. That would lend more
support to GFS guidance. However things shape up late this week, the
common outcome among models is for a much wetter pattern in the
Marianas.

&&

.Eastern Micronesia...
An active portion of the Intertropical Convergence Zone (ITCZ)
is producing showers and thunderstorms over Kosrae, Pohnpei and the
Ralik chain of the Marshalls. Waves of showery weather are likely to
continue for Pohnpei and Kosrae the next several days, with gusty
conditions at times. Majuro still looks to sit on the eastern edge of
this regime with isolated to low-end scattered showers expected
through at least midweek.

Combined seas of 4 to 5 feet look to continue for Pohnpei and Kosrae
through the forecast period, as easterly swell slowly diminishes and
a southeasterly swell strengthens. Slightly higher trade swell is
likely for well-exposed waters around Majuro. Residents should be
aware of the large tides the next several days due to the new moon
phase despite the fact no notable swells are currently forecast.

&&

.Western Micronesia...
Chuuk sits between the primary synoptic players with the monsoon to
the west and the trades and ITCZ to the east. Subtle features
embedded in the low-level flow are likely to trigger bursts of
showers and thunderstorms at times the next couple of days. Models
hint at a more stable, weak trade flow late in the week. Seas lower
through the remainder of the week as local winds and upstream sources
taper off.

Little changes were made to the forecasts farther west with Palau and
Yap likely to see a monsoonal pattern slowly re-establishing. WSO
Yap reported over 2 inches in a short time frame this afternoon,
supporting previous forecasts of locally heavy rains. A quieter
pattern looks to set up for Palau and Yap Wednesday through the
latter half of the week as the monsoon trough shifts north. However,
the weather pattern and positioning of the monsoon trough will
partially depend on the development and movement of a yet-to-form
tropical disturbance, which still looks to assemble near the Marianas
in the latter half of the week. Seas will be benign for Palau and
Yap due to the lack of elevated winds locally or in a distant sense.
Residents should be aware of the large tides the next several days
due to the new moon phase despite the fact no notable swells are
currently forecast.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.

&&

$$

Marianas: Bukunt/W. Aydlett
Tropical: Bukunt/W. Aydlett
East Micronesia: Bukunt
West Micronesia: Bukunt