474
FXPQ50 PGUM 120826
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
626 PM ChST Sun Oct 12 2025

.Marianas Synopsis...
Satellite and radar this evening show isolated showers and partly
cloudy skies across the islands, with fairly uniform conditions
upstream. Buoy and altimetry data indicate combined seas of 4 to 6
feet within coastal waters.

&&

.Discussion...
Fairly quiet conditions continue this evening. A subtle trade-wind
trough just east of Marianas coastal waters looks to slightly
increase showers late tonight as it moves through the area. The
primary forecast concern is the arrival of a potential tropical
disturbance around midweek, originating from a broad, east-west
oriented trough and circulation currently located over Chuuk State.
General model consensus pushes the broad circulation south of Chuuk
Lagoon off to the west, where it looks to weaken as an open surface
trough over far western Micronesia. However, the more aggressive
model solutions continue to show the development of a strong
circulation in the vicinity of the Marianas by midweek, originating
from the elongated trough that extends east-northeast from the
circulation south of Chuuk towards Pohnpei. The latest runs of the
GFS continue to place a potential tropical depression or even a weak
tropical storm in the vicinity of the Marianas around Wednesday
morning. However, latest ensemble model guidance indicates a very low
probability of this, with most models maintaining the disturbance as
a robust surface trough as it lifts northwest across the islands.
The primary forecast concerns will be locally heavy showers and
strong gusty winds, with ensemble guidance indicating rainfall
totals of 1 to 3 inches from Tuesday night through Wednesday night.
While there is plenty of uncertainty with the development of any
circulation, the approaching trough is expected to bring wet
conditions around midweek, with drier conditions to resume for the
latter half of the week.

.Marine/Surf...
Combined seas will remain around 4 to 6 feet for the next few days,
mainly comprised of a long-period north swell and trade swell. Winds
will be gentle to moderate out of the northeast to east for the next
couple days, becoming moderate to fresh around midweek as a tropical
disturbance lifts northwest through the area. This will bring seas up
to 5 to 7 feet for the latter half of the week, due to increasing
wind waves and as another pulse of north swell looks to move in by
the end of the week. A moderate risk of rip currents will continue
along north, east, and west facing reefs of the Marianas through
early next week, as long-period north swell continues to generate
elevated surf along north-facing reefs, with some wrap-around along
adjacent reefs. Although the period and height of the north swell
will gradually diminish, maintain caution as narrow reefs and
channels may still see strong rip currents develop.

&&

.Eastern Micronesia...
A developing circulation is positioned south-southeast of Chuuk.
Extending east-northeast from this feature is a surface trough that`s
currently giving Pohnpei numerous showers. Then closer to Majuro,
another trough is located with a "cusp"-like circulation trying to
develop east of there, locally enhancing convective coverage east of
Majuro. These features will play a role in shower coverage during the
next 24 to 36 hours with the Pohnpei trough heading west, away from
the area. The Majuro trough is also moving west and will pass
overhead as well, with some weak development of a broad circulation
not out of the question, which could boost convective coverage
further if it does happen. For now we took a "middle ground" approach
concerning these features and its associated rainfall
coverage/probabilities. Kosrae will just see isolated showers for the
next 36 hours before the Majuro trough increases rainfall potential.

As for marine conditions, some locally higher wind gusts on an
isolated basis is probable near Pohnpei and Majuro. However,
temporal and areal coverage is expected to remain isolated enough to
include in the forecast. Seas of 4 to 6 feet will continue through
Wednesday, dropping a foot or so as we head into the weekend. Light
to gentle winds will prevail.

&&

.Western Micronesia...
Scatterometer and satellite imagery today shows a broad and
disorganized circulation south-southeast of Chuuk, near 6N152E,
embedded within a broad surface trough that extends across Chuuk
State and into Pohnpei State. Right now the heaviest and area of most
numerous showers are mostly around and south of 6N across Chuuk
State, with some scattered showers passing around the islands. Model
guidance shows this circulation drifting westward towards Palau and
Yap over the next few days, but keeping it broad and lackluster with
the potential for showers at Yap and Palau through the week. The GFS
and ECMWF shows the portion of the trough extending towards Pohnpei
moving west-northwest, first bringing scattered to numerous showers
to Chuuk Monday and then moving up to the Marianas around Tuesday
night and Wednesday. The ECMWF and its ensemble keep this trough an
open trough as it moves through the region, which limits gusty winds
to locally heavy showers and thunderstorms. The deterministic GFS has
been an outlier, even when compared to its ensemble members, for the
past couple of days and shows this trough breaking off from the
broad circulation south-southeast of Chuuk, and developing into a
circulation and organizing as it moves through the Marianas and into
the Philippine Sea. A developing circulation would likely increase
the potential for gusty winds across Chuuk. Right now the forecast is
leaning more towards the ECMWF but did keep the potential for gusts
for Monday, with a upper limit around 30 kt (~35 mph), as both the
GFS and ECMWF show the potential for numerous showers. Also, with the
TUTT and TUTT cell southwest of Wake Island, this upper-level
feature may help create an environment supportive for gusty winds in
heavier showers and thunderstorms. As this feature moves off to the
northwest, winds and showers are expected to decrease Tuesday and
Wednesday, before the next series of troughs move into the area from
near Pohnpei, helping to keep periods of scattered showers in the
forecast for later in the week.

For Palau and Yap, winds were light today and that was enough,
despite the drier air suggested by the Blended TPW satellite product,
for island-effect showers and thunderstorms to develop over Palau
during the peak heating of the day. With the sun setting, showers are
isolated across the area though passing cumulus fields are keeping
skies partly to mostly cloudy. Similar conditions are expected
Monday, so decided to add island effect showers and thunderstorms to
Palau`s forecast. Yap Proper is usually just small enough to support
sustained island-effect, so kept showers isolated. Continuing to
watch the broad circulation south-southeast of Chuuk, but models keep
this circulation very disorganized and so far are lackluster with
shower potential at both Palau and Yap this week. Part of this is due
to the GFS supporting a circulation developing from a trough as it
moves through the Marianas, which supports a ridge pattern across
Palau and Yap later this week. But this same pattern is not present
in the ECMWF, however even this model is not that supportive with
showers over the next few days, but does shows a slight increase
around Thursday due to a passing trough. Regardless, winds are
expected to remain fairly light across the region, so additional
periods of island-effect showers will be possible at Palau during the
daytime hours through the week, so monitor the forecast for these
minor changes.

Altimetry data shows combined seas of 3 to 5 feet extend across the
region. Seas are still expected to increase about a foot or two near
Chuuk Monday as a slight increase in the northerly swell and an
increase in wind waves due to gusts generated by showers and
thunderstorms as a trough or disturbance moves through. This increase
in wind waves and gusts may make seas choppy around Chuuk Monday,
but seas are expected to remain below hazardous levels of 10 feet,
with sustained winds around 20 kt or less. Surf along north facing
reefs will remain elevated across Palau, Yap, and Chuuk this week,
due to pulses of longer-peiod northerly swell moving through the
region. Surf along north facing reefs is expected to remain below
hazardous levels of 9 feet at this time.

&&

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

$$

Marianas: DeCou
East Micronesia: Doll
West Micronesia: Schank