584
FXPQ50 PGUM 130933
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
733 PM ChST Mon Oct 13 2025

.Update...
Forecast updated to reduce showers from widespread to scattered for
Majuro tonight and removed wording for locally heavy showers. Showers
along the fragmented ITCZ have decreased in coverage and are not
expected to redevelop much overnight, as the heavier showers remain
to the east of Majuro. Will continue to monitor if the heavier
showers shift westward into Majuro for Tuesday.

&&

.Prev discussion... /issued 658 PM ChST Mon Oct 13 2025/

Marianas Synopsis...
Satellite and radar this evening show isolated to scattered showers
across the Marianas, with most of the cloud cover and showers focused
over Guam coastal waters. The upstream environment indicates more of
the same, with scattered showers and isolated thunderstorms further
east of the islands moving in. Buoy and altimetry data indicate
combined seas around 5 to 7 feet.

Discussion...
Scattered showers and isolated thunderstorms will continue across
the area overnight and increase through tomorrow as a broad surface
trough begins to build northwest into the area, currently extending
east-northeast from out of Invest 96W (discussed below) to the
northwest of Chuuk Lagoon. This trough is under the influence of the
nearby broad TUTT cell to the northeast, and looks to maintain a wet
and gusty pattern over the Marianas through midweek. However, models
vary on the strength and nature of the disturbance. GFS continues to
persistently and aggressively develop a tight circulation within the
surface trough, possibly as a weak tropical storm over the Marianas
around Wednesday morning. However, ensemble model guidance paints
this solution as an unlikely outlier, with most models maintaining
the disturbance as a robust surface trough as it moves across the
islands. Ensemble guidance indicates a possible 2 to 4 inches of
rainfall, mainly over Guam and Rota waters, from Tuesday night
through Wednesday night. The most widespread and heaviest showers
look to be focused near Guam, which had Probability of Precipitation
(PoP) increased for Tuesday night and Wednesday, with wording for
locally heavy showers. The most likely impacts from this event will
be locally heavy rainfall with near-gale gusts possible at times, but
sustained winds look to remain below levels hazardous to Small Craft
for the time being. However, if the disturbance begins to strengthen
more quickly than expected, or strong gusts become sufficiently
frequent, a Small Craft Advisory will likely be issued. Conditions
dry out during the latter half of the week as the disturbance moves
away, with isolated to scattered showers expected from Thursday into
the weekend.

Marine/Surf...
Combined seas of 5 to 7 feet will continue for the next day or so,
mainly comprised of a long-period north swell and trade swell.
Winds will be moderate and easterly tonight, becoming moderate to
fresh tomorrow through midweek as a tropical disturbance builds
and lifts northwest through the area. The increase in winds,
gusts, and wind waves will bring combined seas up by a foot or
two, mainly Tuesday night through Wednesday. Seas may briefly dip
around Thursday, but another pulse of north to northeast swell
will keep sea heights around 5 to 7 feet through the end of the
week. Winds and seas could rise higher by midweek if the
disturbance intensifies more rapidly than expected. There will be a
moderate risk of rip currents along north, east, and west facing
reefs of the Marianas through at least the next few days, spreading
to south facing reefs Tuesday night as the southerly swell energy
increases. There will also be a moderate risk of lightning starting
tonight, lasting through at least midweek.

Tropical Systems...
The Joint Typhoon Warning Center (JTWC) is monitoring the broad
disturbance across Chuuk State and extending into Yap State as Invest
96W, centered near 6N144E. Scatterometer data shows an elongated
circulation in this area, embedded within a broad trough that extends
eastward from the Philippines, through the center of 96W, and then
curves northeast through Chuuk State. The easternmost portion of this
trough is under the influence of the nearby TUTT and broad TUTT cell
to the northeast, with numerous showers and isolated to scattered
thunderstorms building within it this evening. This morning`s
scatterometry reveals an area of 15-20 kt southwest winds to the east
and southeast of the center of 96W, with stronger convective gusts
of 20-25 kts seen south of Satawal. Convection is disorganized across
96W and models keep the circulation elongated and weak as it shifts
westward, potentially opening back up into a trough, so JTWC has
Invest 96W rated as LOW for development, meaning development into a
tropical cyclone is unlikely within 24 hours. The GFS remains the
most extreme outlier when compared with the other models, with
supporting a second circulation developing in the trough extending
through Chuuk State. However, what overall model consensus does
support is increased rainfall across the Marianas Tuesday night and
Wednesday as this trough moves through the Mariana Islands.

For more information on Invest 96W, see the bulletin issued by the
Joint Typhoon Warning Center under the WMO header ABPW10 PGTW.

Eastern Micronesia...
A surface trough has exited Pohnpei`s area, with the next item of
interest being a trough currently passing trough Majuro that
stretches southwest to northeast from there. This feature denotes
the leading edge of enhanced surface convergence, which the GFS GDI
(Galvez-Davison Index) suggests will mark the resurgence of an ITCZ
that`ll slowly expand west as it gradually intensifies during the
week. As such, we inserted locally heavy rainfall wording at Majuro
for tonight and Tuesday while significantly increasing rainfall
probabilities based on satellite analysis and model forecasts. This
increasing rainfall potential will spread south and west as we head
through the week,

Marine wise, the advertised northerly swell showed an increase to
near 4 feet at Pohnpei today, and the model forecasts show this
swell continuing to increase in height (peaking at 6 feet) as it
spreads east. As such, given a wave period of ~10 to 11 seconds, this
puts surf along north reefs around 8 feet at Pohnpei and Kosrae. As
such, a headline was included in the forecast at these locations. The
northern-most atolls of Majuro will likely see a similar effect,
but, the increased density of the atolls north of Majuro should keep
surf slightly lower there. However, we`ll keep an eye on this.

Otherwise, seas of 5 to 7 feet will increase a foot or so through
Thursday at Pohnpei and Kosrae, and roughly a foot lower at Majuro
(at least for now). These wave heights will gradually diminish from
northwest to southeast across the region as we head into the weekend.
Light to gentle winds will prevail through this week.

Western Micronesia...
The Joint Typhoon Warning Center is monitoring the broad disturbance
across Chuuk State and extending into Yap State as Invest 96W,
centered near 6N144E, with scatterometer data showing an elongated
circulation in this area. Convection is disorganized across Invest
96W, with scattered showers and isolated thunderstorms across most
of Yap State. Showers are becoming more numerous near and north of
Chuuk Lagoon, likely due to the upper-level divergence associated
with the Tropical Upper Tropospheric Trough (TUTT) and TUTT cell that
is positioned east of the Marianas and north of Chuuk State. Some
gusty winds around 25 kt (~30 mph) are still possible around Chuuk
tonight. Model guidance has been consistent that the broad
circulation drifting westward will continue towards Palau and Yap,
potentially opening back into a trough. This will bring periods of
scattered showers and isolated thunderstorms at Palau and Yap through
Thursday, however models continue to be lackluster with precip
totals as the GFS has about an inch of rainfall through Tuesday
night, with the ECMWF closer to 1 to 2 inches during the timeframe.
The main focus for precip is currently around Chuuk State, associated
with a trough extending northeast from Invest 96W and is currently
under the upper-level support provided by the TUTT. This trough is
expected to shift northwest up towards the Marianas Tuesday and
Wednesday, which will pull showers away and lead to a decrease across
Chuuk State trough Wednesday, before the next series of troughs move
into the region, helping to keep periods of scattered showers in the
forecast for later in the week.

Combined seas are around 3 to 5 feet around Palau and Yap. Seas are
pushing up to 6 feet at Chuuk due to the north swell and increased
wind waves due to gusts generated by showers and thunderstorms. As
Invest 96W and the trough extending northeast from 96W pull away
from Chuuk, seas are expected to fall back closer to 3 to 5 feet.
Surf along north facing reefs will remain elevated across Palau, Yap,
and Chuuk this week, due to pulses of longer-period northerly swell
moving through the region. Surf along north facing reefs is expected
to remain below hazardous levels of 9 feet.

&&

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

$$

Update: Schank
Marianas: DeCou
Tropical Systems: Schank/DeCou
East Micronesia: Doll
West Micronesia: Schank