799
FXPQ50 PGUM 151931
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
531 AM ChST Thu Oct 16 2025

.Marianas Update...
A weak trough trailing the departed Invest 96W will maintain showers
and thunderstorms through today before tapering off overnight.
Blended TPW data continues to show a pocket of drier air approaching
from the east. The 12Z RAOB indicated a PWat of 2.27in over Guam
which compared well to the 2.3in seen at 12Z in the Blended TPW. The
BTPW shows the drier pocket to the east with PWats ranging between
1.4-1.9in. However, this difference may be limited to the atmosphere
above 700mb, as noted in the Advected Layer Precipitable Water. While
this suggests there will not be much reduction in humidity, the
potential for showers, and especially thunderstorms, will be reduced
the next few days starting Friday. Mid-range model data suggests the
next wetter period may arrive around Sun-Mon as another trough passes
by the area.

While little change in seas is expected the next several days, did
refresh forecast wave grids to better separate a long-period north
swell from an ENE swell. Adjusting the inherited NE swell to
northerly better aligns with surf and buoy observations, as well as WW3
and GFS-wave model data. Seas will stay between 5 and 7 ft through
the weekend.

&&

.Tropical Systems...
Invest 96W continues to head west, now centered NNW of Yap near
13N137E, and still without a discernible low-level circulation
center. Both physics-based and AI weather models favor a gradual
organization and development of 96W in the coming days as it
continues away from the region. Until it pulls away, peripheral
showers and thunderstorms will affect portions of Yap State through
tonight.

A circulation was noted in overnight infrared satellite imagery and
scatterometer data near 10N175W. Deep convection has persisted near
and just SE of the circulation center for much of the past 12 hrs.
This circulation is slowly drifting westward and is expected to
continue so the next couple of days. Most deterministic and ensemble
model guidance keeps this system weak, opening into a surface trough
over the next 24-36 hrs, though some ECMWF-ensemble members maintain
the circulation and push it through the central-northern RMI around
Sunday. Interestingly, GEFS members initialize with the current
circulation, but all weaken it into an open surface trough Fri-Sat.
Additionally, AI-based models lend no support to a circulation in
the area, either currently, or in the near future. ALPW imagery shows
the circulation within a narrow band of ITCZ moisture with much drier
conditions above 700mb to the north and south. Will keep an eye on
this system and any changes in model trends the next few days in the
event development shows a higher likelihood of occurring.

&&

.Eastern Micronesia Update...
As anticipated, the circulation that was passing to the north of
Kosrae yesterday has opened into a surface trough and is now passing
through Pohnpei State. Fairer weather is on tap for the short term at
Kosrae while Pohnpei will see showers and thunderstorms today with
winds shifting to the SE. Majuro is fairly dry, though nearly
overcast with a deck of mid- to upper-level clouds that should
decrease through the day. East of the dateline, a circulation is
noted near 10N175W in the overnight scatterometer analysis. More on
this circulation and its prospects for development can be found in
the Tropical Systems section, above. As this circulation continues
westward, it will bring increasing showers to the Marshalls around
Saturday, and Kosrae-Pohnpei Sunday-Monday. Little has changed for
regional waters, 6-8 ft seas dominated by a long-period N swell are
expected in the short term for Pohnpei and Majuro. Seas will be about
a foot less for Kosrae, also dominated by a N swell. Seas will edge
downward 1-2 ft by the weekend as N swell diminishes.

&&

.Western Micronesia Update...
A broad trough just NW of Yap and Palau continues to drift west from
the two locations. A large cluster of thunderstorms has persisted to
the north of Yap overnight near 96W, in the vicinity of 13N138E and
still without a discernible low-level circulation center. Despite
southerly winds flowing across Yap island and into the large cluster
of thunderstorms, the massive convective feature has shown a
southward propagation. Have adjusted the Yap forecast to indicate
deteriorating conditions later this morning with increasing gusts
with showers and thunderstorms. Palau, still experiencing fair
weather and light winds could see afternoon showers and thunderstorms
developing today based on island heating. Chuuk, while fairly dry for
today, as noted with a 12Z RAOB PWAT of only 1.65in, will see
showers increasing this evening as a trough approaches from the east
with deeper moisture. No changes were needed regarding marine
conditions, no marine hazards are anticipated though a long-period
N swell will keep surf elevated along north facing reefs.

&&

.Prev discussion... /issued 639 PM ChST Wed Oct 15 2025/

Marianas Synopsis...
Satellite imagery shows overcast skies and radar imagery shows
numerous showers in western Guam and Rota coastal waters and
scattered showers and eastern Guam and Rota waters. Radar also shows
isolated to scattered showers in for Tinian and Saipan and their
waters. Altimetry shows combined seas to the west of the Marianas
coastal waters a 6 to 8 feet and to the east 4 to 6 feet. Ritidian
and Ipan buoys show about 5 feet.

Discussion...
For Guam and Rota, Invest 96W remains the big weather maker for the
region. 96W is producing numerous showers and isolated thunderstorms
over the islands. As 96W continues to move further away from the
Marianas, these showers will continue to diminish. There are
scattered shower over the eastern waters that are expected to make
their way over Guam and Rota around midnight. These showers are
expected to bring a 0.5 to 1 inch of additional rain over the night
time hours. Once 96W moves further away, passing trade-wind trough
are expected to bring periods of scattered showers to the islands
through the middle of next week.

For Tinian and Saipan, these islands will remain north of the heavier
showers produced by 96W. Once 96W pulls a bit further away, a wet
season trade- wind pattern is expected to continue. Passing trade-
wind trough are expected to bring periods of scattered showers to the
islands through the middle of next week.

Marine...
Moderate to Fresh easterlies with strong gusts possible are expected
in Guam and Rota waters through late tonight while remaining
moderate to fresh at Tinian and Saipan as Invest 96W passes by to the
southwest of Guam tonight. Combined seas of 6 to 8 feet near Guam
and Rota, and 5 to 7 feet near Tinian and Saipan, will diminish 1 to
2 feet by Thursday. Over the weekend, seas are expected to fall to 4
to 6 feet. Showers and thunderstorms, primarily across Guam and Rota
waters, will persist through tonight. There will be a moderate risk
of rip currents along north and east facing reefs of the Marianas
over the next few days. Surf is expected to diminish along all reefs
once Invest 96W moves out of the region and distant extratropical
cyclone Nakri, moves further away from the western Pacific. A
moderate risk of lightning is expected the next few days.

Hydrology...
Invest 96W is moving further west and faster than previously
anticipated. Radar shows numerous showers are mainly west of Guam and
Rota with scattered showers to the east of the islands. The lower
category of showers is due to the ingest of drier air which is
weakening the showers. As Invest 96W continues to pull away to the
west, showers are expected to continue to decrease through Thursday.
So far today, 4 to 5 inches of rain has fallen since midnight.
Additional rainfall overnight is expected to be 0.5 to 1.0 inches.
This additional rainfall is not expected to produce widespread flash
flooding.

Tropical Systems...
Invest 96W does have a mid-level circulation and is supporting a
surface trough under it. The circulation is located near 14N141E,
and the axis is located near 18N144E and extends southwest passing
west of the Marianas to 10N141E near Sorol. Convection associated
with this feature is moderate to deep, with the heaviest showers west
of Guam and north Sorol. An upper-level trough is aiding in
thunderstorm development. This feature is expected to continue moving
west into the Philippine Sea and into more favorable conditions.
Joint Typhoon Warning Center (JTWC) has this system as a low. This
means that development is unlikely within the next 24 hours.

For updated information on 96W, please refer to the Significant
Tropical Weather Advisory, issued by the Joint Typhoon Warning
Center, under WMO header ABPW10 PGTW.

Eastern Micronesia...
A trade-wind disturbance embedded within a broad surface trough
remains to be the primary driver for weather across much of eastern
Micronesia. After multiple cycles of intense wind and rainfall near
Kosrae, the system is now in the beginning stages of degrading into
an open wave as it heads toward Pohnpei, where it will bring on-and-
off showers and some thunderstorms the next few days. Overnight,
Kosrae will see showers taper down significantly, with only low-end
scattered showers and gentle to moderate winds expected. The buckling
of the ITCZ against said disturbance from the east, has brought
extensive rainfall to Majuro, with exactly 3.00 inches falling at the
airport in the last 24 hours. Even so, showers will now diminish
considerably there as well, as clouds begin to stratify and the
disturbance moves away, allowing diffusive flow upstream to further
strip the area of moisture needed for shower development. In the
longer term, the region will enter a quieter regime, as a weaker
broad surface trough brings low-end scattered showers through the
weekend.

Combined seas of 6 to 8 feet near Pohnpei and Majuro, and 5 to 7 feet
near Kosrae, are comprised of a significant northerly swell generated
by distant cyclones, and a secondary trade swell. This northerly
swell is producing elevated surf along north facing reefs of all
three forecast points, which may briefly hit hazardous surf criteria
overnight, before beginning to slowly diminish for the rest of the
week thereafter. This northerly swell is expected to taper down
enough to be overtaken by the east swell sometime early next week. As
these swells decrease, so will the seas, which are expected to drop
2 to 3 feet across the region by the weekend.

Western Micronesia...
Satellite imagery shows some scattered showers at Palau, partly
driven by earlier island heating that is starting to diminish as the
sun sets, and from the elongated circulation that is continuing to
open into a broad trough and keeping winds light. At Yap, showers
have decreased to isolated this evening, but are expected to become
scattered again overnight as convergent southerly flow strengthen
slightly as Invest 96W continues to move west-northwest into the
Philippine Sea, west of Guam and well north of Yap. The gentle to
occasionally moderate southerly flow supported by Invest 96W will
help to support occasional upticks in showers and thunderstorms
across both Yap and Palau through at least Thursday. After Thursday
the influence of Invest 96W will diminish, but as the easterly flow
returns to Palau and Yap, passing trade-wind troughs may bring
additional upticks in showers this weekend.

Over by Chuuk, satellite imagery shows isolated showers this
evening, after a slight uptick in showers from a passing trough. The
next trough is currently located near Pohnpei and Kosrae and is
slowly drifting westward. It looks like drier weather will continue
at Chuuk through Thursday, while winds slightly increase to gentle to
moderate tonight. As the trough and area of deeper moisture currently
near Pohnpei approaches Chuuk Thursday night, showers will increase
along with the potential for isolated thunderstorms. Drier weather is
expected to return Friday and Saturday once that trough moves west
of Chuuk, but additional troughs are expected to follow later this
weekend, bringing occasional upticks in showers.

Seas will remain dominated by a long-period north to northeast swell
with combined seas averaging between 4 and 5 ft, occasionally
reaching nearer 6 ft at Chuuk. This trend is currently matching well
with current altimetry data and drifting Sofar buoys located to the
north of the islands.

GU...None.
MP...None.
Marianas Waters...None.

$$

Marianas/Tropical/Hydrology: Bowsher
East Micronesia: Montvila

&&

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

$$


Marianas/East and West Micronesia/Tropical Update: Aydlett
Marianas/Tropical/Hydrology: Bowsher
East Micronesia: Montvila
West Micronesia: Schank