154
FXPQ50 PGUM 271909
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
509 AM ChST Thu Aug 28 2025

.Marianas Update...
A broad monsoonal disturbance lifting NW from Yap and convergent SE
winds over the Marianas will maintain a wet pattern for today. Latest
satellite and radar imagery show the heaviest of showers and
thunderstorms aligned NW-SE just to the north of Saipan. Models show
slightly drier conditions the next day or two before another surface
trough rises from central-east Micronesia bringing another wave of
showers over the weekend. Ensemble model guidance points to the
possibility of 2-4in accumulation from today through Sunday night.
While the more aggressive GEFS and GEPS members remain outliers with
regard to wind and rainfall, the overall forecast does favor
increased showers and moderate, to occasionally fresh, winds over the
weekend. Seas of 3 to 5 feet will continue into the weekend with
occasional wind-driven short-term increases.

&&

.Eastern Micronesia Update...
Overnight satellite imagery shows a very wet pattern persisting for
the region with heavier showers sitting just east of Pohnpei, all
around Kosrae, and just south of Majuro. A 1459Z AMSR2 89GHz
microwave pass showed the very heavy showers surrounding Kosrae and
midway between Pohnpei and Kosrae. Additionally, satellite-derived
moisture data shows PWATs of 2.5-2.8in across the region. This is all
tied to a surface trough perched across the area, extending ENE from
a weak circulation south of Chuuk near 3N153E to the southern RMI.
This trough will continue WNW, sweeping easterly winds to the south-
southeast through the day. Showers at Majuro will briefly taper this
afternoon before an east-west band of ITCZ convection fills in from
the east. Kosrae and Pohnpei will continue to experience very wet
conditions into Friday, possibly with accumulations of 2-4in, before
a brief let up in showers. While winds will generally be moderate,
stronger gusts to around 25 kt, as seen in morning AMSR2 ocean
surface wind data, will be possible with heavier showers. Seas will
stay within the 4-6 ft range the next few days except for short-
lived spikes to 7-8 ft in areas of stronger winds.

&&

.Western Micronesia Update...
Overnight satellite imagery and surface analyses show that the
monsoon trough has lifted to the north of Yap allowing westerly to
southwesterly flow to envelop much of Yap State. Westerlies remain
across Palau with scatterometry and AMSR2 ocean surface wind data
showing 10-15 kt with patches of 20-25 kt within showers. A weak
circulation is noted south of Chuuk, anchoring the broad, wet surface
trough extending ENE across Eastern Micronesia. Made a few short-
term adjustments to delay the very wet conditions at Chuuk another
day or 2. Showers and thunderstorms are still expected today, but the
more widespread convection will hold off just a little while longer.
For Yap and Palau, convection continues to cycle through periodic
flare ups. Currently, the area is in a down pulse of activity with
just cloudy skies, though showers are expected to redevelop later
today. Over the weekend, the trough near Chuuk will trek WNW,
bringing increased moisture to Yap and Palau. Ensemble model guidance
suggests 3-5in of rainfall through Sunday, led by an aggressive GFS.
Seas will hover between 3 and 5 ft the next several days at Chuuk,
and between 4 and 7 ft for Yap and Palau. Highest seas will be found
within the stronger westerly flow south of Palau.

&&

.Prev discussion... /issued 559 PM ChST Wed Aug 27 2025/

Marianas Synopsis...
Satellite observations show partly cloudy skies and isolated showers
across the Marianas. Buoy and altimetry data show combined seas
between 3 to 5 feet.

Discussion...
A rather complex pattern has taken shape across the West Pacific.
Broad cyclonic flow is seen across western Micronesia where a
Westerly Wind Burst (WWB) is present south of Palau. Meanwhile, an
active but fragmented ITCZ is located across eastern Micronesia
where the latest ASCAT shows a weak circulation developing along the
edge of the convection near 160E. Currently, the Marianas are in
between these weather systems where there is a brief lull in the
weather. As the weather pattern shifts northwest, we`re looking at
the potential for some impactful weather this weekend.

In the short-term, we are watching a subtle surface trough approach
the Marianas. Although the trough is not convectively active at this
time, guidance shows a sharp increase in convection later tonight
starting around Guam and Rota and shifting northward through the
early morning hours. These showers and thunderstorms look to
initiate near a weak mid-level circulation south-southeast of Guam
that had initially dissipated this morning. There does appear to be
agitated cumulus north of the weak circulation.

As we head into the weekend, the forecast still remains low
confidence. Guidance remains in a good agreement on either a weak
circulation or sharp trough moving through or near the Marianas.
There has been an uptick in ensemble guidance showing a more
organized storm system near the Marianas with several GEFS, GEPS,
and EPS members showing a tropical cyclone (TC) approaching the
Marianas this weekend. Due to uncertainty on the exact location of
the broad circulation in western Micronesia, the forecast for the
direction of any system varies greatly. What we do know is, whether
this system remains disorganized or becomes a TC, impacts will be
felt well away from the center, with heavy rainfall and gusty winds
becoming increasing likely across the Marianas this weekend. At this
time, an ESF was not considered due to confidence remaining below
50% for rainfall totals that would warrant a Flash Flood Watch for
the Marianas. Those in the Marianas should continue monitor
forecasts for the latest trends.

Marine...
Some minor edits were made to the Marine forecast with this package.
Overall, combined seas look to remain 3 to 5 feet through the
weekend. The main change was to introduce a southeast well this
weekend as the south-southeasterly winds across Micronesia generate
as swell. The marine forecast for this weekend is heavily dependent
on the track and strength of a tropical wave that is expected to
move over or near the Marianas. The current forecast keeps winds and
seas below hazardous to small craft levels, but the GFS does show
the potential for winds to near or above hazardous to small craft
levels as the trough amplifies and cuts off. The forecast carries a
blend of the GFS and ECMWF, but a slight lean towards the GFS could
be necessary if observational data begins to support the potential
for higher winds. A moderate risk for rip currents continues along
east facing reefs the next few days.

Eastern Micronesia...
Pohnpei and Kosrae:
The main changes to the forecast were to increase rainfall potential
and lower temperatures as convection is plentiful near over/near you
and to your east.This is occurring as a lead surface trough is moving
through Pohnpei with another trough stretching from southwest of
Kosrae, through Kosrae, to between Kwajalein and Majuro. Excellent
convergence and deep moisture profiles will lead to numerous showers
and isolated thunderstorm. A brief period of widespread showers is
expected Thursday for Kosrae and the mid shift may need to increase
coverage to similar values at Pohnpei for tomorrow as well.

Majuro:
With good surface convergence present tonight and the trough lurking
in the neighborhood, opted for low-end scattered coverage (30%)
tonight, and scattered (50%) Thursday. Rainfall coverage briefly
drops to isolated for Thursday night, but this will be short-lived as
the next disturbance brings increasing rainfall potential early
Friday morning and continuing into the weekend, as the ITCZ remains
somewhat active.

Marine conditions:
Seas will hold in the 4 to 6 foot range through Friday before
potentially dropping a foot for the weekend. Winds generally will be
in the in the light to gentle range, with brief or isolated periods
into the moderate range when rainfall coverage is more plentiful due
to convective downdrafts. An isolated strong wind gust can`t be
completely ruled out as well.

Western Micronesia...
Yap and Palau remain entrenched within the monsoon pattern, with the
monsoon trough axis positioned just south of Yap. 10-20 kt westerly
winds, with pockets of 20-30 kt gusts, remain just south of Palau
between the equator and 7N. This area is also where most of the
heavier showers and thunderstorm are currently developing, along the
very moist flow visible in Blended TPW imagery. This moist flow will
lift northward as the monsoon pattern evolves and drifts northward,
supporting periods of gusty showers in the coming days, some which
will become heavy at times. Seas are currently around 4 to 6 feet
around Palau and Yap, according to satellite altimetry data. The
southwesterly swell will remain elevated the next few days, though
both the GFS and ECMWF wave models show a slight reduction in the
swell around Friday as both models show a brief decrease in winds,
before potentially rebuilding this weekend. Overall model guidance,
especially the ECMWF, keeps conditions below hazardous levels for
both seas (10 ft), and surf (9 ft). Model guidance continues to
differ on the wind pattern with the deterministic GFS continuing to
be more vigorous then the other deterministic and ensemble data in
showing a notable increase in SW winds. Continuing to keep the wind
forecast closer to the overall model consensus, but will continue to
monitor the overall pattern to see if conditions will become more
favorable for winds to increase this weekend.

Chuuk is still fairly dry this evening, but a robust trough is
currently near Pohnpei and is slowly moving westward towards Chuuk.
As the trough moves towards Chuuk, showers are expected to increase
overnight and continue through Saturday, though the greatest
potential for showers and thunderstorms is expected Thursday and
Thursday night. The GFS continues to point to the potential for
stronger gusts in the vicinity of Chuuk Thursday night associated
with locally heavy showers and thunderstorms. Seas will stay between
4 and 5 feet the next few days but may spike a couple feet higher in
the presence of stronger winds and gusts.

&&

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

$$

Marianas: Aydlett