897
FXPQ50 PGUM 151901
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
501 AM ChST Wed Jul 16 2025

.Marianas Update...
The potential for showers and thunderstorms is increasing through
the end of the week as a couple of surface troughs pass through the
region, along with the TUTT and TUTT cell passing overhead. Periods
of unsettled weather expected to continue into the weekend as the
monsoonal pattern in western Micronesia and near-equatorial trough
near Chuuk move near the Marianas.

Gentle to moderate east to southeast winds will continue across the
region through the weekend, with seas of 4 to 6 feet increasing to 5
to 7 feet by Thursday night as the west-southwest swell builds
slightly. There will be a moderate risk of rip currents along east
facing reefs of the Marianas through at least the end of the week.
Around Thursday, a moderate risk looks to begin along south and west
facing reefs as the west-southwest swell builds.

&&

.Tropical Systems Update...
Invest 96W remains a weak and broad circulation located near
14N133E, north of Palau and northwest of Yap Proper. However,
convection is flaring closer to the center. Invest 96W remains
classified a sub-low by the Joint Typhoon Warning Center and the
potential for it to develop into a significant tropical cyclone over
the next 24 hours is very low. General model trend is for 96W to
slowly develop and become better organized as it creeps west-
northwest into the Philippine Sea and out of the region. As 96W
develops and strengthens, the monsoon flow could over the Republic
of Palau and western Yap State.

&&

.Eastern Micronesia Update...
A moderate Inter Tropical Convergence Zone and a few trade-wind
troughs running through Eastern Micronesia will keep scattered
showers and mostly cloudy skies in play through the weekend. Clouds
have stratified out for now so eliminated the chance of thunderstorms
for the next period or two.

Mainly east winds with a few southeast winds of 5 to 15 knots will be
in the Eastern Micronesian Waters for all 3 marine zones this week.
For seas, Majuro will have 4 to 7 feet, Kosrae a slightly lower 4 to
6 feet, and Pohnpei the lowest at 3 to 5 feet. However, the seas at
Kosrae could start diminishing around Friday and could reach levels
similar to those at Pohnpei for the weekend.

&&

.Western Micronesia Update...
Western Micronesia has seen quite a bit of stratification also, but
not quite to the same extent as Eastern Micronesia, so did not remove
the thunderstorms, just cut the numerous showers at Palau to
scattered. The models are adamant that Yap is going to get some
serious stuff later today, so left Yap at numerous showers. Due to
the proximity of a monsoon disturbance, Koror, Palau will have
scattered showers the entire week, and Yap will have them until
Thursday, then they will come back again that night, so a pretty wet
week ahead overall. Chuuk will also have scattered showers all week,
except for Thursday and Thursday night, so again fairly wet there as
well.

Chuuk will have mainly east winds 5 to 10 knots, and much of their
shower activity will likely be from East Micronesia features
intruding their way into Western Micronesia. Seas at Chuuk will be 3
to 5 feet. Seas at Yap will also be in the 3 to 5 foot range. For
winds, Yap will have southeast varying to southwest as the monsoon
battles the trade winds for supremacy. Meanwhile at Koror, Palau,
that battle is over, and the winds will be mainly southwest as the
monsoon makes its presence known.

&&

.Prev discussion... /issued 600 PM ChST Tue Jul 15 2025/

Marianas Synopsis...
Satellite and radar imagery this afternoon shows isolated showers
across the Marianas with scattered showers off to the east along a
subtle trade-wind trough that is expected to move through tonight.
Latest buoy and altimetry data show seas are around 4 to 5 feet,
increasing to 5 to 7 feet east of 150E.

Discussion...
A subtle trade-wind trough is approaching the Marianas, with
scattered showers visible on the eastern edge of the radar`s range,
and should reach the Marianas by midnight tonight. The potential for
thunderstorms is increasing as the TUTT and embedded TUTT cell west
of Wake Island is nearing the Marianas, with the most favorable
environment for thunderstorms Wednesday through at least Friday.
There were a couple of short-lived thunderstorms west of Saipan and
Guam earlier this afternoon, likely influenced by the localized
heating of the air near the islands. Model guidance also continues to
support the monsoonal pattern across western Micronesia, along with
the near-equatorial trough in near Chuuk and Pohnpei, moving closer
to the Marianas Thursday night and Friday, which will promote a wet
pattern for the Marianas. However, southwest to west monsoon flow is
not expected to extend all the way to the Marianas at this time, as
the GFS continues to trend slightly towards the ECMWF, which pulls
the monsoon trough more northwest as a broad circulation develops in
the Philippine Sea northwest of Yap and swell west of the Marianas,
keeping the Marianas in a east to southeast flow. This supports the
forecast that showers will be more scattered and episodic in nature,
but there is always the potential for locally heavy showers as minor
shifts in the pattern could be the difference between heavy showers
remaining west of the Marianas or developing over the islands.

Looking towards the weekend and early next week, most models
continue to support that the monsoon pattern and potential for
tropical disturbance development will remain west of the Marianas,
but model variability on this high remains quite high. Continue to
monitor the forecast and future discussions to keep up with any
changes in the overall trends.

Marine...
Gentle to moderate southeast to east winds will continue across the
region through the weekend, with seas of 4 to 6 feet building to 5 to
7 feet Thursday night as the west-southwest swell builds slightly.
This will build surf along south and west facing reefs slightly,
likely leading to a moderate risk of rip currents. Not expecting seas
and surf to build much higher through the weekend, as west to
southwest winds upstream from the Marianas and in western Micronesia
have been holding close to 15 kt, with only some gusts to 25 kt near
heavier showers, limiting how much the west-southwest swell will
build. The potential for thunderstorms is increasing as the TUTT and
TUTT cell are moving closer from the east. Scattered showers are
expected to move into the Marianas tonight as a subtle trade-wind
trough moves through tonight, with periods of unsettled weather
expected to continue as the monsoonal pattern in western Micronesia
and near-equatorial trough near Chuuk move near the Marianas.

Tropical systems...
The Joint Typhoon Warning Center has opened Invest 96W on the weak
and broad circulation located near 14N134E, north of Palau and
northwest of Yap Proper. 96W is currently disorganized so the
potential for develop into a significant tropical cyclone over the
next 24 hours is very low. Models have favored a circulation
developing near this location, and predict that 96W will remain
northwest of Palau and Yap as it slowly develops in the Philippine
Sea, slowly moving west-northwest and out of the region. However, as
96W develops it may strengthen the monsoon flow across Palau and
western Yap State, leading to locally heavy showers, gusty winds, and
choppy seas at times.

Eastern Micronesia...
Satellite observations show scattered showers across eastern
Micronesia with numerous showers seen east-southeast of the RMI.
Buoy and altimetry data shows combined seas between 4 to 6 feet
across Pohnpei and Kosrae and 5 to 7 feet across the RMI.

There has not been much change in thought from the previous forecast
packages. Eastern Micronesia looks to remain active through much of
the period with brief pauses in activity. Shower activity for Kosrae
and Pohnpei looks to remain 30 to 40% through much of the period with
Majuro seeing a 40 to 50% chance of showers. Towards the latter half
of this weekend, guidance is starting to show another robust
ITCZ/trade-wind convergence, which would allow for an increase in
shower and thunderstorm activity. For now, kept "likely" PoPs for
only Majuro, but guidance does suggest that higher PoPs could trend
further west towards Kosrae and Pohnpei.

Combined seas are generally between 4 to 6 feet across Pohnpei and
Kosrae and 5 to 7 feet across the RMI. Model guidance shows a
weakening in the trade-winds this week, allowing combined seas and
swells to fall to 3 to 5 feet across Pohnpei and Kosrae and 4 to 6
feet across the RMI. Choppy seas will remain possible near heavier
showers along with occasional gusts to 25 kt.

Western Micronesia...
A building convergence zone along the southern peripheries of a large
developing monsoon gyre will continue to produce convectively active
southwesterlies to Palau and Yap the next few days. The deepest
convection is expected to form Wednesday, with numerous showers and
some thunderstorms carried by fresh to near-gale gusts through the
afternoon. Showers are likely to be locally heavy at Yap, as the
aforementioned monsoon gyre shifts northward. Showers are expected to
diminish considerably at both locations in the second half of the
week as a ridge builds along the lee side of said convergence,
followed by a return of patchy showers toward the weekend. Meanwhile
at Chuuk, Patchy showers will diminish slowly through midweek, with
a brief drier cycle expected Thursday and Thursday night, followed
by another unsettled pattern toward the weekend.

Combined seas of 4 to 6 feet are expected near Palau and Yap the
next several days, with occasional upticks of 1 to 2 feet as
localized wind waves pick up considerably, to then diminish to
between 3 and 5 feet by early next week. Combined seas of 3 to 5 feet
are expected in Chuuk waters for much of the forecast period.

&&

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

&&

$$

Marianas: Slagle
East/West Micronesia: Stanko