416
FXPQ50 PGUM 161903
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
503 AM ChST Thu Jul 17 2025

.Marianas Update...
The forecast remains on track with only minimal updates. The TUTT
with a TUTT cell to our north is passing west of the Marianas this
morning. While showers look to stay isolated today as drier air
passes through the islands, the instability associated with the TUTT
could help initiate a few thunderstorms. Passing troughs will
promote periods of scattered showers and isolated thunderstorms
across the waters beginning tonight and continuing through the
weekend.

Gentle to moderate east to southeast winds will continue across the
region through the weekend. The west-southwest swell has been slower
to build, so seas of 4 to 6 feet are expected to continue. If the
swell comes in a bit higher than projected over the weekend, seas
could rise to 5 to 7 foot. There will be a moderate risk of rip
currents along east facing reefs of the Marianas through at least the
end of the week. The west-southwest swell looks to eventually build
over the weekend, with a moderate risk of rip currents possibly
developing along south and west facing reefs.

&&

.Tropical Systems Update...
Invest 96W continues to develop and become better organized as it
pushes west in the Philippine Sea. 96W is now centered near 14N129E,
which is west of WFO Guam`s Area of Responsibility (AOR). The Joint
Typhoon Warning Center has increased the rating for 96W to high,
meaning development into a significant tropical cyclone is likely
within the next 24 hours. 96W is expected to intensify as it drifts
northwest towards the Philippines, however, no direct impacts are
expected for any of the islands in WFO Guam`s AOR. For more
information on Invest 96W, see bulletins issued by JTWC under WMO
header WTPN21 PGTW.

&&

.Eastern Micronesia Update...
A trade-wind trough is entering the Marshall Islands, and another is
approaching Kosrae. In conjunction with a gradually weakening ITCZ
(Inter Tropical Convergence Zone), these troughs will continue to
spray showers abundantly across Eastern Micronesia. All 3 of the
forecast zones can expect at least scattered shower coverage through
Monday. At Kosrae, showers could ramp up further on Monday, and at
Pohnpei that ramp up could happen Sunday. Looking like a wet weekend.

Majuro will have winds of 5 to 10 knots today that should strengthen
tonight to between 10 and 15, then hold at that level for a while.
Kosrae will have east to southeast trade winds of 5 to 10 knots
through Monday. Pohnpei could have trade winds of 10 to 15 knots
today which will then weaken to between 5 and 10 knots for the rest
of the weekend. Seas will mainly be 4 to 6 feet, but at Pohnpei could
drift downward to between 3 and 5 feet over the weekend.

&&

.Western Micronesia Update...
Chuuk is in the trades, and has a trade-wind trough approaching. This
will promote scattered showers through tonight. Showers could become
isolated Friday, then will likely become scattered again over the
weekend. East to southeast trade winds will continue at 5 to 10 knots
through the period at Chuuk.

Yap is in the border region where the trades are clashing with the
monsoon for the next few days. With lots of convergence, showers will
be abundant, with scattered showers throughout the period. Southeast
winds of 5 to 10 knots with a few 15 knot values will turn south on
Monday as the monsoon starts to build strength and nose further east.
Seas will be 4 to 6 feet and will be a bit complicated with both
trade-wind and monsoon swell flowing through.

Koror, Palau will have south to west winds as the monsoon dominates
here. Scattered to numerous showers will keep things on the wetter
side. Most days winds will be 5 to 10 knots, but they could rise a
bit Friday night. From Saturday through Monday, east trade-wind swell
will combine with the monsoon swells to create complicated seas. Just
like Yap though, the seas will not be overly high, just complicated.

&&

.Prev discussion... /issued 539 PM ChST Wed Jul 16 2025/

Marianas Synopsis...
Satellite and radar imagery show isolated thunderstorms and isolated
to low-end scattered showers moving through the coastal waters of the
Marianas. Scattered showers and isolated thunderstorms are expected
tonight as an upper-level features moves over the Marianas. Buoy
data shows seas are around 4 to 5 feet, with altimetry showing seas
up to 6 feet east of the coastal waters.

Discussion...
The Tropical Upper Tropospheric Trough (TUTT) that has been east of
the Marianas is finally starting to move through the island chain.
TUTTs can be supportive of thunderstorm development and this
afternoon there has been an increase in thunderstorms across the
coastal waters of the Marianas. So far most of the showers and
thunderstorms have remained out in the open waters, but as the sun
sets and a trade-wind trough moves towards the islands, expect
showers and thunderstorms to increase in coverage, increasing the
chance that convection will pass over the islands. As the TUTT moves
west of the Marianas Thursday and Friday, the potential for
thunderstorms is expected to linger as the upper-level environment
remains somewhat favorable for thunderstorm development.

It is becoming more clear that the monsoonal pattern west of the
Marianas will remain off to the west as Invest 96W develops and is
expected to move northwest and out of the region over the next day or
two. A couple of troughs are still expected to move across the region
through the end of the week, but not currently expecting a washout
as showers should be episodic in nature. Winds are also expected to
be gentle to moderate and out of the east-southeast for the next few
days. New model guidance is suggesting light winds are possible late
this weekend or early next week, which could lead to island effect
showers and thunderstorms during the day time hours. It is still to
far out for any firm details, especially for Liberation Day on Guam,
but continue to monitor the forecast for any changes.

Marine...
Gentle to moderate east to southeast winds will continue across the
region through the weekend. The west-southwest swell has been slower
to build so seas of 4 to 6 feet are expected to continue for a couple
more days before building to 5 to 7 feet around Friday. This should
also keep the risk of rip currents low along south and west facing
reefs until Friday as well. The TUTT is now near the Marianas and an
approaching trough will help to generate scattered showers and
isolated thunderstorms across the coastal waters tonight. Passing
troughs will promote periods of scattered showers and isolated
thunderstorms across the waters through the weekend.

Tropical Systems...
Invest 96W is slowly organizing in the Philippine Sea near 14N131E,
with 25 kt winds found on the outer edges of the circulation. The
Joint Typhoon Warning Center now has 96W rated as a medium, meaning
development into a significant tropical cyclone is likely, but
expected to occur beyond the next 24 hours. Invest 96W is expected to
continue to drift northwest into the Philippine Sea, have no direct
impacts any of islands in WFO Guam`s Area of Responsibility (AOR) as
it exits the region. For more information on Invest 90W, see
bulletins issued by JTWC under WMO header ABPW10 PGTW.

Eastern Micronesia...
Satellite observations show isolated showers across Pohnpei and
Majuro and scattered showers and isolated thunderstorms across
Kosrae. Buoy and altimetry data show combined seas between 4 and 6
feet from Pohnpei to the western portions of the RMI and 5 to 7 feet
for the eastern portions of the RMI.

Much of today was quiet across eastern Micronesia, however this
afternoon showers and thunderstorms developed just east-northeast of
Kosrae and has expended across the northern coastal waters and
the the island. Tonight, we`re looking at shower coverage increasing
across the region. Both Pohnpei and Majuro start with isolated
showers in the evening but are expected to become scattered
overnight. The showers for Majuro are quite visible on satellite, but
the showers for Pohnpei are dependent on how much the showers and
thunderstorms grow north of Pohnpei. Both the GFS and ECMWF show an
increase in shower activity for Pohnpei tonight, developing along
the line of showers and thunderstorms that are north of Kosrae this
afternoon.

A chaotic pattern looks to take place late this week into the
weekend. The latest ECMWF shows southwesterly winds approaching
Pohnpei with the ITCZ shifting northward. This pattern would bring
a significant uptick in showers due to converging winds and embedded
circulations. The ECMWF depicts a very active and wet pattern for
eastern Micronesia. The GFS has a similar pattern, but not as
extreme. The current forecast has a slight lean towards the ECMWF due
to model performance over the last few days, but the PoP forecast
remains capped at 60% "likely" showers. If the ECMWF continues to
trend towards a westerly wind or broad circulation across eastern
Micronesia, headlines for heavy rainfall may be necessary.

Little change was made to the marine forecast. Combined seas have
fallen to 4 to 6 feet across much of eastern Micronesia with 5 to 7
foot seas still seen across the eastern half of the RMI. Combined
seas are expected to continue to fall another foot or two through
the week.

Western Micronesia...
Invest 96W remains the primary weather maker in the region for the
next few days. On-and-off showers and some thunderstorms brought by
southwesterlies on the leeside of 96W will continue to develop at
Palau and Yap overnight, followed by a ridge moving in as 96W
continues to move away from the islands. This will bring a brief
period of lesser showers to Palau and Yap Thursday night, followed by
a return of patchy showers and thunderstorms as a vast convergence
zone builds across the region ahead of 96W. Meanwhile at Chuuk, an
average case of trade showers and some thunderstorms are expected for
much of the forecast period, with a brief dryer cycle expected
Friday, followed by the arrival of a broad trade-wind trough
thereafter.

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