111
FXPQ50 PGUM 161846
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
446 AM ChST Wed Sep 17 2025

.Marianas Update...
Little change in the Marianas forecast this morning. Looks like
another wet day today, then it dries out a bit tonight. This allowed
me to recombine the zones, using just a little word smithing to
cover the low-end scattered at Tinian and Saipan versus the high end
scattered at Guam. That`s the only change of any significance this
morning.

Seas will remain at 3 to 4 feet through the weekend. If the
disturbance really develops, we might get enough west swell to
increase the sea heights next week.

&&

.Tropical Systems...
A developing circulation, currently designated Invest Area 90W by the
Joint Typhoon Warning Center, is located near 13N137E. ASCAT shows
winds of 20 to 25 knots near the center. Some vectors show 35 knots
but they could be rain contaminated, as they don`t fit the rest of
the data very well. So for now we`re estimating 20 knot winds near
the center. The likelihood of it becoming a tropical cyclone within
the next 24 hours is still low at this time, it could become medium
soon though. With it already being west of the Marianas, however, the
islands won`t see any direct effects, maybe just some west swell.

&&

.Eastern Micronesia Update...
Little change in forecast philosophy, looks like a pretty wet week
ahead as the ITCZ (Inter Tropical Convergence Zone) remains nearby.
Any breaks will be short lived, so hopefully everyone has been
craving water, because here it comes. Get ready to fill the
reservoirs, if they have any room left. The only update was for
Pohnpei, which has a line of showers bearing down on them, so I
raised their PoP by 10 percent, which makes it numerous at 60 instead
of scattered at 50.

&&

.Western Micronesia Update...
Little change in forecast philosophy, or the forecast itself, this
morning. Right now Koror has entirely too much mid-level cloud for
any island effect convection to get going. Retained the slight chance
of thunder though as it could start to erode/evaporate. The southern
edge of showers south of 90W has made it down to Yap, so happily kept
scattered showers there today. Chuuk is isolated this morning. Kept
the isolated thunderstorms, worded them in the afternoon though. And
then trended in the scattered tonight, as the previous package did,
as the ITCZ arrives.

&&

.Prev discussion... /issued 558 PM ChST Tue Sep 16 2025/

Marianas Synopsis...
A wet pattern has developed across the lower Marianas in response to
a surface trough moving northwest over Guam and Rota. This trough
extends into the Marianas from an elongated circulation, Invest 90W,
centered west of the islands near 15N138E. Latest satellite and radar
imagery show the heaviest of rain and showers centered over Guam and
Rota with winds shifting to the southeast. Local winds have been
rather benign today, but convection is starting to promote stronger
winds with gusts of 30-35 kt being recently observed in Guam
observations. Additionally, the radar vertical wind profile shows
20-25 kt up through 10000 ft. Buoys and recent satellite altimeter
show seas of 3-4 ft around the Marianas, though convective winds will
likely cause short-period waves to spike a few feet higher this
evening.

Discussion...
Today`s weather pattern prompted an infrequently-used approach to
local weather for tonight. Following satellite and radar trends in
the convective leading edge of showers pushing into Rota waters while
convective showers become more stratiform rain over Guam, have split
the local forecasts accordingly. For Guam, steady moderate to
occasionally heavy rain will persist this evening before breaking up
some around midnight with precipitation becoming more showery. The
distinction in the precipitation is that rain is more uniform in
intensity and is long-lasting whereas showers are short-lived and
quickly varying in intensity. Rota will see these conditions this
evening before showers decrease slightly after midnight. Tinian and
Saipan will see scattered showers, just not the widespread coverage
currently seen farther south.

Heading into Wednesday, models show the broad area of moisture
currently soaking the Marianas pushing to the west-northwest as a
slightly drier wedge of air moves in from the east. This is well
supported in recent Blended TPW showing 2.5in PWAT over the islands
with a precipitable drop to 2.2in this side of 150E. Scattered
showers (30-50 POPs) will still persist through Wednesday night
before a drier pattern sets up from the east for late week. After
that, wet season will still remind us of this time of year with
another wet spell on tap as another disturbance approaches with
additional moisture for the region.

Marine...
Generally benign sea conditions will continue through the week,
however, heavy showers this evening and overnight will bring strong
gusts of 25-35 kt, as has been seen at Guam`s tide gauges and the
Guam Int`l Airport. It is possible that today`s convective pattern
will result in a 3-6 hr period of stronger winds in Guam and Rota
waters than what is depicted in models. Such conditions are not well
handled in the global models. For that reason, folks considering
activities on the waters should be mindful of the possibility that
short-period waves could briefly spike to 3-5 ft this evening.

Tropical Systems...
JTWC labeled the circulation west of Guam 90W this afternoon. This
system, formed within the broader trough that moved by the Marianas
Monday, is favored by most models to go on to develop into a tropical
cyclone later this week in the western Philippine Sea. While neither
the Marianas, Yap, nor Palau will see any direct impacts of 90W, wave
models do show a westerly swell developing and reaching the Marianas
over the weekend and early next week from the anticipated tropical
cyclone.

Hydrology...
Today`s rainy weather brought more than an inch of rain to all of
Guam with heavier rainfall amounts of 2-3 inches found across
northern Guam. Ensemble model guidance points to the potential of
another 1-3 inches through Wednesday night for the Marianas.

Eastern Micronesia...
The ITCZ (Inter-Tropical Convergence Zone) remains in place across
the region, centered between 4N and 8N, and it looks to remain in
this area through Friday, before lifting north to near 10N for the
weekend and into the first half of next week. Scattered showers and
isolated thunderstorm will continue developing with this feature
through early next week. Locally heavy rainfall and wind gusts to 30
mph may develop at any time, especially with any surface troughs in
the trades passing through as moisture and convergence with these
features allow colder cloud tops (due to enhanced lift and cloud top
cooling) to develop, enhancing downdraft potential.

Otherwise, seas generally in the 3 to 5 foot range will persist,
with light to gentle winds prevailing through the period.

Western Micronesia...
Chuuk remains in a wet, convergent southeasterly flow pattern as a
surface trough pushes to the west. The next several days will favor
more wet weather than dry as clusters of convection push westward
along the ITCZ over eastern Micronesia. Passing troughs will act to
consolidate the moisture through the weekend. Around Sunday or
Monday, one of these surface troughs is expected to develop into a
circulation to the west that could turn Chuuk winds out of the south
or even southwest early next week. Combined seas will generally
remain between 3 and 5 feet with only short-term spikes due to
convective winds.

For Yap and Palau, a shallow ridge of high pressure extends east-to-
west across Palau. This has greatly reduced Palau`s winds to light
and variable and brought winds to the southwest for Yap. Made a few
short-term adjustments for a generally drier forecast for the two
locations, though Palau could see some afternoon showers and
thunderstorms in the light wind afternoon heating. Invest 90W, to the
north of Yap, will tend to dominate regional weather the next few
days with the weak ridge holding steady in the vicinity of Yap and
Palau. Showers and thunderstorms will occasionally push westward, but
with little true organization or timing. More notable, over the
weekend, both GFS and ECMWF depict a circulation taking root in
eastern Yap State. While long-range model guidance shows little
support for a notable tropical cyclone, its presence would
nonetheless lead to an uptick in regional showers, thunderstorms and
gusts, while also potentially linking with the monsoon trough over
the Philippines, introducing a stronger southwest monsoon flow over
Western Micronesia sometime next week. For sea conditions, through
the weekend, models show rather benign conditions with seas of 2 to 4
feet.

&&

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

$$


Marianas/East and West Micronesia/Tropical Update: Stanko
Marianas: Aydlett
East Micronesia: Doll
West Micronesia: Decou