731
FXPQ50 PGUM 091907
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
507 AM ChST Thu Jul 10 2025

.Marianas Update...
An unsettled weather pattern remains close to the region. Satellite
shows the Marianas in an area of relatively quiet weather, between
convergent monsoon flow to our west and convergent trades to our
east. Current forecast remains on track, and no major changes were
made. A mix of isolated to scattered showers and isolated
thunderstorms are expected over the next couple days with another
round of heavier showers possible over the weekend and into next
week.

Buoy and altimetry data reveal seas remain between 2 to 3 feet,
comprised of minor trade swell.

&&

.Eastern Micronesia Update...
Eastern Micronesia is being affected by a an Inter-Tropical
Convergence Zone (ITCZ) and a couple of trade-wind troughs. Eastern
Micronesia is firmly in the grip of a trade-wind regime, but there`s
nothing dry about it. A lack of features near Majuro is contributing
to a drier day there today, however. Moisture is expected to return
as the ITCZ builds back toward them. Showers will remain scattered
except for a pulse over the weekend, forecast likely showers on
Sunday to try to show that trend. For Kosrae and Pohnpei, the ITCZ
is already present and has some trade- wind troughs moving through
it. These features are providing plenty of focus for shower activity.
Kosrae will probably have numerous showers through Friday, then
scattered for the rest of the forecast period (through Monday). This
activity hasn`t reached Pohnpei yet, but soon will. So, gave them
isolated this morning and scattered this afternoon, then numerous
tonight through Saturday, then scattered Sunday and Monday. Generally
a wet week ahead for Eastern Micronesia.

Pohnpei will start with winds of 10 to 20 knots, and seas of 5 to 7
feet. Later in the forecast period, winds diminish about 5 knots and
seas will shrink about 1 foot.

Kosrae and Majuro start with winds of 10 to 20 knots, and seas of 6
to 8 feet. At Kosrae, these diminish to around 4 to 6 feet. At
Majuro, these seas don`t really diminish at all, and the winds barely
do so, while they shrink 5 knots at Kosrae in the same few days.

&&

.Western Micronesia Update...
A convectively active southern flank of the monsoon trough will
continue to bring unsettled weather to Palau and Yap for the rest of
the week, with the most significant pulses in showers and
thunderstorms to occur over Palau the next several days. As mentioned
previously, there is higher uncertainty in the exact timing of these
heavier showers over Palau due to the disorganized nature of the
regime, but continuing trends in the ensembles indicate for peak
shower and thunderstorm potential today and during the weekend.
Showers are not likely to taper down at Palau much until sometime
next week, becoming patchy in nature as the synoptic regime phase
changes. Even so, these consistent showers are likely to saturate the
soils, so mudslide potential must be reassessed over the next few
days as these showers develop. Yap is expected to see higher-end
scattered showers and some thunderstorms carried by southerly winds
through Friday night, followed by another uptick in showers and gusty
winds as the monsoon trough briefly reorients itself, to then enter
a regime similar to Palau`s. In the meantime, a leading edge of a
building convergence zone and embedded trade-wind disturbance in
central Micronesia is still expected to bring a wetter regime to
Chuuk during the weekend, with numerous showers and isolated
thunderstorms brought by gusty trades to make their way across
beginning tonight.

A primarily southwest swell will continue to remain dominant at Palau
and Yap for much of the forecast period, followed by an intrusive
east well mixing in beginning Saturday at Yap, and by early next week
near Palau. Meanwhile at Chuuk, a 3 to 5 foot trade swell is expected
through the weekend into early next week.

&&

.Prev discussion... /issued 603 PM ChST Wed Jul 9 2025/

Marianas Synopsis...
Satellite observations show thinning cirrus across Guam and Rota
with mostly cloudy skies across Saipan and Tinian. Ritidian and Ipan
buoys show combined seas of 2 to 4 feet.

Discussion...
Scattered showers and isolated thunderstorms will trend downward
this evening, becoming isolated across the Marianas overnight. The
next couple of days look to be drier across the region, however,
there does look to be some potential for island convection during the
day time. PoPs were capped at low-end (30%) scattered due to ECMWF
showing slightly higher winds near 10 kts compared to the GFS winds
of 5 kts. Additionally, with southeast winds near the surface and
aloft, this would favor the west coast of the islands for showers. As
we move into the weekend, another unsettled pattern looks to take
place as monsoonal flow once again encroaches on the region just west
of the Marianas. This looks to put the Marianas under the
convergence zone of southeasterly winds meeting southwesterly winds.
Another round of heavier showers will be possible as we head into
next week.

Marine...
The marine forecast was adjusted to remove mention of the westerly
swell. Ritidian buoy 9-band plot does show a very weak long period
westerly swell, but this swell is well below a half foot. There does
appear to be a 2 to 6 second westerly wind wave at Ritidian, but
with general flow being south-southeasterly, this appears to be a
locally generated wind wave. Both the GFS and ECMWF show an increase
in combined seas this weekend emanating from an easterly swell. Seas
only look to rise 1 to 2 feet, but the increase in swell is expected
to lead to a moderate risk for rip currents along east facing reefs
this weekend.

Eastern Micronesia...
Satellite this afternoon reveals several bands of scattered showers
extending westward across the region in a disorganized ITCZ-like band,
from the Date Line near 5N passing over the southern RMI to end just
east of Kosrae. Showers are focused along areas of subtle troughing
centered just east of Kosrae and just southeast of Majuro, with
scattered to numerous showers and isolated thunderstorms seen further
east near the Date Line. The weak trough and associated showers east
of Kosrae looks to push through this evening and maintain scattered
coverage across the area overnight, merging with a broader developing
trough and convergence zone between Chuuk and Pohnpei come Thursday
night. The developing convergence zone looks to focus showers near
Pohnpei and Kosrae through the remainder of the week, with locally
heavy rainfall and strong gusts possible at times. Conditions will be
relatively quieter near Majuro with most of the convergence and
showers focused to the south and west.

Surface observations and scatterometry show moderate to fresh
easterly flow near Majuro, with gentle to moderate east to southeast
winds near Kosrae and Pohnpei. Areas of fresh to strong winds
associated with convection are seen east-northeast of Majuro (around
23-28 mph) and within the heavy showers east of Kosrae (20 to 25 mph).
Models indicate a broad sustained increase in trades across eastern
Micronesia tonight through the next few days, bringing moderate to
fresh winds throughout the region. This will lead to increases in
trade swell and sea height, and subsequently an increase in surf,
mainly for east-facing reefs. For Kosrae, surf is expected to reach
hazardous levels of 8 ft beginning Thursday lasting until around
Sunday, and a High Surf Advisory has been issued.

Buoy data off Kosrae`s southern shoreline shows combined seas of
around 4 to 6 feet, which are expected to increase another foot or so
through Friday. Altimetry passes show combined seas of 5 to 7 feet
between Kosrae and Majuro, and 6 to 8 feet east of Majuro. For
Pohnpei, latest buoy and altimetry data show around 3 to 4 foot seas.
Seas will continue to rise tonight through the next day or two, but
then trade winds and associated wind waves and swell are expected to
relax late this weekend into early next week.

Western Micronesia...
Short Term...
(Tonight and Thursday)

Palau and Yap:
A southwesterly monsoon flow continues across the region before
turning more southerly across Guam and the CNMI. Based on satellite
trends we started off with numerous shower wording, becoming
scattered Thursday, with Yap seeing scattered showers through the
period. We opted for lower temperatures at Palau tonight, given
current and expected rainfall coverage and intensity.

Chuuk:
Currently a subtle trough is sitting atop the region without much
fanfare convectively speaking, but, this should change around or
shortly after midnight as nocturnal influences interact with the
slow-moving trough.

Long Term...
(Thursday night through Monday)

Palau and Yap:
The southwesterly monsoon currently in place across the region is
expected to weaken slightly as it becomes more westerly, before
backing to the southwest and gradually increasing for the first half
of next week. Although this signal has been consistent in the model
data, which would result in perhaps a brief period over the weekend
of isolated to low-end scattered shower coverage, we didn`t want to
get "too cute" in varying the coverage of showers over the weekend.

The numerical models struggle with these transitions and any
potential effects from Mesoscale Convective Complexes, or MCC for
short, that are essentially a low to mid-level circulation of spin
(vorticity) resulting from the large area of persistent moderate to
heavy precipitation. This typically occurs as a complex 150 miles in
circumference or more with persistent cloud top temperatures cooler
than -70 (more so -75) degrees Celsius. This is a result of good
directional shear in the 0-6km layer, which helps increase
convergence. To have one of these situations develop during
transitional periods like this, during nocturnal timing especially,
is not uncommon. However, confidence in this occurring is not high.
We`ll continue to monitor this potential though, as timing when the
monsoon flow begins to increase would prove better environmental
conditions for this to develop.

Chuuk:
An area of stronger convergence is seen in the latest GFS streamline
analysis, overlaid with Infra-red satellite imagery. Both show a
subtle trough (a different trough that the one currently over Chuuk)
approaching from the east. Pattern recognition and model forecasts
would indicate this may not arrive until Thursday night. However,
nocturnal influences at times can accelerate this process. Given that
the trough is roughly 6 degrees east/~360 miles east-southeast of
Chuuk, moving est at ~20 knots (a little under 25 mph) it should
"cover that area" (close the distance) by Thursday afternoon or
evening. Thus, rainfall probabilities will gradually increase to 70
percent probabilities (numerous shower coverage) Thursday night, with
scattered showers continuing Friday through the first start of next
week, becoming numerous at times.

Aviation...
(Marianas)
Stratiform rain-cooled effects have put the ki-bosh to any
convective elements that were observed earlier. It remains possible
though that precipitation may become more convective and locally
intense during the overnight hours (~10Z through 18Z). That said, any
reductions to MVFR or IFR CIGS and/or VIS should be confined to less
than an hour temporarily.

(Western Micronesia)
Ceilings and visibilities will become MVFR ,possibly IFR at times
for Palau, Yap, Guam, and the CNMI as a south to southwest monsoonal
flow across the Marianas, and southwesterly across Palau and Yap
continues. These reductions may be more prone to this potential
during the night (~10Z through 18Z). Chuuk will see VFR conditions,
but they may briefly drop to MVFR during the night as shower coverage
and intensity possibly increases given the typical "nocturnal bump"
(increase) in shower coverage we typically see, also between 10Z and
16Z. However, the potential and duration is less that at terminals
mentioned above.

(Eastern Micronesia)
A series of weak surface troughs crossing the region will generally
allow VFR conditions to prevail. The exception to this would be
Kosrae, as a more saturated atmospheric profile and better surface
convergence along a trough approaching from the east, may allow MVFR
CIGS and VIS to briefly develop later tonight. Should this occur, and
duration of such will likely be less than an hour.

&&

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

$$

Marianas Update: Cruz
East Micronesia Update: Stanko
West Micronesia Update: Montvila
Marianas: Williams
East Micronesia: DeCou
West Micronesia/Aviation: Doll