837
FXPQ50 PGUM 090803
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
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: Williams
East Micronesia: DeCou
West Micronesia/Aviation: Doll