743
FXPQ50 PGUM 121956
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
556 AM ChST Thu Aug 13 2026

.Marianas Update...
A monsoon trough remains to our north, passing through newly formed
Tropical Depression Nangka (16W) to the distant north-northeast. A
area of diffluent flow remains in place across the Marianas, leading
to a fairly dry pattern over the next few days. No changes were
needed to the forecast. A Small Craft Advisory remains in effect
through late this afternoon and a High Surf Advisory and a High Risk
of Rip Currents remain in effect through late tonight and into the
early morning hours on Friday. Significant westerly monsoon swell
continues over the next few days, which may warrant an extension to
the coastal hazard products over the weekend.

&&

.Tropical Systems Update...
At 100 AM, Invest 91W was upgraded to Tropical Depression Nangka
(16W). Nangka was located near 23N145E, about 260 miles east-
southeast of Iwo To and well northwest of Agrihan in the northern
Marianas. Latest satellite imagery continues to show a cloud covered
low-level circulation center (LLCC), with the leading edge of the
monsoonal belt curving along its northeastern periphery, while deep
convection and dense cloud cover remains relatively sparse along its
western periphery. Nangka is moving northeast at 21 mph and is
forecast to have a dramatic decrease in forward speed through Friday.
Maximum sustained winds are 35 mph and Nangka is forecast to
intensify through Friday possibly becoming a tropical storm later
this morning. Nangka continues moving away from the Marianas, likely
moving north of 25N and exiting our region tonight. For more
information on Nangka (16W), please see bulletins issued by the Joint
Typhoon Warning Center (JTWC) under WMO header WTPN31 PGTW and by
the National Weather Service under WMO header WTPQ31 PGUM.

A weak tropical disturbance, Invest 90W, has slowly drifted westward
with an elongated center located near 9N165E, based on overnight
scatterometry data. It is still rated "sub-low", meaning development
is possible, but is not expected over the next day or so. The main
area of convection remains along the western periphery of the system,
enhancing northwesterly convergence across Pohnpei and Kosrae states.
Invest 90W is expected to drift westward, possibly becoming better
organized late tonight and Friday.

&&

.Eastern Micronesia Update...
A trough extends from Invest 90W, west of Kwajalein, to an ill-
defined circulation south of Majuro near 4N172E, then stretches
northeast to cross the Date Line at 8N. This pattern is producing
scattered to numerous showers and isolated thunderstorms across
Pohnpei and Kosrae states, within a broad area of northwesterly
convergence. Farther east, patchier showers are seen across much of
the RMI this morning. Showers look to continue to build across
Pohnpei and Kosrae today, so the only change was to increase shower
coverage, becoming scattered to numerous later this afternoon and
tonight.

The High Surf Advisory was allowed to expire early this morning along
south-facing reefs of Kosrae, however, surf will still be elevated
at 5 to 7 feet through Friday. Then, an increase in south to
southwest swell emanating from a circulation moving near the Solomon
Islands may boost surf back to hazardous levels over the weekend.

&&

.Western Micronesia Update...
A fairly quiet pattern continues across the region. Convergent
southeasterlies is producing an area of thunderstorms nearing southern
Yap State, however, this convection is not expected to shift much
further north due to the broad surface ridge that has remained in
place across far western Micronesia over the past few days. For
Chuuk, kept isolated showers in for today, but may see a quick
increase in showers late tonight or early Friday morning as the
unsettled pattern in eastern Micronesia shifts closer.

&&

.Prev discussion... /issued 607 PM ChST Wed Aug 12 2026/

Marianas Synopsis...
Partly cloudy skies with spotty showers prevail across the Marianas
this afternoon. Moderate to fresh winds with strong gusts continue,
with strongest winds over Tinian and Saipan. Seas remain around 10
feet over western Tinian and Saipan waters.

Discussion...
A fairly dry pattern is now in place across the Marianas, with only
a few showers seen across the region. This dry pattern looks to
continue through the end of the week. By Friday night, inflow into
Invest 91W looks to remain mostly outside of the area, however, it is
expected to bring an increased potential for thunderstorms to the
Marianas. Then, during the last half of the weekend, convection
associated with Invest 90W looks to move into the region, increasing
POPs (Probability of Precipitation) into the 30 percent range across
the Marianas. A drier pattern is expected to return by the middle of
next week.

Marine...
Moderate to fresh winds will prevail across the Marianas for the
next couple of days, with strong gusts over Tinian and Saipan
waters. Winds are expected subside further, becoming gentle to
moderate for the weekend. Large west swell will gradually subside
through Thursday, with the highest combined seas found over Tinian
and Saipan waters. The west swell will combine with wind waves and a
small southeast swell to maintain 10 foot seas across western waters
of both Tinian and Saipan through Thursday afternoon. For Guam and
Rota, seas will hover at around 9 feet through Thursday.

Tropical Systems...
A weak disturbance, Invest 90W, is centered northwest of Kwajalein
near 12.1N 164.8E, moving slowly west-northwest. The Low-Level
Circulation Center (LLCC) remains relatively cloud free, with the
associated convection seen to the west of the center, along a band
of convergence south of a col region. Invest 90W is expected to
continue to move slowly west-northwest over the coming days with
little change in intensity. Convection, however, may increase closer
to the center of 90W.

A more organized disturbance, Invest 91W, is centered west-northwest
of Maug Island near 21.8N 141.9E. Invest 91W has been upgraded to
high and is now the subject of a Tropical Cyclone Formation Alert by
the Joint Typhoon Warning Center (JTWC), which means that tropical
cyclone development is expected within the next 24 hours. Latest
satellite data and surface analysis show a more organized cloud
covered LLCC. Other factors remain favorable for TC development as
well, with low vertical wind shear, good upper-level outflow and
warm sea surface temperatures. The latest ASCAT analysis shows 25 to
30 knot winds to the south of 91W. Although Invest 91W is not
expected to have a direct impact on the Marianas, increasing
convection south of the monsoon trough and 91W could move into the
Marianas, especially Tinian and Saipan northward, over the weekend.
Invest 91W will continue to be monitored closely, and advisories will
be issued as needed.

For more information on Invest 91W, please see bulletins issued by
the Joint Typhoon Warning Center (JTWC) under WMO header ABPW10
PGTW and WTPN21 PGTW.

Eastern Micronesia...
A fairly dry pattern continues for Majuro with only isolated showers
in the area. Precipitable water remains high enough for showers to
become scattered tomorrow through Friday before a surge of moisture
from the south moves north from the Equator. This will likely stall
south of Majuro and keep showers scattered, though the more unstable
airmass will allow for isolated thunderstorms. Winds look to remain
very light the entire forecast period.

For Pohnpei and Kosrae, a wetter pattern is moving in due to the
proximity of Invest 90W with high end scattered showers already
visible over Kosrae. Showers are forecasted to become numerous by the
weekend for both islands as 90W becomes more organized and the surge
of moisture from the Equator looks to move right over the islands.
This will also cause the light to moderate winds to shift from the
northwest to the southwest by Friday night.

Strong southeast trades and a circulation south of the equator
continue to generate south to southeast swell into eastern
Micronesia. As the circulation meanders towards the Solomon Islands,
swell turns more southwest late week and over the weekend. For now,
hazardous surf up to 8 feet is expected along south facing reefs of
Kosrae through early Thursday morning, with another boost in surf
heights likely to begin Saturday or Saturday night. A High Surf
Advisory remains in effect along south facing reefs of Kosrae through
at least early Thursday morning.

Western Micronesia...
The pattern remains fairly quiet across western Micronesia, well
south of the monsoon trough and associated disturbances. Mostly
cloudy skies developed over Yap and Palau this afternoon as swaths of
upper-level clouds swept overhead, debris from deep convection far to
the north. Meanwhile, mostly cloudy skies prevail across Chuuk,
associated with patchy showers in the area, which models are
trending downward for tonight. Isolated showers are expected to then
continue through Thursday night before a wetter pattern arrives
Friday. A broad disturbance, the eventual merger between Invest 90W
and the area of vorticity just east of Majuro, will gradually shuffle
westward over the next few days and promote heavier showers and
thunderstorms across eastern and central Micronesia. Showers look to
increase for Chuuk especially late Friday night as the center of 90W
shifts past Pohnpei, becoming numerous and locally heavy at times
through the weekend. Latest guidance shows 90W stalling near Chuuk to
the northeast of Weno until around Tuesday night, before it
eventually shifts northwest in the general direction of the Marianas.
90W remains very weak and disorganized which yields higher
uncertainty in the forecast, especially this far out. For now, a
generally wet pattern is expected for Chuuk from around Friday
through the middle of next week. Depending on how this pattern
evolves, wetter conditions will then spread westward to Yap and Palau
during the early half of the week.

Latest altimetry and Yap buoy data show combined seas between 3 and 5
feet for Yap and Palau, and between 4 and 6 feet for Chuuk. Benign
sea conditions are expected to continue through the forecast period.
Seas will increase by a foot or so for Yap late this weekend as the
west swell and wind waves build slightly.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk through late Thursday night for GUZ001.

     High Surf Advisory until 4 AM ChST Friday for GUZ001.

MP...High Rip Current Risk through late Thursday night for MPZ001>003.

     High Surf Advisory until 4 AM ChST Friday for MPZ001>003.

Marianas Waters...Small Craft Advisory until 5 PM ChST Thursday for PMZ153-154.

&&

$$


Marianas/East and West Micronesia/Tropical Update: Cruz
Marianas/Tropical: Kleeschulte
East Micronesia: Ogle
West Micronesia: DeCou