079
FXPQ50 PGUM 150823
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
623 PM ChST Tue Sep 15 2026

.Marianas Synopsis...
Visible satellite and radar imagery depicts a line of numerous
showers and isolated thunderstorms over eastern Rota coastal waters
propagating southward. Altimetry indicates combined seas of 4 to 6
ft in the region and 5 to 7 ft to the east of the Marianas.

&&

.Discussion...
Invest 90W`s influence continues to bring unsettled weather to the
Marianas. A Mid-Level Circulation (MLC) is apparent to the
southeast, producing the convective outbursts encroaching upon the
region from the east. As 90W continues to consolidate (discussed in
the Tropical Systems section below), the moisture influx will persist
despite the center remaining well away from the islands. CIMSS MIMIC
indicates precipitable water (PWAT) values around 2.5 inches, with
some pockets reaching up to 3.0 inches to the east. Vorticity and
upper-level divergence continues to be favorable for convection, so
isolated thunderstorms are not expected to let up, and locally heavy
showers are possible at least for tonight for all islands. Since
there are slight spatial and intensity differences in shower coverage
as well as wind directions and speeds, the zones remained split. As
for the long-term, the monsoon trough will trail behind 90W and move
over the Marianas should 90W lift to the northwest. Model guidance
indicates this effect to be delayed, however, so showers and
increasing winds may be concentrated over Guam and Rota before
including Saipan and Tinian sometime over the weekend. Due to the
uncertainty regarding timing and direction of 90W`s evolution,
showers remain scattered with slight chances of thunderstorms for all
islands. This may be refined as confidence naturally increases with
time.

&&

.Marine...
Combined seas are currently 4 to 6 ft according to altimetry data.
The northeasterly and south to southwesterly swells are expected to
build due to Invest 90W intensifying to the east-northeast and a
monsoon surge to the south. Therefore, combined seas will increase a
foot or two in the coming forecast periods, but is expected to remain
below Small Craft Advisory criteria. There is a moderate risk of rip
currents along north, east, and south facing reefs tonight.
Beginning Wednesday night, the moderate risk of rip currents will
extend to all reefs, but surf will most likely remain below any
hazardous criteria as the largest swells and wave heights are
concentrated near 90W, which is forecasted to remain well to the
northeast of the Marianas. This situation will continue to be
monitored.

&&

.Tropical Systems...
A developing disturbance, Invest 90W, is centered east of the
Marianas near 15N151E, moving NW. 90W is now rated high and is the
subject of a Tropical Cyclone Formation Alert (TCFA). This means
that Invest 90W is expected to develop into a significant tropical
cyclone (tropical depression) within the next 24 hours. A distinct
Low-Level Circulation Center (LLCC) is now more evident and
indicates the system is organizing. The environment remains
conducive to strengthening and Invest 90W is expected to become a
tropical depression within the next day. 90W should continue to move
northwest and strengthen over the coming days.

For more information on Invest 90W and the TCFA, see bulletins
issued by the Joint Typhoon Warning Center under WMO header ABPW10
PGTW and WTPN21 PGTW.

&&

.Eastern Micronesia...
A very dry pattern remains over Kosrae and Majuro this afternoon.
For Pohnpei, a weak band of convergence is seen over Pohnpei,
extending southwest. This band is producing scattered showers over
Pohnpei this afternoon. These showers will slowly dissipate through
the evening, allowing Pohnpei to return to a dry pattern along with
Kosrae and Majuro. Not much is expected across the region, with POPs
(Probability of Precipitation) remaining in the 20 to 30 percent
range through the weekend. So there will be periods of increased
showers through the week, but nothing spectacular.

No big changes in the marine forecast. Light to gentle winds are
expected through the weekend, with Kosrae seeing occasional moderate
winds through Thursday.

&&

.Western Micronesia...
Looks like the higher winds and seas are continuing to pull away from
Chuuk, so went ahead and expired the high surf and small craft
advisories on schedule.

Mostly debris cloudiness left over Palau this evening, so opted for
isolated showers tonight, then likely scattered showers Wednesday and
Wednesday night, then isolated showers through Sunday as the ribbon
of showers bends to the north away from Koror. Winds will be from the
southwest or the west, with speeds of 10 to 15 knots most of the
time. Some 20 knot values may be possible Wednesday and Thursday,
with some 25 knot gusts Wednesday night and Thursday night. Seas will
be between 4 and 6 feet pretty much the whole period (through
Sunday). This suggests challening marine conditions through Thursday
night, then benign conditions starting Friday.

At Yap, winds will be from the west, possibly turning to the
southwest Sunday. Speeds will be 10 to 15 knots tonight and
Wednesday, possibly with 25 knot gusts on Wednesday. Winds will be 15
to 25 knots Wednesday night through Thursday night, and 10 to 20
knots Friday through Sunday. Seas will be 5 to 7 feet through most of
tonight through Sunday time period, possibly 6 to 8 feet Friday. This
suggests benign marine conditions tonight, becoming challenging on
Wednesday, outright hazardous Wednesday night through Thursday night,
and challenging Friday through Sunday.

For Chuuk, winds will be from the southwest. Speeds will be 10 to 15
knots through Friday, with gusts up to 25 knots tonight. Showers will
be isolated through Friday, then scattered for the weekend. Seas will
be 6 to 8 feet tonight, diminishing to between 5 and 7 feet Wednesday
and Wednesday night, then 4 to 6 feet Thursday and Thursday night,
then 3 to 5 feet Friday through Sunday. This suggests marine
conditions will be challenging tonight, then benign the rest of the
week.

&&

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

$$

Marianas: Mesa
East Micronesia/Tropical Systems: Kleeschulte
West Micronesia: Stanko