198
FXPQ50 PGUM 191939
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
539 AM ChST Wed Aug 20 2025

.Marianas Update...
Satellite and radar show mostly cloudy skies and isolated showers
for the Marianas Waters. There were some thunderstorms a few hours
ago. This fairly closely matched the forecast of mostly cloudy with
isolated showers and thunderstorms, so no significant changes were
necessary.

&&

.Marine...
Ipan and Ritidian buoys show combined seas of 3 to 5 feet, which are
expected to remain the same or show a slight decrease over the next
several days. A moderate risk of rip currents is expected along east
facing reefs of the Marianas through Friday, then the risk could
become low along all reefs for the weekend.

&&

.Tropical Systems...
The center of poorly defined Invest Area 90W is north of Koror Palau,
somewhere near 12N135E based on ASCAT wind data. Most winds are 10 to
15 knots, with a few 20 knot vectors and a maximum of 24 knots. ASCAT
did not detect a closed circulation, so the potential for development
of a significant tropical cyclone continues to be low, which means it
is unlikely within the next 24 hours.

&&

.Eastern Micronesia Update...
Showers have increased in Eastern Micronesia beyond what was
expected. Put in afternoon wording for Pohnpei to take it from
isolated to scattered. Brought Kosrae up to numerous, and took Majuro
up to scattered. After today, merged it back into the previous
forecast.

&&

.Western Micronesia Update...
Little change for the forecast in Western Micronesia. Koror is near
the edge of an area of showers, these will then clear up a bit, then
showers will build back in. Yap is in a fairly dry area at this time,
but will see increasing showers around Thursday. Chuuk will have
residual scattered showers today, then isolated tonight and Thursday,
then showers will build back in again Thursday night.

&&

.Prev discussion... /issued 434 PM ChST Tue Aug 19 2025/

Marianas Synopsis...
Satellite imagery shows the majority of convection west of Guam with
some showers and thunderstorms building in western Guam waters this
afternoon. Patchier showers are seen across Guam and over eastern and
southern waters. Over the CNMI, mid to upper-level clouds persist,
with lesser shower development over the islands.

Discussion...
A broad tropical disturbance, Invest 90W, is about 350 miles
southwest of Guam. Based on last night`s airport observations, wind
gusts did reach up to 36 mph on Guam and up to 31 mph on Saipan.
As 90W continues to move westward away from the region, conditions
will continue to improve. Due to more active weather to the south and
southwest, Guam can expect scattered showers and isolated
thunderstorms throughout the night. Meanwhile, showers are expected
to increase to scattered for the CNMI closer to midnight. Lack of
organized convection across model guidance makes it somewhat
difficult to time showers for the remainder of the forecast period,
but generally expect brief periods of scattered showers and isolated
thunderstorms this week and over the weekend due to weak troughs and
convergence in the trade-wind pattern..

Marine/Surf...
Buoy data showed seas reaching up to 5 to 7 feet last night, but has
gradually subsided since this morning. Combined seas of of 3 to 5
feet and light to gentle winds are expected through the week and over
the weekend. There will be a moderate risk of rip currents along
east facing reefs through at least Thursday. A slight decrease in the
swell may allow the risk of rip currents to become low along all
reefs late week or over the weekend.

Tropical Systems...
JTWC`s Invest 90W was located near 11N137E as of 06z. Invest 90W
continues to remain disorganized with no clear low level circulation.
The developmental chances for 90W have been downgraded to low, which
means that development into a tropical cyclone is unlikely within 24
hours. Little to no additional impacts are expected for the Marianas
or western Micronesia beyond scattered showers over the next day.
For more details on Invest 90W, please see the JTWC bulletin ABPW10
PGTW.

Eastern Micronesia...
The main items of interest are a trough currently passing overhead at
Pohnpei, an area of enhanced convergence south of Majuro that`s
approaching Kosrae, and a trough crossing the Date Line. All these
features are associated with a weak ITCZ that is currently forming.

Pohnpei:
As the trough moves out during the evening, showers will become
isolated in coverage. Isolated showers will remain possible through
Thursday, before the Date Line trough brings increasing rainfall
potential towards the weekend.

Kosrae:
An area of convergence to your east will move overhead tonight into
Wednesday, increasing your potential for shower and thunderstorm
development, especially after midnight tonight. Scattered showers and
the occasional thunderstorm will occur through late Wednesday
night/Thursday morning, before rainfall becomes more isolated
Thursday afternoon. However, the Date Line trough then moves in for
Friday and into your weekend, with scattered showers returning.

Majuro:
An area of deeper moisture and convergence has bypassed you to the
south. The next forecast challenge was trying to determine if and
when enough moisture and convergence with the trough to your east
would be sufficient to increase shower coverage. We were tempted to
bring scattered showers into the forecast Wednesday night, but opted
to hold off until Thursday for this as upstream satellite imagery
isn`t overly impressive, and the models show the best juxtaposition
of moisture and convergence occurring Thursday as the ITCZ becomes
more focused. Then, scattered showers and isolated thunderstorms will
persist into the weekend.

Marine (all locations):
Mainly light to gentle winds with seas of 4 to 6 feet will prevail
through the weekend. However, locally higher winds and waves will
occur as the ITCZ develops, mainly near any heavier shower or
thunderstorm that develops.

Western Micronesia...
Satellite imagery depicts isolated showers over Palau, scattered
showers over Yap, and numerous showers entering Chuuk. Altimetry
data shows combined seas between 3 to 5 feet across Palau and Yap
and 4 to 6 feet across Chuuk.

Overall, not much has changed from the previous forecast package.
Invest 90W is now located near 11N137E and will continue to move
away from the region. Scattered showers related to Invest 90W will
continue tonight for Yap and through tomorrow for Palau. The next
trough that is set to move across western Micronesia is currently
moving across Chuuk today, bringing numerous showers to Weno
tonight. This trough is expected to reach Yap Proper on Thursday and
Palau on Thursday night. This trough looks to kick off an active
period for Palau and Yap where guidance shows a weak and broad
circulation developing within the trough. This will lead to high end
(50%) scattered to numerous showers. The pattern looks to remain
active into the weekend for Yap and Palau as a monsoon like pattern
looks to develop. For Chuuk, a downtrend in shower activity is still
expected for the second half of the week but another tropical wave
looks to move across the region this weekend, bringing increased
shower coverage.

Little change was made to the marine forecast. Combined seas are
forecasted to fall to 3 to 5 feet for Chuuk on Wednesday with little
change in sea height expected through the week for western
Micronesia.

&&

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

$$

Marianas/East and West Micronesia/Tropical Updates: Stanko
Marianas: Cruz
East Micronesia:Doll
West Micronesia: Williams