799
FXPQ50 PGUM 041900
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
500 AM ChST Sat Oct 5 2024

.Marianas Update...
Minor edits made to the forecast this morning. Invest 99W looks to be
arriving and entering the coastal waters earlier than expected, so
have increased winds and rain potential for this afternoon. Recent
model trends show slow development of Invest 99W and passing it over
or east of Tinian and Saipan. However, gusty winds and scattered to
numerous showers expected for all the Marianas through Sunday night.
The potential for active weather is likely to continue into next
week as a monsoon-like pattern looks to impact the islands.

Invest 99W is quickly approaching from the southeast and is expected
to bring heavy rainfall, gusty winds and elevated seas starting
later today and continuing through at least Sunday night. Winds and
seas could become hazardous to small craft with 99W`s passage tonight
and tomorrow, especially over the waters of Tinian and Saipan, but
for now look to stay just below advisory criteria.

A moderate risk for rip currents continues along north and east
facing reefs. Surf and rip risk could rise and possibly become
hazardous late weekend into early next week as 99W moves across the
region, especially along north and east facing reefs. Surf and rip
risk look to rise along south and west facing reefs around the middle
of next week as a southwest swell sets up in the wake of 99W`s
passage.

&&

.Hydrology Update...
Tropical disturbance Invest 99W and the monsoon-like pattern in its
wake will bring locally heavy rainfall of 4 to 8 inches across the
Marianas, beginning this afternoon and continuing into early next
week. Some uncertainty remains regarding potential rainfall impacts,
due to the disorganized nature of this system, and expected rainfall
totals may change. Even so, flash flooding remains possible from
this afternoon through Tuesday.

&&

.Tropical Systems Update...
Invest 99W, centered near 12N148E, has been upgraded to low by the
Joint Typhoon Warning Center, meaning that development into a
significant tropical cyclone withing 24 hours is unlikely. Invest 99W
will trek northwestward towards the Marianas and pass over or east
of the main islands through Sunday. The mid to long-term guidance
indicates better organization is most likely once the system embeds
itself into the building monsoon trough in the region once it passes
over the far Northern Marianas.

&&

.Eastern Micronesia Update...
Satellite imagery shows a weak circulation to the west of Kwajalein
near 9N165E, embedded within surface trough that extends across the
northern RMI and westward north of Kosrae and Pohnpei. The bulk of
the showers and thunderstorms are on the western edge of the Marshall
Islands and just east of Kosrae. With showers pulling away from
Majuro, scattered showers will linger around Majuro today but become
isolated by tonight, while scattered showers and isolated
thunderstorms will move into Kosrae later this morning and continue
through Sunday. Over at Pohnpei showers, are very spotty, but since
winds are still light, island convection will bring scattered
showers and possibly thundestorms to the island during the afternoon
hours through at least Sunday.

Benign marine conditions are expected to continue with combined seas
of 5 feet or less and mostly light to gentle winds. Occasional
moderate winds are possible for Kosrae through Sunday night as winds
will be higher within heavier showers associated with the surface
trough and embedded circulation.

&&

.Western Micronesia Update...
As Invest 99W continues to move northwestward towards the Marianas,
the weather across Chuuk State looks to be fairly dry, especially
near Chuuk Lagoon over the next few days, though weak troughs may
bring some spotty showers as they pass near the islands. Over by
Palau and Yap, a ridge has developed south of Palau and Yap, but
this is creating some convergence along the northern edge of the
ridge, so scattered showers continue to develop around Palau and Yap
this morning. This risk of thunderstorm looks limited, but with some
cooling cloud tops near the waters of Yap Proper, isolated
thunderstorms was added to the coastal forecast for Yap today.
Overall, on and off scattered showers are expected through the rest
of the weekend through the middle of next week as ridging affects the
region, with no more than low-end scattered showers likely.

Combined seas of 3 to 5 feet across the region may increase another
foot or two near Yap and Chuuk late weekend or early next week due to
a slight increase in northerly swell. This swell may filter into
Palau later in the week.

&&

.Prev discussion... /issued 537 PM ChST Fri Oct 4 2024/

Marianas Synopsis...
Gentle to moderate southeasterlies prevail across the region.
Satellite imagery and Doppler radar indicate stratiform fragments
from convection earlier, now making their way across the CNMI.
Ritidian buoy indicates seas are around 5 feet.

Discussion...
Little change has been made to the forecast. The main concern for the
region coming into next week remains the largely disorganized
tropical disturbance Invest 99W well to the southeast. Tapered down
the shower coverage and winds slightly for the time of passage late
Saturday night through Sunday night, due to a potential shift
northward of the storm track; this may allow the bulk of the showers
and potential wind impacts to dodge the main population centers of
the Marianas. Even so, some uncertainty remains at this time due to
the lackluster condition of the disturbance as observed on recent
satellite imagery. The public should continue to watch this
development over the next several days, as the forecast may change
considerably.

Marine...
Combined seas of 4 to 6 feet are expected into early next week, as
primary swells diminish and wind waves increase with the passage of
99W. Gentle to moderate easterlies will prevail in the region
tonight, before becoming more south-southwesterly over the next few
days. A moderate risk of rip currents will continue along north and
east facing reefs, with a low risk along south and west facing reefs.
A moderate risk of lightning is expected through at least the
weekend. Surf and rip risk could rise and possibly become hazardous
late weekend into early next week, as tropical disturbance 99W moves
across the region, especially along north and east facing reefs.

Tropical systems...
A loosely organized tropical disturbance, known as JTWC`s Invest 99W,
is centered near a general area well northwest of Fananu, near
11N153E. Model guidance indicates the system will continue to trek
northwestward, moving across or east of the Marianas as it develops
further. The window of potential development continues to diminish as
the system attempts to consolidate, so wind impacts remain highly
uncertain at this time. The mid to long-term guidance indicates
better organization is most likely once the system embeds itself into
the building monsoon trough in the region once it passes over the
Far Northern Marianas.

Hydrology...
New estimates indicate that tropical disturbance Invest 99W will
bring locally heavy rainfall of 4 to 8 inches across the Marianas
late this weekend through early next week. Some uncertainty remains
regarding potential rainfall impacts, due to the disorganized nature
of this system. This feature will be monitored closely over the next
several days by various agencies, so expected rainfall totals may
change. Even so, flash flooding remains likely during the Saturday
night through Tuesday time frame.

Eastern Micronesia...
A weak circulation is centered just south of Kwajalein near 9N169E
with a trough extending eastward to beyond the Date Line. Strong
convergence associated with this trough is producing widespread
showers and isolated thunderstorms across Majuro. This system will
slowly move west over the next couple of days. Showers will decrease
in coverage over Majuro by Saturday morning, becoming isolated by the
evening. For Pohnpei and Kosrae, a dry pattern remains across the
region this afternoon. Pohnpei looks to remain mostly dry over the
next couple of days, however, due to the light winds and abundant
low-level moisture, island convection will bring scattered showers
and possibly thunderstorms to the island through the weekend. For
Kosrae, the weak circulation and associated trough look to bring
increased showers to Kosrae Saturday night into Sunday. Since the
system is weakening significantly, Kosrae is only expected to see
scattered showers as the trough, and possible circulation, passes by.
Early next week, models show an ITCZ (Inter-Tropical Convergence
Zone) developing across the Marshalls and spreading westward. At the
same time, a series of trade-wind troughs look to move across the
Date Line and interact with the ITCZ, bringing increased convection
to Majuro around Monday, Kosrae Monday night and Pohnpei Tuesday
night.

Benign marine conditions are expected to continue with combined seas
of 5 feet or less and mostly light to gentle winds. Occasional
moderate winds are possible for Majuro through Saturday afternoon.

Western Micronesia...
Scattered showers and isolated thunderstorms are seen across Koror`s
waters developing along a boundary of convergent southwest winds.
Yap`s waters are expected to remain in isolated showers with shower
activity expected south of their waters tonight, then increased
clouds and showers Saturday. Overall on and off scattered showers for
the weekend through the middle of next week as ridging affects the
region, with no more than low end scattered showers likely. Same goes
for Chuuk. Late week looks possible for scattered showers and
southwest winds for Koror and Yap as a possible system develops near
the Marianas and pulls to the north. For Chuuk, showers are expected
to develop again late week as well, as a trough moves through.

Combined seas of 3 to 5 feet across the region, may increase another
foot or two near Yap and Chuuk late weekend or early next week.

&&

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

$$

Marianas Update: Slagle
Micronesia Update: Schank
Marianas/Tropical/Hydrology: Montvila
Eastern Micronesia: Kleeschulte
Western Micronesia: Hong