224
FXPQ50 PGUM 021941
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
541 AM ChST Sat Oct 3 2026

.Marianas Update...
Choi-wan (26W) was upgraded to a typhoon late last night as it began
to pivot and move northward, with the typhoon-force wind field
expanding across several of the far northern Marianas. A Typhoon
Warning was issued for Alamagan, Pagan, and Agrihan, and a Tropical
Storm Warning continues for Tinian and Saipan this morning. Strong
winds with frequent gale-force gusts continue across the Marianas as
the surging monsoon inflow, south of Choi-wan, strengthens over the
area. Within the moist monsoon flow, lines of heavy showers
continued to sweep the islands overnight, which produced damaging
gusts up to 60 mph on Guam and up to 75 mph on Saipan. Winds have
only seen a very slight decrease early this morning, and are expected
to remain strong to tropical storm force on Tinian and Saipan for the
next few hours, and fresh to near-gale force on Guam and Rota
throughout the day, decreasing moreso for Tinian and Saipan as
Typhoon Choi-wan moves farther away, and surging monsoon conditions
dominate in its wake. However, windy conditions are expected to
continue through at least Monday. There was little change in the
Public or Marine zone forecasts overnight, and all surf and marine
hazards remain in effect.

&&

.Tropical Systems Update...
As of 5 AM ChST, Typhoon Choi-wan was centered near 18.1N 144.7E, or
70 miles west of Pagan, 215 miles north-northwest of Saipan, or about
320 miles north of Guam. Choi-wan continues to move northward at
around 9 mph, and is forecasted to turn north-northeast this morning
as it shifts toward Agrihan and Pagan, looking to pass just to the
north of Agrihan by this afternoon with little change in forward
speed. Choi-wan strengthened rapidly late last night, and the latest
forecast from the Joint Typhoon Warning Center (JTWC) shows it
continuing to strengthen through the remainder of the weekend,
becoming Category 3 by midnight tonight as the wind field expands
considerably. This will maintain damaging tropical storm conditions
across the far northern islands for longer, potentially through
Sunday afternoon.

For more information on TY Choi-wan, see bulletins issued by JTWC
under WMO headers WTPN32 PGTW, and by the National Weather Service
under WMO headers WTPQ32 PGUM and WTPQ82 PGUM.

A weak disturbance, Invest 94W, is centered northwest of Kwajalein
near 11N166E. 94W is rated sub-low, meaning development is not
expected over the next 24 hours. Invest 94W remains a very weak
circulation. However, latest model guidance does favor 94W slowly
developing and strengthening over the next several days as it moves
north-northwest to north. Invest 94W will be monitored closely over
the next few days.

&&

.Eastern Micronesia Update...
No major changes were made to the previous forecast. An overall dry
pattern is expected to continue for Pohnpei through Sunday night and
Kosrae through Sunday. Scattered showers with a possibility of
isolated thunderstorms will continue at Majuro for the next few
days. No changes to marine conditions.

&&

.Western Micronesia Update...
A couple of minor short-term changes for the western Micronesia
forecast. For both Yap and Palau, only isolated showers are currently
found over both locations. This looks to change through the morning,
with scattered showers expected at both locations near noon. Yap will
also have the potential for isolated thunderstorms. For Chuuk, no
changes to the previous forecast.

&&

.Prev discussion... /issued 708 PM ChST Fri Oct 2 2026/

Marianas Synopsis...
Strong to gale-force southwesterlies prevail across the Marianas,
largely influenced by the outgoing Tropical Storm Choi-wan (26W) and
enhanced monsoon surge from the southeast. Satellite imagery and
Doppler radar indicate a plume of showers anchored just outside the
northwestern coastal waters of Guam, Rota, Tinian and Saipan. Recent
altimetry passes and surf zone observations indicate combined seas of
at least 12 feet.

Discussion...
Tropical Storm Choi-wan continues to move away from Anatahan and is
inching closer to Alamagan, pivoting slowly to the north-northeast
through Saturday. During this time, the wind field will continue to
expand, exposing Tinian and Saipan in Tropical Storm conditions a
little longer, now through Saturday. As such, a Tropical Storm
Warning remains in effect for Tinian and Saipan. While showers have
tapered a little due to a dry slot overhead, rainbands that pummeled
Guam and Rota earlier this morning are now being pulled toward
Tinian and Saipan, with more intense showers and thunderstorms
expected tonight. After a lull in showers and thunderstorms expires
soon, periods of rain, widespread heavy showers and some
thunderstorms are expected to return and continue to pummel Tinian
and Saipan tonight, to then taper down thereafter as Choi-wan moves
away. Similar at Guam and Rota, albeit only patchy showers and some
thunderstorms expected overnight brought by very fresh to strong
southwesterlies. A drier monsoon pattern will return early next week,
with low-end scattered showers and a thunderstorm or two expected to
develop through next Friday. For more information, please refer to
the local statement issued by the National Weather Service under WMO
header WTPQ82 PGUM.

Marine...
Hazardous seas of 15 to 20 feet and dangerous surf of 10 to 14 feet
along south and west facing reefs will persist through Saturday,
then begin to gradually diminish as Tropical Storm Choi-wan (26W)
moves away from the northern CNMI and brings elevated southwest
monsoon swell to the Marianas. In addition to a secondary northerly
swell and massive localized wind waves, hazardous seas, gale-force
winds and dangerous surf are expected through the weekend. There will
be a high risk of rip currents along north, west and south facing
reefs of Guam and Rota. The north facing reefs could diminish to
moderate along north facing reefs while continuing to be high along
south and west facing reefs through at least Sunday night.

Hydrology...
Multiple flood advisories were issued across the Marianas today, with
a Flash Flood Warning issued for the southern portions of Guam where
the heaviest cells made their way across this morning. Preliminary
satellite-based rain estimates indicate a total of 3 to 5 inches of
rain may have fallen across the Marianas, while various sensors
across Guam have measured between 2 and 3 inches of rain during this
rain event so far, with a peak observed at American Memorial Park,
Saipan at just under 4 inches so far. After Choi-wan`s passage this
weekend, a trailing monsoon inflow band will maintain wet conditions
into early next week, resulting in 4 to 6 inches of rain for Guam and
Rota, and 5 to 8 inches for Tinian and Saipan during the Watch
period.

Tropical Systems...
At 1 PM ChST, the center of Tropical Storm Choi-wan (26W) was located
near latitude 17.2N and longitude 144.7E. This was about 260 miles
north of Guam. Choi-wan is moving west-northwest at 5 mph, slowing
down before pivoting toward the north-northeast with little change in
forward speed through Saturday as it moves up the Far Northern
Marianas. Maximum sustained winds have increased to 65 mph. Choi-wan
is forecast to intensify through tonight, possibly becoming a typhoon
later today. Tropical storm force winds continue to extend outward
from the center up to 125 miles, however, the wind field is expected
to expand farther south over the next few hours as Choi-wan continues
to indulge in very favorable shear conditions over the northern
CNMI. At this time, a Tropical Storm Warning remains in effect for
Saipan, Tinian and their adjacent coastal waters, as well as Alamagan,
Pagan, and Agrihan. In addition, a Typhoon Watch remains in effect
for Alamagan, Pagan, and Agrihan.

Further south and east, Invest 94W was dubbed by JTWC (the Joint
typhoon Warning Center) earlier this morning, as a low-level
circulation has become somewhat better organized north of Kwajalein.
This feature looks to meander in the area for a few days (through
Sunday or Monday), possibly allowing another weak circulation
northeast of Majuro to merge with it. Then, all of the ensembles and
AI-driven models lift this system off to the north and then northwest
as it organizes for the middle to end of next week. At this point,
the main impacts from 94W will be potentially prolonging and
enhancing a westerly swell across a goof portion of the FSM and RMI,
potentially leading to some minor inundation concerns developing
over the weekend. Also, the GFS has hinted at some convective gusts
near 30 mph, with todays scatterometer pass showing similar values
south and southeast of Majuro. With that said, the models show this
burst of winds dampening out over the next 24 to 36 hours, but this
is something we`ll keep an eye on.

For more information on Tropical Storm Choi-wan, please refer to the
local statement issued by the National Weather Service under WMO
header WTPQ82 PGUM, the Tropical Cyclone Public Advisory issued by
the National Weather Service under WMO header WTPQ32 PGUM, the
Tropical Cyclone Warning issued by the Joint Typhoon Warning Center
under WMO header WTPN32 PGTW, and the Significant Tropical Weather
Advisory under WMO header ABPW10 PGTW.

Eastern Micronesia...
The main forecast concerns were two fold. First, determining
rainfall probabilities at majuro were assessed in terms of keeping
shower coverage at scattered (50 percent probabilities) or increasing
them to numerous shower coverage (60 to 70 percent probabilities).
Given recent satellite trends of warming cloud tops we opted to keep
things scattered for now, and the bulk of the numerical guidance
continues this theme well into next week. This is due to good
moisture pooling and surface convergence along the monsoon trough
that stretches from well north of Pohnpei, to Invest 94W north of
kwajalein, to another circulation northeast of Majuro. As mentioned
above, 94W looks to meander in the region through the weekend
(possibly longer) while also potentially absorbing the Majuro
circulation.

This not only prolongs the high-end scattered shower coverage, but
also a westerly surface wind and swell affecting much of the FSM and
RMI. We checked with the local Weather Service Office in majuro and
they have not received any impact reports from the wind gusts or
westerly swell thus far, but we`ll continue to monitor trends for
this potential.

Otherwise, for Pohnpei and Kosrae, with high pressure centered south
of Pohnpei very dry air is being driven around its periphery from
Chuuk east through Pohnpei and southeast through kosrae. The 6-Layer
AlP (All Precipitable Water) layered moisture values were generally
under an inch below 500mb, with MIMIC showing total PWATs
(Precipitable Water Values) near 1.5 inches, which is fairly dry for
this region. This combined with a dearth of surface convergence will
keep show potential to isolated through most of the forecast period,
with the exception being at Kosrae as you could get "clipped" be
increased moisture and convergence on the back side of 94W as it
develops.

As for marine conditions winds will generally increase as we head
into the first half of next week, when general to moderate winds will
be fairly common, perhaps a tad higher at majuro and Kosrae depending
how 94W evolves. Seas look to hold in the 4 to 6 foot range.

Western Micronesia...
The main focus was on whether to continue the Special Weather
Statement for Palau and Yap or not. Given unimpressive satellite
morphology today, as evident by abundant convective debris but
little in the way of robust convective development, it appears the
models were initally overestimating the rainfall potential. Todays
numerical runs have come more in line for Koror and Yap, with 84
hour rainfall totals ending Monday evening generally in the 1.5 to
2.5 inch range. This is about a third of what we were looking at a
few days ago. Part of the reason for this may be tied to the slower
than expected development of Tropical Storm Choi-wan, as it`s been
fighting some northeast shear. This shear has kept the vorticity
centers slightly tilted over the past 48 to 72 hours, inhibiting the
rate of strengthening. As this occurred, despite good monsoon
strength across Palau and Yap as evident by soundings showing 15 to
30 knots in the lowest 2 km, convection has been fairly sparse.

Then, with Choi-wan being slower to develop, the models have started
to back off on the swell heights while delaying the arrival time
somewhat of the northeasterly pulse of wave energy to Yap and Palau.
With this in mind, we decided not to reissue the Special Weather
Statement. There remains some concern for high surf at yap, but if it
does materialize it won`t be until late Monday at the earliest, and
given model trends we`ll refrain from inserting a headline until
Choi-wan becomes better organized. Even then, it`ll be moving away
from the region which will help negate these concerns.

With that said, we did maintain scattered shower coverage at Palau
and Yap through the middle of next week. Conditions look to be much
drier at Chuuk, as you`re caught in a "ribbon" of dry air with MIMIC
PWATs of 1.5 inches, much of the moisture above 500mb. Given a
persistent ridge of high pressure to your south, rainfall prospects
are looking fairly bleak through the middle of next week.

As for marine conditions, moderate to fresh winds at Palau and Yap
will continue this weekend, becoming gentle to moderate by early next
week. Chuuk will see light to gentle winds as you`re further removed
from the pressure gradient. Seas of 4 to 6 feet may build a foot or
two as increased swell from Choi-wan arrives early next week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Wind Advisory until 4 AM ChST Sunday for GUZ001.

     Flood Watch through late Sunday night for GUZ001.

     High Rip Current Risk through late Sunday night for GUZ001.

     High Surf Advisory until 4 AM ChST Monday for GUZ001.

MP...Wind Advisory until 4 AM ChST Sunday for MPZ001.

     Flood Watch through late Sunday night for MPZ001>003.

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

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

     Coastal Flood Watch through Monday evening for MPZ002-003.

     Tropical Storm Warning for MPZ002-003.

Marianas Waters...Tropical Storm Warning for PMZ153-154.

     Gale Warning until 5 PM ChST this afternoon for PMZ151-152.

 Hazardous Seas Watch from this afternoon through Sunday
 afternoon for PMZ151-152.

&&

$$

Marianas Update: DeCou
Tropical Systems Update: DeCou/Kleeschulte
East and West Micronesia Update: Kleeschulte
Marianas/Tropical/Hydrology: Montvila/Stanko
East/West Micronesia: Doll
IDSS: Mesa