Flood Watch 1, NWS Guam

Area Forecast Discussion
National Weather Service Tiyan GU
748 AM ChST Wed Sep 22 2021

.Marianas Synopsis...
Scattered showers and thunderstorms are moving into the Marianas this
morning as a tropical disturbance, JTWC's Invest 99W, approaches from
the east-southeast. Winds are moderate east to northeast, and Ipan
buoy is reporting 3-4 ft seas.

&&

.Discussion...
GFS and ECMWF are still in raucous disagreement on the future of 99W,
with the GFS treating it much more aggressively than the European
model. For now, the ECMWF looks more credible in developing 99W at
a more realistic pace and taking it on a more southern track south
of Guam. So for now, the forecast for the Marianas is based heavily
on the ECMWF, which brings scattered showers and thunderstorms in
for today and tonight, followed by widespread showers and some
thunderstorms Thursday into Friday as 99W passes by, with potential
for locally heavy rain. It should be noted that uncertainty with
this forecast is high, and changes are likely.

&&

.Marine...
Wave guidance for today's forecast is derived from the ECMWF, which
yields lower seas than the aggressive GFS based WW3 guidance. Even
so, seas peak around 8 feet on Thursday, mainly in eastern Saipan
waters, and surf approaches hazardous levels on Thursday as well.
Swell starts out from the east, but should shift around to southeast
on Friday, south Saturday and southwest on Sunday. As mentioned
above, this forecast is subject to uncertainty and changes are
likely.

&&

.Tropical Systems...
Based on ASCAT scatterometer winds and the latest IR satellite loop,
the weak center of 99W appears to be near 11N 150.5E. On satellite
it appears to be isolating itself from surrounding convection and
moving west. It is too early to tell with confidence which model is
to be preferred, but the European solutions seems more realistic,
as the GFS version requires an abrupt turn to the northwest this
evening, followed by rapid intensification into a tropical storm
on Thursday.

&&

.Eastern Micronesia and Chuuk...
For Chuuk, scattered showers today as the tropical disturbance to
the north, still disorganized, slowly moves west-northwest. We're
still seeing some buildup of clouds and showers where there's
convergent winds south of the tropical disturbance. Kept scattered
showers for this morning, but by Thursday should see more clearing
out.

For Pohnpei and Kosrae, not likely for any heavy rains today, but
more spotty showers as there's a weak ridge in the vicinity.

Surface trough is right over Majuro, scattered showers will continue
through tonight as the trough moves west and flattens out by
tomorrow.

Combined seas will be between 3 and 5 ft through Sunday.

&&

.Western Micronesia for Koror and Yap...
For Yap, recent satellite shows spotty showers coming in from the
east with a surface trough and some divergence aloft. Updated Yap's
forecast for scattered showers later in the day.

Koror, convergent winds brought a few good showers the coastal
waters last night, and the models and satellite show this feature
tracking west. Forecast was for isolated showers trending to
scattered by Thursday, but may need to adjust the forecast for
increasing showers coming in earlier.

Combined seas will be between 3 and 5 ft through Sunday.

&&

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

&&

Marianas: Middlebrooke
Micronesia: Hong

Special Weather Statement
National Weather Service Tiyan GU
102 PM ChST Wed Sep 22 2021

GUZ001>004-221000-
Guam-Rota-Tinian-Saipan-
102 PM ChST Wed Sep 22 2021

...DEVELOPING TROPICAL DISTURBANCE IS NOW THE SUBJECT OF A
   TROPICAL CYCLONE FORMATION ALERT...

The developing tropical disturbance north of Chuuk is now the
subject of a Tropical Cyclone Formation Alert (TCFA) issued by the
Joint Typhoon Warning Center. The TCFA means that the formation of
a tropical cyclone is possible within the next 24 hours.

The circulation is centered north of Chuuk near 11N152E, or roughly
480 miles east-southeast of Guam. The circulation is moving
west-northwest at 13 mph.

This disturbance is expected to strengthen as it heads near or
through the Marianas by the end of this week. This disturbance is
known as Invest Area 99W and is still in the formative stage, and
questions remain in the track, how quickly the circulation will
strengthen, and how quickly will it move. The latest guidance
suggests some type of circulation passing near or through the Marianas.

Showers, thunderstorms, and gusty winds are likely Wednesday
through Friday. Although it'll highly depend on how organized the
system is and where it tracks, current estimates call for 1 to 3
inches of rainfall, with locally higher amounts possible.
Sustained winds of 15 to 25 mph, with gusts to 35 mph are possible
in the stronger showers and thunderstorms. Continue to monitor
the forecast, as changes are likely.

Doll/Ziobro

Special Weather Statement...UPDATED
National Weather Service Tiyan GU
304 PM ChST Wed Sep 22 2021

GUZ001>004-221000-
Guam-Rota-Tinian-Saipan-
304 PM ChST Wed Sep 22 2021

...DEVELOPING TROPICAL DISTURBANCE IS NOW THE SUBJECT OF A
   TROPICAL CYCLONE FORMATION ALERT...

The developing tropical disturbance north of Chuuk is now the
subject of a Tropical Cyclone Formation Alert (TCFA) issued by the
Joint Typhoon Warning Center. The TCFA means that the formation of
a tropical cyclone is possible within the next 24 hours.

The circulation is centered north of Chuuk near 11N150E, or roughly
400 miles east-southeast of Guam. The circulation is moving
west-northwest at 13 mph.

This disturbance is expected to strengthen as it heads near or
through the Marianas by the end of this week. This disturbance is
known as Invest Area 99W and is still in the formative stage, and
questions remain in the track, how quickly the circulation will
strengthen, and how quickly will it move. The latest guidance
suggests some type of circulation passing near or through the Marianas.

Showers, thunderstorms, and gusty winds are likely Wednesday
through Friday. Although it'll highly depend on how organized the
system is and where it tracks, current estimates call for 1 to 3
inches of rainfall, with locally higher amounts possible.
Sustained winds of 15 to 25 mph, with gusts to 35 mph are possible
in the stronger showers and thunderstorms. Continue to monitor
the forecast, as changes are likely.

Doll