195
FXPQ50 PGUM 210808
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
608 PM ChST Mon Sep 21 2026

.Marianas Synopsis...
Satellite imagery and doppler radar show light showers across Guam
and Rota, with little to no showers across Saipan and Tinian.
Altimetry shows combined seas are between 4 and 6 ft across the
region.

&&

.Discussion...
Overall, the main change driving this forecast package is the
designation of Invest 91W by the JTWC and its subsequent upgrade to
"Low" status. Currently, guidance does not develop 91W into a
tropical cyclone until well west of the Marianas. During its
disorganized stage, it likely will bring heavy rainfall to Guam over
the next couple of days. Numerous locally heavy showers and isolated
thunderstorms are expected for Guam Tuesday through Tuesday night,
with high-end (50%) scattered showers and isolated thunderstorms for
the CNMI. Model guidance ranges from 2 to 5 inches of rain across
Guam through Wednesday evening and 1 to 3 inches of rain across the
CNMI. See the Hydrology section for more details.

As Invest 91W lifts off to the northwest, the monsoon trough looks
to lift north of the region, shifting the active wet phase of the
monsoon over the Marianas. See Tropical Weather Discussion for more
details. Currently, scattered showers and isolated thunderstorms
through the period are forecasted. Shower coverage could increase
across the region late this week and into the weekend depending on
the setup monsoon trough. If Invest 91W tracks southerly, it could
keep the monsoon trough further south over the Marianas. This would
put under a col where island effect showers would be possible,
similar to what we saw this past weekend. For now, the focus is heavy
rainfall on Tuesday and Wednesday, but if the Marianas see
additional rainfall during the first half of the week, flooding
concerns would increase heading into the weekend. &&

.Marine...
Benign seas of 4 to 6 feet will continue before falling 1 to 2 feet
during the second half of the week. Some choppy seas would be
possible for Guam near heavier showers Tuesday and Wednesday. There
is a moderate risk for rip currents along all reefs through Tuesday,
and then for north and east facing reefs through Friday.

&&

.Hydrology...
A Flash Flood Watch has been issued for Guam beginning 10 AM Tuesday
until 10 PM Wednesday, calling for 2 to 5 inches of rain across Guam
during the timeframe. Much of Guam saw 2 to 4 inches of rain since
this past Saturday, which has kept soils wet. An additional 2 to 5
inches of rain would lead to flash flooding due to the wet
antecedents. The main challenge is for the potential for Invest 91W
to shift further west during its development, leading to less
rainfall for Guam. Some of the recent 00z runs coming out after the
forecast package have shown this shift.

The main culprit of this shift is displacement of the low-level
vorticity to the west of a Mid-Level Circulation (MLC). Models are
keeping the MLC weak while quickly ramping up the low level vorticity
into a Low-Level Circulation (LLC). If the focus of convection
shifts to the LLC, then rainfall amounts would be considerably lower
for Guam, keeping the risk for Flash Flooding low. Latest satellite
trends seem to be favoring both with new towers going up along the
developing LLC, but some towers have also developed along the
southern and eastern edges of the MLC. This adds uncertainty in the
forecast. Currently, residents of Guam should prepare for heavy
rainfall starting Tuesday as Invest 91W develops. Conditions will
continue to be monitored through the night for significant changes.

&&

.Tropical Systems...
A developing disturbance, Invest 91W, is found south of Guam,
centered near 11N144E. 91W is now rated low, meaning that
development into a significant tropical cyclone is not expected over
the next 24 hours, but will be possible over the next few days. 91W
resides on the northern edge of the monsoon trough. Convection is
sparse to the northern half of 91W, however, showers are increasing
in this area. Convection remains very active on the south side.
Invest 91W will slowly move northwest over the coming days.

For more information on Invest 91W, see bulletins issued by JTWC
under WMO header ABPW10 PGTW.

&&

.Eastern Micronesia...
The main concern this afternoon is a strong monsoon surge over the
Marshall Islands, producing scattered showers and isolated
thunderstorms across Majuro with reports of gale-force winds and
gusts across the atoll. A short-term Gale Warning was issued for
majuro coastal waters through Tuesday at 3 AM ChST. The latest ASCAT
does show an area of 30+ knot winds south of and over Majuro. This
surge will maintain near-gale to gale force winds and gusts through
the night. Stronger gusts are likely with heavier showers. Winds
should decrease early in the morning, falling below gale force around
3 AM ChST.

For the remainder of the forecast, an overall unsettled pattern looks
to continue across Majuro into the weekend. For Pohnpei and Kosrae,
an overall dry pattern is in place and looks to continue through most
of the week. As the monsoon trough drifts a bit farther south late in
the week and over the weekend. expect a weak monsoon surge to
develop across Pohnpei and Kosrae, bringing increased convection to
Pohnpei Friday and to Kosrae Saturday.

As mentioned earlier, a Gale Warning is now in effect for Majuro,
through Tuesday at 3 AM ChST. Seas look to remain below hazardous
levels, though seas will be choppy due to large wind waves produced
by the monsoon surge. Winds will subside quickly by Tuesday morning,
dropping below 25 knots by 3 AM. For Pohnpei and Kosrae, benign winds
and seas look to continue through the week.

&&

.Western Micronesia...
A special weather statement remains in effect for Yap State and has
expanded to include Palau. Heavy showers and thunderstorms are
expected through the middle of next week, and gusty winds at times
near these heavy showers.

The main features of the region are Invest 91W and the monsoon flow.
91W is located near 11N144E and is embedded in the monsoon flow over
the northern portion of Western Micronesia. Within both features
heavy showers and isolated thunderstorms are to be expected. Winds
may approaching the frequent 22 knots or frequent gusts to 25 knots
threshold for small craft, especially on Tuesday night at Yap.

For Palau, showers are currently scattered and expected to build into
widespread over the evening hours as the monsoon pattern is expected
to move south over the region. Model guidance suggests and 1 to 2
inches of rain to fall per day through Wednesday, then taper down to
0.5 inches for Thursday. Benign weather is anticipated to return for
the weekend. Seas are expected to remain 4 to 6 feet with winds
light to moderate.

For Yap, there will be widespread locally heavy showers and isolated
thunderstorms. Wind gusts near these heavier showers may reach to 25
knots (or 30mph) at times. Showers may begin to wane as early as
Wednesday. Rain totals are anticipated to be 1 to 2 inches a day,
then showers are anticipated to taper down around Thursday. Benign
weather is anticipated to return for the weekend. Seas are expected
to be 5 to 6 feet through Wednesday. tapering down to 3 to 5 feet for
the latter half of the week.

For Chuuk today, a surface trough interacting with some mid-level
convergence has produced scattered showers for the region. These
showers have weakened to isolated and is mostly cloud cover. There
still remains some convergence surface, however, without more
tightening of the streamline or another trough passing through,
showers are expected to be isolated through the remainder of the
week. Seas are expected to be 4 to 6 feet through about Wednesday,
then drop by a foot for the later half of the week.

&&

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

$$

Marianas: Williams
East Micronesia: Kleeschulte
West Micronesia: Bowsher