850
FXPQ50 PGUM 191833
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
433 AM ChST Sun Oct 20 2024


.Marianas Update...
Satellite and radar trends show showers being held at bay for the
most part across the Marianas. Scattered showers are seen across the
eastern and western coastal waters of the Marianas. Current thinking
is for shower coverage to increase today based on both the GFS and
ECMWF showing the showers filling in this morning. The forecast was
kept at numerous 70% PoPs with both the GFS and ECMWF in fair
agreement. The risk for flash flooding looks to increase next week
with the grand ensemble (A combination of the GFS, ECMWF, and
Canadian ensembles) showing a mean of 7 inches of rain through
Wednesday night for the Marianas. A Flash Flood Watch may be
necessary as confidence in the rainfall forecast increases.

Combined seas will remain around 5 to 8 feet through Monday as winds
become moderate to fresh at times and slowly turn to the south and
southwest as a monsoon trough develops. Gusty winds and choppy seas
are expected near heavy showers and thunderstorms. Winds and seas
may become hazardous to small craft mid week as another tropical
disturbance and/or monsoon surge impacts the region.

&&

.Tropical systems Update...
A Tropical Cyclone Formation Alert has been issued for Invest 96W,
which replaced Invest 94W. Invest 96W is currently
located near 14.1N and 138.1E, about 443 Statute Miles west of
Guam. Deep convection with numerous showers and thunderstorms are
seen southwest and northeast of the center. Latest guidance
indicates that TCFA 96W will drift slowly west and strengthen over
the next several days, but models vary greatly on strength and
location. Invest 96W is not expected to directly impact the
Marianas, Yap or Palau, but a monsoon type wind flow will cause
inclement weather across these areas.

Another monsoon disturbance, Invest 95W, is centered just south
of Guam near 13.0N 140.1E. 95W is showing as a broad circulation
with an ill-defined center at this time. A broad area of moderate to
deep convection is seen southwest of the Marianas into Yap Proper.
Models vary greatly on the movement and development of 95W over the
next few days, so confidence in the development and location of 95W
over the next few days is very low. The probability of 95W becoming a
significant tropical cyclone over the next 24 hours is low.

For more information on Invest 96W and Invest 95W, see bulletins
issued by the Joint Typhoon Warning Center under WMO header ABPW10
PGTW

&&

.Eastern Micronesia Update...
Continued previous forecast trends. Relatively quiet weather is over
Pohnpei and Kosrae this mourning, but the surface trough and showery
weather over the Marshall Islands is expected to move westward into
Kosrae and Pohnpei states during the next couple of days.

&&

.Western Micronesia Update...
Invest 96W near 14N138E is now the subject of a TCFA from JTWC. The
monsoonal flow over far western Micronesia will become much stronger
through about midweek as 96W gradually develops and drifts westward.
The Small Craft Advisory for Yap was moved up and is now in effect
early this morning, just like the SCA which is in effect for Palau,
and both in effect through Wednesday afternoon.

&&

.Prev discussion... /issued 627 PM ChST Sat Oct 19 2024/

Marianas Synopsis...
A wet monsoonal pattern is developing across the Marianas and will
increase winds, seas and surf around midweek, potentially creating
hazardous surf and conditions to small craft.

Discussion...
Satellite imagery shows mostly cloudy to cloudy skies with radar
imagery showing scattered showers this evening. The coverage and
intensity of showers has been decreasing since early this morning,
likely do a temporary stabilization of the atmosphere from earlier
showers and the decrease in convergence over the Marianas. An active
monsoonal pattern still extends across the region and models depict
showers and the potential for thunderstorms increasing overnight and
into Sunday, so have showers becoming numerous and added thunderstorms
after midnight into the forecast. It is difficult to try and provide
similar trends in the forecast after tonight as models struggle to
capture the ups and downs in convection and the exact location where
a line of heavy showers will set up within the monsoonal pattern.
Model trends continue to support breezy to possibly windy conditions
around midweek, however there is still a lot of uncertainty and it is
compounded by the potential for circulations to develop and
strengthen within the monsoon trough, and how the models still differ
from each other. There are currently two open invests, 95W located
southwest of Guam near 13N143E and the second being invest 96W well
west of the Marianas near 14N139E. Both the GFS and ECMWF show a
consolidation into one circulation west of the Marianas over the next
couple of days with the potential of a second one developing mid
week near or just east of the Marianas. The movement of this second
circulation looks odd so it hard to really trust that these models
are correctly handling the exact evolution of this pattern, however
the overall trend supports wet and potentially breezy to windy
conditions. Until a discrete circulation develops and models remain
consistent on a potential solution, there is still plenty of room for
the forecast to change.

Marine...
Combined seas will remain around 5 to 8 feet through Monday as winds
become moderate to fresh at times and slowly turn to the south and
southwest as a monsoon trough develops. Gusty winds and choppy seas
are expected near heavy showers and thunderstorms. Winds and seas
may become hazardous to small craft mid week as another tropical
disturbance and/or monsoon surge impacts the region.

Tropical systems...
JTWC closed 94W today, but have opened two more invest areas
throughout the day. The first is 95W located southwest of Guam near
13N143E and it is currently very weakly defined circulation that is
rated low for development, meaning development into a tropical
cyclone is unlikely over the next 24 hours. The second is 96W, located
even further west near 14N138E and is a more defined circulation
visible within scatterometer data. 96W is rated as a medium meaning
tropical cyclone development is likely, but expected to occur beyond
24 hours. Overall model guidance suggest 95W and 96W will merge into
one circulation as slowly drift westward into the Philippine Sea,
away from the Marianas and not impacting any of the major islands in
WFO Guam`s area directly. The overall monsoonal pattern will however
continue to bring a wet pattern to the Marianas, Yap, and Palau, with
breezy to potentially windy conditions developing first at Yap and
Palau and then spreading to the Marianas around mid week. Long-term
model predictions continue to suggest additional circulations
developing in the monsoon pattern, mainly north of 10N and west of
160E, but confidence in any other details is low.

Eastern Micronesia...
An overall dry pattern is currently in place across eastern
Micronesia. A few showers are lingering over Majuro as a weak trough
continues west, moving away from the atoll. This trough looks to
move into Kosrae Sunday morning and into Pohnpei by Monday morning,
bringing a round of scattered showers to both locations. A weak ridge
west of Pohnpei will help to slow the progression of the trough as
the trough slowly erodes the ridge. Another trough, currently east of
Majuro looks to bring increased showers and thunderstorms to Majuro
Sunday. As the trough moves west, it is expected to elongate and
orient more southwest to northeast, with an area of convergence
lingering over Majuro after the trough passes, maintaining the
increased convection through Monday. This trough, and the trailing
convergence, will move into Kosrae by Monday, maintaining the
convection there through around Monday night. This same feature will
maintain increased convection at Pohnpei through Tuesday night.
Majuro will then see a dry spell Tuesday into Wednesday, with a band
of convergence and a series of weak troughs moving through the
Marshalls, maintaining increased convection at Majuro from the middle
of next week into next weekend. This will spread west- southwestward
into Kosrae, maintaining a chance of showers there into next weekend
as well. The band of convergence and weak troughs look to remain
south of Pohnpei allowing Pohnpei to see partly cloudy skies and a
slight chance of showers by midweek.

The marine conditions look to remain fairly unimpressive over the
next several days. The one thing of note is an increasing west swell
moving into Pohnpei near the end of next week as a monsoon swell
pushes into the region. Seas could reach up to 11 feet north of
Pohnpei, though only around 6 to 7 feet is expected in the Pohnpei
coastal waters. Otherwise, combined seas of 3 to 5 feet through the
next several days. Gentle to moderate winds at Majuro will push into
Kosrae early next week. After Tuesday, light to gentle winds will
prevail at both locations. Winds will remain light to gentle for the
next several days at Pohnpei.

Western Micronesia...
A small craft advisory has been issued for Sunday for Palau, and for
Sunday night for Yap, and a Special Weather Statement has been issued
to highlight the upcoming conditions expected with the monsoon surge.

Currently for Chuuk, showers are building back in. For Yap and Palau,
scattered shower are developing along the convergence flow into the
tropical disturbances to the north. Babeldaob Buoy shows 3 to 4
feet, however this buoy will be protected from the incoming monsoon
surge. Seas on the north and west of Palau can expect seas to be
around 4 to 6 feet and building.

The monsoonal surge is starting to pick up over Palau as invests 96W
starts to intensify. There is still some uncertainty with this
system. This mean concerns will primarily be the winds, and
eventually the seas and heavy rainfall. Models suggest the winds
will be 20 to 30 mph with possible gusts of 40 mph. Yap is
anticipated to lag behind Palau by about 12 hours. Once the monsoon
swell builds up, surf on north and west facing reefs of Palau and
Yap may become hazardous during this event, reaching to at least 9
feet. Palau could see about 2 to 4 inches of rain, while Yap could
sea 3 to 5 inches with potential for more depending on how the
tropical disturbances develop.

Chuuk is in the convergence zone to the south east of invest 95W.
This convergence zone will bring mostly cloudy to cloudy skies and
scattered showers. These showers will likely linger this upcoming
week due to the ongoing series of disturbances to the north.

&&

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

$$

Marianas: Williams
Micronesia: Simpson