668
FXPQ50 PGUM 191840
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
440 AM ChST Sat Jul 20 2024

.Marianas Update...
JTWC issued a Tropical Formation Alert last night for Invest 92W. At
the current time it is just moving northwest across 130E and will
likely be a Tropical Depression within a few hours.

Otherwise, a few moderate to locally heavy showers spread across Guam
overnight from east toward the west then southwest. Recent Satellite
shows fewer clouds upstream and significant drying trend developing
over the region as a ridge of high pressure arrives. Father east,
there is still a trade-wind trough just north to northeast of Chuuk
that could arrive on Sunday.

&&

.Tropical systems Update...
JTWC issued a Tropical Formation Alert last night for Invest 92W. At
the current time it is just moving northwest across 130E, probably
near 14N129.5E, and will likely be a Tropical Depression within a
few hours. It is expected to move farther northwest and gradually
develop during the next few days. By the way, Longitude 130E is the
western boundary of Guam`s Area of Responsibility.

&&

.Eastern Micronesia Update...
Satellite imagery continues to shows a clearing region. The remaining
showers over Majuro and Kosrae are beginning to rain themselves out.
Showers for both locations are expected to be scattered today and
tonight. Pohnpei, on the other hand, can expect to see rather fair
weather today. Winds are expected to be moderate to fresh, but
diminish to gentle to moderate as the day progresses. Seas between
Kwajalein and Majuro are expected to be 4 to 6 feet today, and will
gradually diminish to 3 to 5 feet.

&&

.Western Micronesia Update...


&&
.Prev discussion... /issued 557 PM ChST Fri Jul 19 2024/

Marianas Synopsis...
Satellite imagery shows partly cloudy to mostly cloudy skies across
the Marianas, with most of the showers associated with the monsoon
trough, now to the west to southwest of the Marianas this evening.
Observations from the airports are reporting wind around 10 to 20 mph,
and latest buoy and altimetry data are showing combined seas between
5 and 7 feet.

Discussion...
With the monsoon trough lifting further northwest, it has pulled the
development showers and thunderstorms mainly to the west of the
Marianas and allowed some drier weather to filter back in. There is
some weak convergence south of Guam which may generate some scattered
and isolated thunderstorms within the southern waters of Guam, but
overall expecting isolated showers across all the islands through
Saturday night. Models show showers and thunderstorm chances
increasing Sunday and Sunday night as the trade-wind trough, which
satellite imagery currently shows passing north of Pohnpei this
evening, moves into the Marianas. This trough will likely produce
damp conditions for Liberation Day on Guam, but this currently does
not look like a complete washout just some minor impacts to outdoor
events when scattered showers pass over Guam, though do watch out for
lightning if any isolated thunderstorms develops near Guam. Looking
towards next week, models show some weak troughs passing near the
Marianas which may bring some periods of scattered showers to the
Marianas, but nothing to heavy is currently expected.

Marine...
The monsoon trough is being pulled northwestward by Invest 92W,
located well west of the Marianas, so most of the showers and
thunderstorms are currently developing west and south the coastal
waters of the Marianas. Next main potential for scattered showers and
isolated thundestorms is expected Sunday when a trade-wind trough,
currently well east of the Marianas, is expected to move into the
Marianas. Combined seas of 4 to 7 feet are expected, with a weak
secondary southwest swell through at least Monday. Winds are expected
to be gentle to moderate through the weekend and into early next
week.

Tropical systems...
JTWC Invest 92W continues to drift in a northwest direction with
latest scatterometer visible satellite showing 92W centered around
13N133E, embedded within the monsoon trough, and located northwest of
Palau and Yap and well west of the Marianas. Convection around 92W
has become more organized and the low-level circulation is more
defined, so JTWC has increased the risk of further development to
medium, meaning development into a tropical cyclone is likely but
expected to occur beyond 24 hours. Models are still fairly consistent
that 92W will continue to move away from our region and continue to
move northwest into the Philippine Sea.

Eastern Micronesia...
Satellite imagery shows shower activity has greatly decreased across
eastern Micronesia today. Scattered showers and isolated
thunderstorms are seen near Pohnpei and just east of Majuro. Isolated
showers are seen near Kosrae.

Numerous showers and isolated thunderstorms have persisted well to
the east of Majuro as a trade-wind trough approaches the region.
Both the GFS and ECMWF dissipate convection as it approaches Majuro.
As a result, scattered coverage remains on the low (30%) end through
the weekend. Changes may be needed in future forecasts if convection
becomes self sustaining. For Kosrae and Pohnpei, models show a pulse
up later this weekend as the trade-wind trough approaches from the
east. High end (50%) scattered is in place for Kosrae and Pohnpei
Saturday night into Sunday. An upgrade to numerous (60%) showers may
be needed if convection persists along the trade-wind trough.

Scatterometer data shows gentle to moderate winds across the region.
Altimetry data shows combined seas have lowered to 3 to 5 feet for
Pohnpei and Kosrae, and 4 to 6 feet around Majuro.  Seas may drop an
additional foot across the region as the easterly trade swell
weakens, with winds remaining gentle to moderate across the region
overall, though some locally stronger gusts are possible near
showers.

Western Micronesia...
Satellite observations show scattered showers near Chuuk and Palau
with scattered to numerous showers near Yap.

Invest 92W has lifted northwest out of western Micronesia and is
moving towards the Philippines Sea. The system has looked better
organized today and both Yap and Palau are now in a region of low
level convergence. A secondary weak circulation is located northeast
of Yap and has been proving additional shower coverage today, leading
towards an increase to numerous coverage for tonight. High end (50%)
scattered showers will be prevalent for Palau tonight before tapering
off to low end (30%) scattered showers on Saturday. Locally heavy
rain and gusty winds of 25 mph will be possible for Palau and Yap
tonight and through Saturday night for Yap. A quieter pattern will
set in across western Micronesia next week after the monsoon trough
and 92W lift to the north. Models hint towards a resurgence of the
monsoon trough towards the end of the week, allowing for a higher
coverage in showers and thunderstorms. Low end (30%) scattered
showers and isolated thunderstorms are forecasted for Chuuk tonight
through Saturday night. A trade-wind trough from eastern Micronesia
will approach Chuuk on Sunday and will bring high end (50%) scattered
showers and isolated thunderstorms.

Latest altimetry data shows seas generally between 3 and 5 feet for
all of western Micronesia. Wind gusts to 25 knots will be possible
near showers for Palau through tonight and Yap through Saturday
night.

$$

&&

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

$$

Marianas: Simpson
Micronesia: Bowsher