174
FXPQ50 PGUM 181848
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
448 AM ChST Fri Jul 19 2024

.Marianas Update...
Only minor changes were made to the forecast. The axis of a surface
trough has moved just west overnight, but a moist flow from the east
to southeast will likely persist over the Marianas. JTWC Invest 92W
is moving into the Philippine Sea and the monsoon trough to our south
is moving slowly north to northwest.

&&

.Tropical Systems Update...
Invest 92W was roughly near 12N132E overnight with a monsoon trough
east-southeastward to just north of both Yap and Chuuk. 92W seems to
be drifting slowly toward northwest and the monsoon trough has edged
slowly northward.

&&

.Eastern Micronesia Update...
Currently, over most of Eastern Micronesia, mostly cloudy skies with
a few cells of showers in the region, most notably over Majuro and a
smaller one over Kosrae. Both of these cells are expected to clear up
by the morning commute. Pohnpei is expected to clear up today.
Showers are expected to build back in later in the weekend.

Seas are expected to be 3 to 5 feet today for Pohnpei and Kosrae, and
5 to 7 feet for Majuro. Winds are generally going to be light to
gentle today and building up to fresh by this evening.

&&

.Western Micronesia Update...
Continue previous forecast trends, although some heavy showers have
unexpectedly remained just southwest of Koror overnight. Invest 92W
is roughly near 12N132E with a monsoon trough east-southeastward just
north of both Yap and Chuuk. Weather conditions will remained very
unsettled for Western Micronesia.

&&

.Prev discussion... /issued 607 PM ChST Thu Jul 18 2024/

Marianas Synopsis...
Heavy showers arrived in the Marianas late this morning with the
passage of a surface trough, bringing 30+ kt wind gusts. Otherwise,
surface winds reported from local airports have been east to
southeast at 10 to 15 kt with occasional higher gusts. Recent
satellite shows patchy showers approaching from the southeast, at
least one of which has produced lightning.

Discussion...
Very unsettled weather persists across the region. The monsoon
trough south of the Marianas is beginning to shift north, as a broad
circulation slowly develops to the west of Yap near 10N136E, now
designated as Invest 92W by the Joint Typhoon Warning Center.
Additional showers are developing along the northern edge of the
monsoon trough and are lifting north towards the Marianas tonight.
Isolated thunderstorms are in the forecast as some of the heaviest
showers have produced occasional lightning at their peak. Models
still show the monsoon trough lifting northwest following Invest 92W.
This favors the heaviest showers developing west of the Marianas,
though scattered showers are still expected across the Marianas
through Saturday. Showers are expected to become isolated Saturday
night, before the next trade wind trough pushes into our area Sunday,
with an overall wet pattern expected through early next week.

Marine/Surf...
The surface trough expected for the Marianas has passed to the west
after bringing showers this morning. However, the monsoon trough to
our south is expected to lift north, bringing additional rounds of
showers through at least Friday night. Winds will be gentle to
moderate tonight, increasing to moderate to fresh on Friday with
gusts to around 25 kt near showers. Combined sea of 4 to 6 feet will
be choppy at times with a weak secondary swell arriving from
southwest.

There will be a moderate risk of rip currents along east and south
facing reefs tonight. There will also be a moderate risk for
lightning through Friday night.

Tropical systems...
A broad circulation has developed within the monsoon trough, and is
located near 10N136E, or west- northwest of Yap Proper and north of
Palau. The Joint Typhoon Warning Center (JTWC) is monitoring this
feature as Invest 92W. 92W is very disorganized and is expected to
slowly drift northwestward into the Philippine Sea and away for Yap
and Palau. Further development of 92W is very unlikely over the next
24 hours, with any long-term development to take place west of Palau
and Yap, out in the Philippine Sea.

Eastern Micronesia...
Satellite imagery shows scattered showers and isolated thunderstorms
across the region, driven by the monsoon trough that enters the
region from western Micronesia, passing just south of Pohnpei and
through Kosrae this evening, while the ITCZ has redeveloped near and
east of the Marshall Islands. The monsoon trough is expected to
slowly lift northwest over the next few days, causing the bulk of the
showers to lift northward as well, this should allow showers to
become isolated at Kosrae tonight and into Friday, with Pohnpei
becoming isolated Friday. Showers are expected to remain scattered
across the Marshall Islands as the ITCZ slowly breaks down as it
loses its southeasterly winds and devolves more into a series of
troughs and general trade convergence that will move across the
Marshall Islands. As troughs, left over by the ITCZ, drift westward,
scattered showers and isolated thunderstorms will rebuild across
Kosrae and Pohnpei Friday night, with an overall wet pattern expected
for the region through the weekend and into early next week.

Scatterometer data shows gentle to moderate winds across the region
some fresh gusts associated with showers are expected across the
Marshall Islands tonight. Altimetry data shows combined seas have
lowered to 4 to 6 feet for Pohnpei and Kosrae, but are still around 6
to 8 feet around Majuro. Since seas have decreased around Kosrae, it
looks like the easterly swell has eased enough to allow surf along
east facing reefs of Kosrae to drop below 8 feet, so the High Surf
Advisory was allowed to expire earlier this afternoon. Seas may drop
an additional 1 to 2 feet 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...
Scatterometry and satellite observations show two circulations
circulations across western Micronesia. The first is invest 92W near
10.5N135.6E. The second is located near 9.2N144E. Numerous showers and
isolated thunderstorms are seen near Palau with scattered showers
and isolated thunderstorms near Chuuk and isolated showers and
thunderstorms near Yap. Latest altimetry shows seas in the 3 to 5
feet range for Palau and Yap.

The main feature across the region is invest 92W which is embedded
within the monsoon trough. GFS streamlines show low-level
convergence across Palau where showers and thunderstorms have
persisted through the day. The latest forecast package increased PoPs
to numerous to cover the area of convergence tonight. On the other
hand, low-level diffluence has left Yap with isolated showers
throughout the day. There are some signs of increasing convergence
near Yap with a cumulus field having developed over the last couple
of hours, but decided to decrease coverage to scattered 50 percent
PoPs for Yap Friday. Numerous locally heavy showers are still
expected for Saturday and Saturday night for Yap. Scattered showers
were seen near Chuuk today and will remain persistent over the next
few days. The monsoonal trough and invest 92W will slowly shift
northwestward over the coming days allowing showers and thunderstorms
to concentrate closer to Yap by Saturday. The GEFS, EPS (ECMWF
ensembles), and Canadian Ensembles all show gradual development of
92W, but at this time significant development appears to hold off
until the system is well to the northwest of Yap and Palau.

Seas may rise another foot or so but should remain in the 3 to 5 foot
range for Palau and Yap and 4 to 6 feet for Chuuk. Seas will start to
subside by Saturday as the monsoon trough shifts away from the
region. Wind gusts of 25 knots will be possible near showers for
Palau and Yap through Saturday.

&&

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

$$

Marianas/Western Micronesia: Simpson
Eastern Micronesia: Bowsher