537
FXPQ50 PGUM 172021
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
621 AM ChST Wed Sep 18 2024

.Marianas Update...
Weather radar shows scattered showers and isolated thunderstorms
moving through the Marianas Waters. The buoys reveal combined seas
of 5 to 7 feet, composed of a moderate west swell and a smaller east
swell with some wind waves mixed in.

&&

.Tropical systems...
Tropical Storm Pulasan is moving west-northwest and will be exiting
Micronesia later today. Other than some west swell, there is little
effect on Micronesia as there are no Micronesian islands in its
path.

&&

.Eastern Micronesia Update...
An ill-defined ITCZ stretches across the Dateline to the Marshalls,
but is largely broken up into a series of trade-wind troughs from
Majuro to north of Pohnpei. The main change was to increase POPs and
add isolated thunderstorms for Majuro`s forecast today based on
deepening convection across the Marshalls. Quieter conditions are
over Pohnpei and Kosrae being slightly south of the main activity,
then ITCZ showers are expected to build back in Thu-Sat.

&&

.Western Micronesia Update...
Satellite shows heavy showers and thunderstorms over Palau and just
west and southwest of Yap early this morning. Due to strong
divergence aloft, this area of strong convection has been nearly
stationary between Palau and Yap. Decided to bump up POPs for today
since models guidance and satellite trends still keeps these areas of
robust convection too close for comfort. Well to the north, TS
Pulasan is quickly moving farther west-northwest, steering the
monsoon flow along with it. Yap and Palau will see gentle to moderate
winds out of the south-southeast today, indicating a transition away
from the strong monsoon pattern consuming the region the past few
days. No changes needed for for Chuuk`s forecast. Isolated to
scattered showers are expected for the next few days until a trough
at the leading edge of the ITCZ increases showers late week.

&&

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

&&

$$

/Previous AFD issued at 602 PM ChST Tue Sep 17 2024/
.Marianas Synopsis...
Satellite imagery shows a thick mid- to upper-level cloud deck over
the region, left over from the heavy showers yesterday that have
pulled well west of the Marianas. The remaining showers are spotty
to scattered in coverage. Winds continue to turn towards southeast
and are gentle to moderate. Buoys are reporting seas around 6 to 7
feet with a primary southwest to west swell.

&&

.Discussion...
As Tropical Storm Pulasan (formerly Invest 97W) continues to move
northwest and away from the Marianas, cloud cover will slowly thin
and the lingering scattered showers will also decrease in coverage
overnight. Compared to the past 10 days, the weather for the rest of
the week will be much more pleasant as winds become easterly and are
expected to be 15 mph or less. The chance for showers will also not
be as high as it has been, allowing us to dry out a little bit. The
next best chance for showers will be around Thursday as a weak trough
approaches from the east. Showers will not be frequent but scattered
in nature, however an upper-level low currently northwest of Wake
Island will be passing north of the Marianas around the same time and
the extra upper-level support it will provide may help to trigger
some isolated thunderstorms. Once the trough passes to the west
Thursday night, Friday and the weekend look to be fairly pleasant
with only some minor upticks in showers at times.

&&

.Marine/Surf...
Buoy and surf observations still indicate the westerly swell is
around 5 to 7 feet with a period of 9 to 10 seconds, which is
capable of producing surf up to 9 feet along west facing reefs, so
the High Surf Advisory was extended to 6AM Wednesday morning. The
west swell has been decreasing since yesterday and this trend is
expected to continue allowing surf along west facing reefs to drop
below 9 feet Wednesday morning. Seas are 6 to 7 feet in the open
waters, but slightly lower in the shadow of the islands. As the west
swell decreases, so will sea heights with seas expected to become 4
to 6 feet by Friday. Gentle to moderate southeast winds tonight will
continue to become more easterly over the next few days with little
change in wind speeds expected.

&&

.Tropical systems...
Tropical Storm Pulasan (15W) is quickly moving away from the
Marianas. Satellite imagery this evening an exposed low-level
circulation, located near 22N136E, that looks to have developed just
north and just shot out from the broader vorticity or circulation
near 19N138E, but this broader circulation is getting harder to
distinguish via satellite. Convection remains displaced to the east
and south of both circulations along with tropical storm or gale
force winds, based on scatterometer data, giving this system more of
a monsoon depression structure. As Pulasan continues to move away
from the Marianas and Micronesia, no direct impacts are expected as
this system exits the region over the next couple of days and
continues to drag the monsoon trough northwest and away from Palau,
Yap, and the Marianas.

&&

.Eastern Micronesia...
A few trade-wind troughs are moving across the region along a weak
and fragmented ITCZ, which sits over and just east of the Marshall
Islands. The troughs are oriented southwest to northeast, with the
first located just between Chuuk and Pohnpei, and the second to the
east of Majuro. Satellite shows scattered showers building to the
west and north of Pohnpei, and isolated showers over Pohnpei itself
along the backside of the trough. Quieter weather will continue for
Pohnpei and Kosrae over the next few days as this trough shifts away
to the northwest, with better chances for showers during the second
half of the week as a stronger ITCZ wave transits the region.

Skies are mostly clear over Majuro for now with only a few isolated
showers seen across the area, but convection is increasing within the
approaching trough to the east, and models indicate an increase in
showers over Majuro mainly after midnight tonight. Adjusted PoPs to
reflect this, keeping scattered showers in the forecast for the next
few days as a few weak troughs move through the area. A stronger wave
enters the region by Friday morning, which looks to bring scattered
to numerous showers over the Marshalls by the coming weekend.

Marine conditions remain fairly benign. Recent altimetry and buoy
data indicate 3 to 5 ft seas. Easterly swell is dominant, but
Pohnpei buoy data still indicates a significant westerly swell
component. Wave models show pulses of longer-period westerly swell
over the next few days, emanating from the monsoon surge now moving
away from the Marianas and far western Micronesia, but should stay
around 2 feet with no hazards expected at this time.

&&

.Western Micronesia...
The monsoon surge has significantly weakened over far western
Micronesia as Tropical Storm Pulasan continues to move northwest over
the Philippine Sea, away from the region. Surface observations reveal
gentle to moderate southerly flow with frequent gusts to near 20 kts
at Palau, and gentle southwesterly winds at Yap with occasional fresh
gusts. Satellite imagery shows bands of thunderstorms and numerous
showers over the region, south of the monsoon trough and TS Pulasan.
Thunderstorms are seen between Palau and Yap, stretching further to
the west and northwest within the broad area of convection that has
persisted over nearly the last 24 hours, enabled by strong divergent
flow aloft and fed by convergent, moist monsoon flow at the surface.
Despite a decrease in sustained winds, this morning`s scatterometry
shows a few areas of stronger winds within the storms, which may
still be capable of producing a few 24-30 kt gusts as they gradually
shift northward overnight. Conditions will see gradual improvement
on Wednesday as the monsoon disturbance moves away, with gentle to
moderate southeasterly flow expected. However, on-and-off scattered
showers remain possible over the next few days, enabled by
convergent trade flow behind the departing disturbance.

Further east, weak trade wind convergence is triggering patchy
showers near Chuuk State, which sits under a mid-level cloud deck
keeping skies mostly cloudy for tonight. Showers increase late in the
week with the arrival of a strong trough riding along the ITCZ.

Altimetry data shows 4 to 6 ft seas to the east of Palau, increasing
to 6 to 8 ft seas directly west of Yap. However, winds have greatly
decreased and are seeing a downtrend as indicated in the surface
observations and scatterometry data, and the Small Craft Advisory has
been allowed to expire at 6 PM ChST this evening. Southwest swell
will diminish over the next few days, making way for a 3 ft east
swell by this weekend.

&&

Marianas: Schank/Stanko
Micronesia: DeCou/Cruz