209
FXPQ50 PGUM 311944
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
544 AM ChST Fri Aug 1 2025

.Marianas Update...
The trough mentioned in the previous discussion has arrived a little
sooner and more convectively active, producing numerous showers and
isolated thunderstorms in the waters, which will trickle across the
islands near noon today. Some of these showers may be heavy at times
and carry gusty winds. Otherwise, the weather and marine forecasts
remain unchanged.

&&

.Eastern Micronesia Update...
As mentioned in the previous forecast, showers and thunderstorms are
mainly concentrated around trade-wind troughs passing through the
Intertropical Convergence Zone (ITCZ) this morning, which are
currently located near Kosrae and Majuro. Pohnpei starting out this
morning fairly dry so the forecast was updated to delay showers and
thunderstorms to the afternoon. Also delayed numerous showers at
Majuro to this afternoon as the trough that moved through Majuro
overnight is pushing off to the west and showers have remained
scattered for the past few hours. Models guidance still shows showers
rebuilding in coverage this afternoon as the trade wind trough and
area of convergence between Majuro and the Date Line pushes into the
Marshall Islands. This pattern will continue into the weekend, then
the ITCZ will weaken, allowing drier air to move into the region and
lower the potential for showers early next week.

Buoy data at Pohnpei and Kosrae show a mixed east swell, driven by
the weak trade-wind pattern, and a weakening northwest swell that was
triggered by the monsoon flow and distant TC Krosa well northwest of
the region. Given the fairly gentle winds and swell, seas are
expected to remain around 3 to 5 feet through the weekend and into
next week. Continuing to monitor the southeast to south well that
was generated by American Samoa earlier in the week, but models
continue to support most of the energy remaining east of the Date
Line, so not expecting a major change in seas or surf by the middle
of next week.

&&

.Western Micronesia Update...
Scatterometer data still shows a ridge-like wind pattern over Palau
and Yap, but with the high amount of upper-level clouds due to a
weak upper-level trough passing nearby it looks like the ridge is
starting to erode. The increase upper-level cloud cover will help to
limit island-effect showers that sometimes occur when winds are
light, but increased instability aloft could trigger some isolated
showers and thunderstorms over the next few days. As the ridge
erodes, shower chances will increase by early next week as trade-wind
troughs are able to push back into Palau and Yap.

A broad surface trough extends between the Marianas and Chuuk,
generating scattered showers and isolated thunderstorms, mainly near
the Marianas and Chuuk Lagoon, where convergent flow along the outer
edge of the trough is the greatest. Scatterometer data shows weak
circulation potentially embedded within the trough near 11N149E but
this signature is weak and difficult to confirm with infrared
satellite due to the cirrus deck over the trough. These features look
to maintain at least scattered showers and isolated thunderstorms
near Chuuk over the next day or two, gradually pushing northwest
toward the Marianas into the weekend with little change in strength
or organization.

Benign marine conditions continue across the region. Babeldaob buoy
data and recent altimetry data indicate combined seas of around 2 to
3 feet near Palau, 2 to 4 feet near Yap, and 3 to 5 feet further east
near Chuuk. All three islands are still seeing a mixed swell pattern
of a weakening north-northwest swell from distant monsoonal and
tropical influences, and the weak background trade swell. By early
next week the north-northwest swell will subside and the trade swell
will be the primary swell across the region.

&&

.Prev discussion... /issued 421 PM ChST Thu Jul 31 2025/

Marianas Synopsis...
Satellite and radar imagery is showing modest cumulus buildups with
a band of thick cirrus pushing over the islands, along with very
spotty showers in the waters. Latest altimetry and nearby buoys are
indicating seas of 4 to 6 feet.

Discussion...
A thick band of cirrus, along with low to mid level subsidence, has
limited the heating and destabilization needed for island-effect
showers to develop this afternoon, even though the morning sounding
indicated plenty of low-level moisture (PWAT 2.32") and a high K
index. Only modest cu buildups along and west of the islands has been
observed. May see a isolated to low-end scattered island-effect
showers develop tomorrow if enough clearing and heating can occur as
winds will be light again. Models continue to show a surface trough
to our southeast with an embedded broad and weak circulation
approaching the region tomorrow. Expect low-end scattered showers
and isolated thunderstorms beginning Friday night and continuing
through the weekend as the trough and circulation slowly pass across
the islands. The elevated potential for showers and thunderstorms
will continue next week with moist, converging south to southeast
flow over the region along the backside of the departing low.

Marine/Surf...
Generally light winds are expected through Friday with a ridging
pattern over the region. Winds will increase slowly and become gentle
to at times moderate over the weekend. Combined seas of 4 to 6 feet,
comprised mainly of a lingering west-northwest swell, will continue
through the weekend, then decrease next week as said swell slowly
subsides.

A moderate risk of rip currents will continue along west and north
facing reefs as the elevated west-northwest swell persists. Rip
current risk is likely to drop to low along east and south facing
reefs by Friday as the aforementioned swell and associated wrap-
around effects diminish just enough to lower surf below the moderate
5-foot threshold.

Eastern Micronesia...
A trio of troughs is found across the region today. The first trough
is exiting trough Kosrae, with little convective development. The
next trough has a stronger convergence signature with it and widely
scattered showers with isolated thunderstorms have started to develop
recently. The third trough is crossing the Date Line and has the
"best" convergence with it and convective coverage.

This sets the theme for the next few days across all of Eastern
Micronesia through Saturday, where mainly scattered showers (locally
higher for Majuro) and isolated thunderstorms will persist. Then, the
ITCZ weakens with drier air moving into Majuro and Kosrae by Sunday
night or Monday, waiting until just after Monday at Pohnpei.

Mainly light to gentle winds will persist across the area. A
northwesterly swell mixes with an easterly trade swell at Majuro
through Friday night, so this area was "broken out" into two distinct
swells mentioned in the forecast. At Kosrae and Majuro, just a
predominate east swell is present and will remain. Swell heights will
remain in the 3 to 5 foot range.

Western Micronesia...
Satellite this afternoon shows a fairly messy pattern across western
Micronesia. A broad low-level ridge extends southwest into the
region from over the Marianas, passing over Yap and the Republic of
Palau. Skies have been mostly clear over Palau through mid-day,
which, in combination with light to gentle east-southeast winds,
allowed for some island showers to form along the western sides of
Babeldaob and Koror. To the northeast, moderate to deep convection
has been present near Yap Proper since the early morning, moving
west-southwest through the early afternoon. These showers and
thunderstorms have been maintained by very strong, divergent
easterlies aloft, but models have not picked up on them well. This
convection looks to continue moving away from Yap and dissipate
through the evening hours, with just isolated showers expected for
Palau and Yap overnight. The ridging pattern across the region looks
to persist through the weekend before shifting away to the north
early next week.

To the east, a broad east-to-west oriented trough persists to the
north of Chuuk, having been semi-stationary over the last day or two
with the ridge in place just further north. Embedded within this
trough, northwest of Chuuk Lagoon, a very weak circulation can be
seen in the visible satellite loop this afternoon and in the latest
scatterometry analysis. This is co-located with an area of enhanced
850-700 mb vorticity. Scattered to numerous showers are seen over Yap
State to the south of this weak circulation, including over Eauripik,
Ifalik, and Satawal. Scattered showers are seen east of the
circulation, to the north of Weno, Chuuk. These features look to
maintain at least scattered showers near Chuuk over the next day or
two, gradually pushing northwest toward the Marianas into the
weekend with little change in strength or organization.

Benign marine conditions continue across the region. Babeldaob buoy
data and recent altimetry data indicate combined seas of around 2 to
3 feet near Palau, 2 to 4 feet near Yap, and 3 to 5 feet further east
near Chuuk. For Palau, this is comprised of mainly an east trade
swell, while Yap continues to feel a primary west to northwest swell
steered by distant tropical and monsoon influences. Chuuk is also
experiencing a primary west swell, but this looks to be gradually
replaced by a primary east swell by early next week as the westerly
swell subsides.

&&

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

$$

Marianas: Montvila
East/West Micronesia: Schank