973
FXPQ50 PGUM 292000
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
600 AM ChST Wed Jul 30 2025

.Marianas Update...
Little change has been made to the forecast. While the winds have
become light and variable, as well as the atmosphere being
destabilized enough to allow potential island convection to develop
during peak heating hours this afternoon, cloud cover from nearby
cells stratifying is likely to keep Guam and Saipan clear of any
sporadic convection today. As such, mostly cloudy skies and isolated
showers are expected today, followed by a minor trough gradually
pushing in from the southeast, allowing low-end scattered showers to
develop overnight. In addition, the marine forecast remains
unchanged; a moderate risk of rip currents is expected along all
reefs, as a dominant westerly swell and secondary easterly swell
continue to produce elevated surf along all reefs. Rip current risk
is likely to drop to low along east and south facing reefs by the
weekend, once the westerly swell weakens enough to minimize wrap-
around effects across the Marianas.

&&

.Eastern Micronesia Update...
Pohnpei will have scattered showers for the next couple of days,
thanks to a trough moving slowly through Eastern Micronesia. Overall,
this is a slight increase in the PoP`s from this morning`s forecast,
except for today, which is a slight decrease. Some east winds at 5
knots also became variable. Little change for Pohnpei that is of any
real substance, mainly cosmetic.

Based on satellite and the many synoptic alerts from Kosrae, worded
today numerous becoming isolated. After this, it should be isolated
through Thursday, then trends back to scattered Thursday night. This
is an increase in PoP for Thursday night.

Majuro has a fair amount of speed convergence in its trade-wind flow,
based on satellite went with numerous showers developing, meaning
they`re not there yet, but they likely will be, probably by noon. The
numerous showers persist tonight then become high end scattered for
Thursday and Thursday night. This is an increase in PoP`s for the
Majuro Waters.

Trade winds will rule Eastern Micronesia, with all winds being some
flavor of east. Majuro will be capped at 15 knots on Thursday and
Thursday night, 10 knots other times, and at 10 knots on other days.
Pohnpei and Kosrae will be capped at 10 knots through the period.
Seas will be capped at 6 feet for Majuro, 5 feet for Kosrae, and 4
feet for Pohnpei. These are all benign marine conditions, so just
watch for the isolated thunderstorms that can be found enroute if you
are engaging in marine travel.

&&

.Western Micronesia Update...
There is a trough just west of Chuuk, otherwise, Koror Palau and Yap
are continuing to be dominated by a ridge that has built in south of
the monsoon trough.

Chuuk will have westerly winds of 5 to 10 knots under the influence
of a possible circulation a bit to their northwest. The proximity of
the trough and circulation will contribute to scattered showers today
becoming numerous tonight and Thursday, then widespread Thursday
night. This is an increase in PoP`s tonight and Thursday night. For
marine travel, Chuuk`s winds through the weekend will be capped at 15
knots Friday, 10 knots for the rest of the period today through
Sunday, and seas will be capped at 5 feet, which is benign.

Yap will have south to southeast winds of 5 to 10 knots through
Friday, which will then turn east then north over the weekend.
Showers could become scattered Thursday, but will mainly be isolated
through Friday, then could become scattered again for the weekend.
For marine travel, Yap`s winds will be capped at 10 knots tonight,
and 5 knots the rest of the period, and seas capped at 4 feet through
Thursday, then 3 feet through Sunday. This is a slight increase of
PoP on Thursday.

Koror Palau will have southeast winds of 5 to 10 knots which could
become variable Sunday. Koror Palau will have isolated showers
through Saturday, which could become scattered Sunday, which is
unchanged from the previous forecast. For marine travel, winds will
be capped at 10 knots, and seas will be capped at 3 feet through
Friday, then at 2 feet through the weekend.

Seas are getting really low in Western Micronesia. The Tabular
guidance is set to auto delete if there is no swell of 3 feet or
greater in the entire grid, and the grids only extend through August
5th (they would normally go through August 8th). Could be interesting
to watch.

&&

.Prev discussion... /issued 557 PM ChST Tue Jul 29 2025/

Marianas Synopsis...
Satellite and radar imagery show light isolated showers drifting over
Marianas coastal waters. Widespread cloudiness is seen in the mid
and upper levels across much of the region, leftover from moderate to
deep convection to the north and southeast of the Marianas.
Altimetry data and Ritidian buoy data indicate combined seas between
6 to 8 feet with a dominant west-northwest swell.

Discussion...
Conditions will remain quiet through Wednesday as a broad ridge,
extending southwest into the region from near 25N163E to around
3N130E over the Republic of Palau, builds slightly westward over the
islands. Meanwhile, a southwest-to-northeast oriented trough located
to the northwest of Chuuk, looks to gradually push northwest into the
area, increasing showers near Guam around Wednesday night. Gentle to
moderate winds will gradually diminish through midweek as the ridge
builds westward, with periods of light and variable winds expected
from Wednesday night through Friday. A consideration for island
effect showers was included in the forecast for Thursday afternoon as
deterministic and ensemble model guidance indicate light and
generally southwesterly flow, becoming variable at times. There
looks to be a slight increase in moisture over the next few days with
precipitable water amounts between roughly 2.2 and 2.5 inches,
enough to support island convection given sufficient daytime heating
and a light wind field oriented roughly parallel with the islands.
Winds may begin to shift and increase Friday with increasing shower
chances, as models support a weak trough or disturbance developing
north of Chuuk and shifting west across the Marianas through
Saturday.

Marine...
Gentle to moderate southwesterlies will continue across the Marianas
tonight, tapering down to light to gentle Wednesday as a ridge
builds further west into the area. Buoy and altimetry data indicate
combined seas of 6 to 8 feet in the coastal waters, which are
expected to relax over the next few days as the dominant west-
northwest swell and wind waves diminish.

Eastern Micronesia...
A surface trough in the trades that was approaching Pohnpei yesterday
has lifted north of there and is mainly providing just some mid to
high level cloud cover. Meanwhile, another more active surface
trough stretches from east of Kosrae to Kwajalein. This trough is
continuing to foster scattered shower and isolated thunderstorm
development, with showers becoming numerous in coverage from time to
time, with this activity stretching the length of the trough axis and
behind it to include Majuro and portions of the FSM.

Further upstream towards the Date Line and beyond, nothing
significant is occurring convectively speaking, with just subtle
troughs and associated areas of somewhat increased convergence seen
in the streamline analysis. Taking a look at the Galvez-Davison
Index (typically a good indicator of at least scattered showers when
above 50), "waves of moisture" are seen in the numerical fields,
allowing the GDI to spike to 50 or above from time to time, dropping
below 30 in between. This suggests that shower coverage will
oscillate back and forth between isolated and scattered coverage over
much of the next week, which looks reasonable when looking at
upstream satellite and streamline analysis.

Light to gentle winds will prevail across the region through the
weekend, with seas remaining in the 3 to 5 foot range.

Western Micronesia...
Satellite analysis shows isolated showers and partly cloudy skies
across Palau, Yap, and Weno with several weather systems seen across
Yap and Chuuk States. Altimetry and buoy data show combined seas
between 2 to 5 feet across the region with seas greater than 7 feet
seen just north of Yap State.

The lull in active weather continues for Palau and Yap as they are
both under the influence of low level divergence. Additionally,
upper level convergence is seen across Palau and Yap and is
forecasted to continue through much of the week. The process of
convergence aloft and divergence at the surface leads to sinking air.
Thunderstorms were removed from the forecasts at Palau and Yap
through Wednesday night, coinciding with the period of sinking air.
Guidance shows convergent surface flow returning late this week,
which would bring the return of scattered showers and isolated
thunderstorms to Yap and Palau.

Chuuk remains in a general convergent surface flow, but there are
some weak surface circulations embedded within the surface trough
just north of Weno. Short periods of surface divergence can occur
between these circulations, leading to lower shower activity at
times. Both the GFS and ECMWF show an increase in surface
convergence for Chuuk the next few days, leading to numerous showers
and isolated thunderstorms Thursday and Thursday night. The active
pattern looks to trend downward late this week into the weekend as a
trade-wind pattern returns.

No changes were made to the marine forecast. Benign seas of 2 to 5
feet continue across the region. The westerly to northwesterly swell
will continue through the weekend before shifting easterly due to a
weak trade-wind swell entering the region.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.

MP...None.

Marianas Waters...None.
&&

$$

Marianas Update: Montvila
East/West Micronesia Update: Stanko
Marianas: DeCou
East Micronesia: Doll
West Micronesia: Williams