619
FXPQ50 PGUM 131933
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
533 AM ChST Thu Aug 14 2025

.Marianas Update...
Satellite imagery this morning shows mostly clear to partly cloudy
skies over the islands, with most of the showers and thunderstorm
currently staying east coastal waters, along an area of weak
vorticity at 850 mb, according the the CIMSS 850mb vorticity product.
Hi-res models continue the potential for island effect convection
along western shores and adjacent waters today and potentially
Friday, especially for Guam, due to light winds and sunshine during
the morning and early afternoon hours heating up the island.
Otherwise, a dry pattern pattern favored for the region into the
weekend. Looking towards early next week, models show a trough
passing south of Guam, which may bring showers to Guam, with a lower
potential for Rota northward.

Light to gentle winds are expected to become gentle to moderate this
weekend. Combined seas of 2 to 3 feet are expected through the
workweek and then become 3 to 4 feet this weekend. The potential for
isolated thunderstorms is expected to continue through today for the
Marianas, with a slight chance of thunderstorms Friday night and into
the weekend.

&&

.Eastern Micronesia Update...
A fairly dry pattern continues to extend across most of the region,
with some spotty showers east of Kosrae and Majuro, along some weak
troughs passing through the region, but the precipitable water (PWAT)
content with these troughs is barely reaching above 2.0 inches on the
TPW and CIMSS MIMIC satellite products, so not expecting showers to
increase much at Majuro or Kosrae today, even if these troughs pass
near the islands. The best potential for rainfall is still not
expected until Friday or the weekend, when models show that the ITCZ
(Intertropical Convergence Zone) will attempt to build fairly rapidly
westward across the region. Timing and location continue to be the
main forecast challenge, but made no changes to the previous forecast
for this time period.

Light to gentle winds will increase into the gentle to moderate range
at times, especially for the weekend and into early next week. Seas
look to mainly hold in the 3 to 5 foot range.

&&

.Western Micronesia Update...
Besides for a trough moving through western Chuuk State and into
eastern Yap State, and weak convergent flow generating some scattered
showers and isolated thunderstorms north of Palau waters, showers are
isolated across Palau, Yap and Chuuk this morning as most of the
notable weather features have moved away from the islands. A gentle
and dry trade wind weather will continue for Chuuk until the weekend,
when the ITCZ is expected to build across eastern Micronesia and
start to push towards Chuuk, increasing the potential for showers and
increasing winds slightly.

For Palau and Yap, a light south to southwest flow will slowly become
easterly over the next few days. The weather pattern is expected to
also be on the drier side, but brief periods of scattered showers
cannot be ruled out, especially for Palau, due to the influence of
convergent south to southwest flow.

Seas continue to remain fairly benign at 2 to 4 feet across the
region with light to gentle winds. Palau may see a brief period of
southwest swell Friday and Friday night, but this swell is expected
to be weak with a short period, so not expecting any impacts. Winds
may increase slightly this weekend, especially around Chuuk as the
trade winds strengthen.

&&

.Prev discussion... /issued 552 PM ChST Wed Aug 13 2025/

Marianas Synopsis...
Radar shows a speckling of isolated showers over the Marianas.
Satellite imagery shows partly cloudy skies. Both the buoys and
altimetry show 2 to 3 foot seas. Winds have been light to gentle
today. Overall a fairly pleasant day.

Discussion...
Overall pleasant weather is expected for the Marianas through the
weekend. With the light to gentle winds that are expected over the
next few days, island convection remains a distinct possibility.

Marine...
Benign seas of 2 to 3 feet with occasional 4 foot are expected
through the rest of the week. Near the end of the weekend the trade
swell may strengthen bring the seas up a little. Winds are expected
to remain light to gentle. Surf remains below 5 feet on all reefs,
this will allow the risk of rip currents to be low. A moderate risk
of lightning remains due to island convection.

Eastern Micronesia...
The weather remains quiet and will continue to be that way for the
next 24 to 36 hours. We thought about bringing a brief period of
scattered showers to Majuro with a surface trough currently crossing
the Date Line. However, Infrared satellite imagery shows rapid
warming of cloud tops, MIMIC water vapor imagery shows a narrow
precipitable water surge, and visible imagery suggests the cloud
cover is mainly mid-level instability with lots of altocumulus. For
now, will defer to the swing shift to monitor for any change in
forecaster thinking and analysis regarding any potential brief
increase tonight or tomorrow.

Otherwise, isolated showers will continue for another day or two,
with perhaps a thunderstorm mixed in. Then, an ITCZ (Intertropical
Convergence Zone) will attempt to build fairly rapidly westward
across the region by the weekend. Upstream satellite analysis shows
some support of this. Thus, rainfall potential looks to definitely
increase, with timing and location being the main forecast challenge.
For now, we went with a consensus (blend of models) approach in the
precip amounts, and leaned more heavily on the GFS as it had a better
representation of what`s currently occurring.

Light to gentle winds will increase into the gentle to moderate range
at times, especially for the weekend and into early next week. Seas
look to mainly hold in the 3 to 5 foot range. There was less support
for 6 foot waves at Pohnpei, and somewhat less at Majuro, for the
weekend into early next week seen in today`s model runs versus
yesterday.

Western Micronesia...
The main weather feature in Western Micronesia is a trade-wind trough
that is currently just west of Weno Atoll or Chuuk Lagoon. Since the
trough is already past, Chuuk will start with isolated showers
through Friday night. An ITCZ building in from Eastern Micronesia
will ramp up the rainfall chances to scattered for Saturday through
Monday, while providing moderate winds as opposed to light to gentle
as we expected to have through Friday night. For the marine forecast,
winds will top out at 10 knots through Friday night, then at 15 knots
through Monday. Seas of 3 to 4 feet will build to between 3 and 5
feet on Saturday. All of these values are counted as benign.

For Yap, they will remain near the border of the trades and the west
winds, which while not a monsoon yet, could try to build up to that
in the far extended forecast. So winds will tend to be light and
variable here, slightly favoring east. Isolated showers will dominate
through Friday thanks to a ridge, then scattered will encroach as the
west winds approach and begin to try to build up to a monsoon flow
pattern. For the marine zones, winds will be capped at 10 knots with
the exception of a possible 15 knots on Sunday. Seas will be 3 feet
through Saturday, then 3 to 4 feet for the new week, all of which is
benign.

For Koror, Palau, the monsoon trough tracks south of them, however it
is on the way down. It could disappear entirely from our area
Saturday and Sunday, then reappear next week. Thanks to the same
ridge affecting Yap right now, showers will be isolated through
Thursday night, then scattered Friday through Monday, with possibly a
brief break Saturday as the monsoon trough departs briefly. Winds
will be capped at 10 knots the entire forecast period, and seas will
be 2 to 4 feet, the 4 foot values possible Friday and Friday night as
a southwest monsoon swell. All these values are also benign.

&&

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

$$

Marianas/East/West Micronesia Update: Schank
Marianas: Bowsher
East Micronesia: Doll
West Micronesia: Stanko