407
FXPQ50 PGUM 081919
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
519 AM ChST Wed Jul 9 2025

.Marianas Update...
Satellite and radar imagery showed moderate to heavy showers
developing across the region overnight due to a shift in a south to
southeasterly convergence zone. Through this afternoon, showers may
be locally heavy at times with gusts reaching up to 25 mph. This was
the main change to the forecast, along with bumping up showers to
numerous for Tinian and Saipan this morning. As this convergence zone
shifts back to northwest, conditions are expected to settle back
down to isolated late tonight.

Buoy and altimetry data reveal seas remain between 2 to 3 feet,
comprised of minor trade swell.

&&

.Eastern Micronesia Update...
Only slight changes were made to the forecast. A slightly more active
trade-wind regime is expected to bring occasional upticks in showers
and thunderstorms across all three forecast zones for much of the
week. In addition, some of these showers are expected to be heavy at
times, especially today at Pohnpei and Majuro, and overnight at
Kosrae. A leading edge of a trade-wind surge is carrying fresh to
strong winds, with occasional gale-force gusts near collapsing
outflow boundaries just upstream from Majuro; as such, a Small Craft
Advisory has been issued for Majuro today. Once this feature passes
Majuro to the west and weakens, winds are likely to fall below levels
hazardous to small craft tonight. Other than that, the marine
forecast remains unchanged for all three locations.

&&

.Western Micronesia Update...
The tail end of a long convergence zone spanning southward from the
Marianas across western Micronesia, is building just upstream of
Palau and moving in quickly. This is expected to produce higher-end
scattered showers and some thunderstorms today, with said
thunderstorms likely to increase during the afternoon hours as the
local atmosphere destabilizes further and builds even more overnight.
Even with this happening in close proximity, Yap is expected to only
observe patchy showers through tonight, with an isolated
thunderstorm or two to develop sometime in the evening. Chuuk is
still expected to see low-end scattered showers increase over the
next few days, as the leading edge of a trade-wind surge in eastern
Micronesia makes it to Weno by Thursday night. In the meantime, the
marine forecast remains unchanged for all three locations.

&&

.Prev discussion... /issued 631 PM ChST Tue Jul 8 2025/

Marianas Synopsis...
Over the Marianas, partly cloudy skies reign over Guam and Rota
while mostly cloudy skies reign over Tinian and Saipan. Isolated
showers are seen on radar over the region. Winds have been light to
moderate today with a few fresh gusts.

Discussion...
There is a weak disturbance near 23N139E that is stirring up
convection over the Marianas as it pulls the winds and moisture to
its central low. As this disturbance pulls further away, so to will
will the showers diminish. Showers are expected to peak tomorrow as
the winds will be light enough to allow island enhancement to take
place. The feature is expected to be far enough away to have little
effect on the Marianas by Wednesday night. For the rest of the
forecast periods a drier wet season pattern is expected to take hold
with scattered showers during the day and isolated during the night.

Marine...
Altimetry shows combined seas of 1 to 3 feet and both buoys show 2 to
3 feet. Seas are expected to remain fairly docile at 2 to 4 feet
throughout the forecast period. Surf and the risk of rip currents are
expected to remain low for much of the forecast period also, with the
caveat of a potential rise over the weekend as a distant disturbance
may produce a swell that would bring the north and west facing reefs
up a bit.

Eastern Micronesia...
A fairly active pattern will persist across eastern Micronesia
through the period. A more noted trough along the leading edge of
the ITCZ which helped bring numerous showers and isolated
thunderstorms for Majuro today is pushing west. As the trough pushes
away from Majuro and the ITCZ weakens, expect a downward trend of
showers tonight, with mainly low-end scattered trade showers for the
rest of the week. The ITCZ looks to build back into the Marshall
Islands over the weekend, with showers and thunderstorm potential
increasing.

A weak trough that brought scattered showers and isolated
thunderstorms for Kosrae today looks push into Pohnpei tonight and
bring scattered showers overnight. This trough and the trough pushing
away from the Marshalls will pass through Kosrae and Pohnpei over the
next 24-36 hours, followed by the emergence of the ITCZ building
into the region. Scattered to numerous, at times heavy, showers and
isolated thunderstorms are expected through the weekend.

Trade flow will increase and winds will become moderate to fresh for
Majuro by Wednesday, and for Kosrae and Pohnpei by Wednesday night.
Winds will slowly decrease by the weekend and become light to gentle
again. Buoy and altimetry passes are showing seas of 2 to 4 feet at
Pohnpei, 3 to 4 feet at Kosrae and 3 to 5 feet at Majuro. Seas will
be on the rise beginning tonight due to an increase in trade winds
and swell, with seas climbing 2 to 3 ft by Thursday and remaining
elevated through the weekend. Surf will rise, especially along east
facing reefs. A High Surf Advisory may be needed for Kosrae beginning
Thursday as surf along east facing reefs may reach 8 ft or higher.

Western Micronesia...
Western Micronesia will be the stage where a grand drama plays out
over the next couple of weeks, and perhaps longer. It will be a
struggle between opposing forces of nature, both gigantic in size.
Thankfully, they are also spread over large areas of the earth, so in
any one area the power won`t overwhelm everything. It will keep the
background weather quite active in Western Micronesia though, as the
trade winds and the southwest monsoon grapple for supremacy. In the
first part of the period, the southwest monsoon has taken over the
Republic of Palau and much of Yap State. The trade winds remain
entrenched in eastern Yap State and Chuuk State. Starting on
Wednesday night, the ITCZ (Inter-Tropical Convergence Zone) from
Eastern Micronesia could make an appearance near Chuuk, so especially
Thursday night through Saturday, hit the showers hard there.

There will also be much shower activity between Palau and Chuuk, as
the easterlies and the westerlies interact, giving rise to
convergence. Palau and Yap are fairly dry for tonight as the showers
are either hanging south of them or being drawn to the north of the
tropical disturbance northwest of the Marianas. The current forecast
has the showers returning Wednesday, although that could be
premature, it could be later than that. It shouldn`t be too much
later though, at some point the northern branch of the monsoon trough
will weaken and allow it to redevelop in the south near Micronesia.
The rather low key Canadian Ensemble has the monsoon trough
stretching to south of Chuuk, but gradually losing ground over the
next couple of weeks, until Koror Palau and Yap are nearly on the
front lines. The Global Ensemble Forecast System is rooting for the
other team, and in a monsoon surge nearly sweeps the trade winds
completely out of Western Micronesia, with the monsoon flow reaching
Chuuk and even the Marianas during the last third of July.

They can`t both be correct, but just to further muddy the waters and
inject a little more salt in the wound, the ECMWF-HiRes paints a
middle of the road picture through the middle of the month at least.
It has the monsoon trough losing some geographical ground like the
Canadian Ensemble, but after the middle of the month invokes a fairly
beefy surge like the GEFS. Unfortunately the output ends after day
10. It could be quite enlightening to see where it eventually cast
its vote. So, the key take away is, Western Micronesia is in a rather
active pattern (except for the three main forecast sites!) tonight,
and it`s pretty likely to stay that way. The models are not yet
pinging on any tropical cyclone development, we will need to keep an
eye on that, especially if we get a fresh to strong monsoon surge
though.

In the meantime, once the showers move back into the 3 main
locations, it should stay showery for most of the rest of the week.
Winds at Chuuk will be mostly east at 5 to 10 kt, although they could
become southeast 10 to 15 kt as a trough passes at the height of the
showers on Thursday night. Seas will be 3 to 5 feet through the
period at Chuuk. Winds at Yap will be south to southwest at 5 to 10
knots. Seas will be 3 to 4 feet. Winds at Koror, Palau will be west
to southwest at 5 to 10 knots. Seas at Koror, Palau will be mostly 2
to 4 feet, although there could be occasional 5 foot values Thursday
and Thursday night.

&&

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

$$

Marianas Update: Cruz
East/West Micronesia Update: Montvila
Marianas: Bowsher
East Micronesia: Slagle
West Micronesia: Stanko