212
FXPQ50 PGUM 271811
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
411 AM ChST Fri Mar 28 2025

.Marianas Update...
Satellite and radar show partly cloudy skies and isolated showers
moving through the Marianas Waters. Upstream advection and numerical
weather prediction agree that fairly dry weather will remain in place
through the forecast period. The KBDI will likely cross the high fire
danger threshold on Sunday, thankfully the winds should remain below
red flag criteria. The buoys are reading around 4 feet this morning.
The forecast is worded for 4 to 6 feet as they should begin gradually
increasing. The seas are expected to peak Sunday or Monday at 6 to 8
feet, with winds at 15 to 20 knots, so a Small Craft Advisory is not
expected. The GEFSwave shows about a 3% chance of such an advisory,
barely above zero.

&&

.Eastern Micronesia Update...
A couple of areas of increased convergence, one east of Kosrae and
another southeast of Majuro, will continue to be the focus where we
can expect to see the "best" shower coverage, and maybe a stray
thunderstorm. Kosrae is the only location that looks to see any
potential increase of shower coverage in the next 48 hours, as Majuro
is removed from the better lift and moisture. Pohnpei is on the
northern edge of the deeper moisture, but with a surface trough to
your southwest pulling away, there`s a lack of a focus for more
widespread convective development.

&&

.Western Micronesia Update...
The "main area of convection" is located well to the south of Central
Yap State, well to the west-northwest of Kapingamarangi. This will
continue to pass harmlessly west over the next 24 to 36 hours, not
affecting any land masses. Here too moisture depth isn`t overly
impressive, with the best "quality" of moisture at Palau where it`s
near what we`d expect to see this time of year, less than that at Yap
and Chuuk. given a dearth of lift, aside from near the trough
mentioned above, look for just isolated showers at Yap and Chuuk,
scattered at Palau, during the next day or so.

&&

.Prev discussion... /issued 445 PM ChST Thu Mar 27 2025/

Marianas Synopsis...
Himawari visible satellite shows patchy showers and cloud cover over
Tinian and Saipan coastal waters this afternoon, likely associated
with weak trade-wind convergence as evidenced by the latest
scatterometry data. Surface observations show mostly moderate trade
winds with occasional fresh gusts on Guam, with slightly higher
winds observed to the north near Saipan. Conditions remain mostly
clear over Guam and Rota.

Discussion...
CIMSS MIMIC Precipitable Water (PW) imagery reveals a north-south
moisture gradient across the Marianas this afternoon. PW values
range between 1.5 to 1.7 inches near Tinian and Saipan, where
converging trade winds are producing isolated showers and mostly
cloudy skies across the area. To the south over Guam and Rota, PW
values are around 1.0 to 1.3 inches and skies are mostly clear.
Overall, these ranges of values indicate fairly dry conditions for
the tropics, and any showers tonight are expected to be brief and
light. Model guidance points to a slight increase in showers after
midnight through early Friday morning. For the rest of the forecast
period, similar such weak, transient features may briefly interrupt
what will otherwise be a mild trade-wind regime. This weekend, trade
winds will see an increase across the region due to a slight
strengthening in the subtropical ridge to the north. This will lead
to a larger trade swell and corresponding surf along north and east
facing reefs, especially Sunday through Monday.
such

Marine/Surf...
Benign marine conditions will continue through the remainder of
the week as generally moderate trades prevail. Nearby fixed buoys,
SOFAR drifting buoys, and altimetry data show combined seas of around
4 to 5 feet. Currently, there is a moderate risk of rip currents for
east facing reefs, and a low rip current risk for all other reefs.
This weekend, trade swell and wind waves will begin to increase in
response to a broad increase in trade winds across the region,
bringing combined seas into the 6 to 8 foot range on Sunday. Surf
along north and east facing reefs will increase in response, bringing
the risk of rip currents into the moderate category for north facing
reefs, and potentially a high risk for east facing reefs. These
conditions will continue into early next week, with swells and surf
expected to subside somewhat come Tuesday.

Eastern Micronesia...
The Intertropical Convergence Zone (ITCZ) that was fairly active
yesterday and started to fragment overnight has remained fragmented
as scatterometer data showed the southeast flow that supported the
low-level convergence has weakened faster than what models were
previously depicting. Satellite imagery this evening shows spotty to
scattered moderate showers lingering around Pohnpei and Kosrae, while
remaining well south of Majuro. However, there is some increased
cloud cover moving across the Date Line and towards Majuro, so added
scattered showers to Majuro`s forecast later tonight. Over the past
couple of hours, Global Lightning Detection (GLD) network has only
detected a few thunderstorms, one just south of Pohnpei`s coastal
waters, one well north of Kosrae, and the last well south of Majuro.
Since thunderstorms are very limited, the mention of isolated
thunderstorms was reduced in the forecasts for all three forecast
points. New GFS and ECMWF model guidance is starting to initialize
better with current satellite imagery, and both continue to show
fairly dry weather for Majuro and the islands/atolls to the north,
while spotty to scattered showers will linger south of 7N across the
Marshall Islands and around Kosrae and Pohnpei, along what remains of
the convergence within the ITCZ and the weak trade-wind troughs
moving through the region. Over the next several days, models do not
really support the ITCZ redeveloping, but they do depict an upper-
level jet max descending through the sub-tropics to near or just
north of 10N late this weekend and into next week, north of the
Marshall Islands. This would put the right entrance region near the
region, providing some upper-level divergence and helping to trigger
showers along any surface disturbance, such as troughs or
convergence, within the region. Right now, models do not show a
strong surface signature but this pattern will be something to
monitor as it will have an impact on any changes to the forecast for
the next few days.

According to buoy and satellite altimetry, combined seas have
decreased slightly to around 4 to 5 feet, with some peaks up to 6
feet across the region as the east swell has decreased about a foot.
With the lack of heavy convection, the risk of gusty winds and choppy
seas are now very low, making marine conditions fairly favorable as
winds are fairly moderate (around 15 kt) across the region. No major
changes in sea height or swell are expected through the rest of the
week, with models showing a weak north swell entering the region
early next week. There will be a new moon around Saturday, so high
tide will be higher than usual for a few days before and after the
new moon, but the risk of coastal inundation will be limited and
minor due to the lack of high surf, seas, and swells.

Western Micronesia...
The Near-Equatorial Trough (NET) and dissipating circulation (Invest
96W) continue to shift towards the Philippines. Trade convergence
along its northern and eastern flank will also shift southwest.
After scattered showers this evening, Yap looks to stay well north
of the convergence zone and will likely see mainly isolated trade
showers through the weekend into next week. For Palau, showers look
to remain low-end with isolated thunderstorms due to proximity to the
convergence zone and a better moisture profile. Shower potential
increases for Palau by later this weekend into next week as the NET
begins to redevelop to the southwest and a possible disturbance
develops within the NET. For now, cap Probability of Precipitation at
50 percent for next Tuesday and Wednesday based off of forecast
uncertainty this far out. For Chuuk, periods of low-end scattered
showers and isolated thunderstorms possible through the period as the
trade convergence south of the lagoon fluctuates north at times.

For Yap and Palau, combined seas of 4 to 5 feet will climb 1 to 2
feet this weekend with an increase in trade swell. Moderate to fresh
winds expected through the weekend with a slight decrease next week,
especially for Yap, as the NET re-emerges and creeps north. For
Chuuk, expect seas 5 to 7 feet and moderate to fresh trades for the
period.

&&

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

$$

Marianas Update: Stanko
East/West Micronesia Update: Doll
Marianas: DeCou
East Micronesia: Schank
West Micronesia: Slagle