132
FXPQ50 PGUM 101812
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
412 AM ChST Tue Mar 11 2025

.Marianas Update...
The main change this forecast update was to go ahead and hoist a High
Surf Advisory for north-facing reefs, and a High Rip Risk for north
and east-facing reefs. The combination of a significant northerly
swell, and a northeasterly trade swell will cause seas, surf, and rip
risks to increase.

Currently, we expect north-facing reefs to become hazardous for
surfing Wednesday, when 7 to 9 foot surf is expected. This looks to
increase further Wednesday night into Thursday morning, when surf
heights peak at 10 or perhaps 11 feet. As the seas and surf
increase, a High Rip Risk also develops for north and east-facing
reefs.

There remains uncertainty with regard to how long a shear line
remains in the area, which is responsible for the increase in seas,
surf, and rip risk. The GFS wants to weaken and fragment the shear
line that stalls near or just north of Saipan by Thursday afternoon,
lifting it back north. If this was to occur, seas, surf, and rip
risks would subside. However, the ECMWF (European) model wants to
maintain shear line integrity, letting it hang out near Saipan
through Saturday before it weakens and retreats north-northwest.

Given the uncertainty, it`s possible that additional headlines for
small craft may be needed, and for the current headlines to be
extended. Hopefully, better model agreement will develop with later
model runs, which is anticipated. Until then, we wanted to hoist the
headlines a "tad early" to give more time to "get the word out", as
drowning due to getting caught in rip currents is an unfortunate
possibility the next few days.

Precipitation-wise, the zones split was maintain in the forecast to
allow for higher rainfall potential, as scattered showers look to
develop for Saipan and Tinian by Wednesday afternoon, with isolated
showers for Guam and Rota.

&&

.Eastern Micronesia Update...
Little to no changes were made to the zone forecast. The dry trade-
wind pattern looks to continue for the next day or two before a
noticeable uptick in showers and thunderstorms will be possible
later this week.

The Small Craft Advisory was extended another 12 hours for Majuro.
The latest altimetry date showed combined seas of 7 to 9 feet west
of Majuro and 8 to 11 feet east of Majuro. The forecast continued
seas between 7 to 10 feet as a result. Guidance continues to show
seas fall below 10 feet later today.

&&

.Western Micronesia Update...

Only minor changes were made to the forecast. Dry conditions
continue across western Micronesia. Shower activity looks to
increase late this week into next week as trade-wind convergence
moves into the region.

No changes were made to the Marine forecast. A northerly swell looks
to impact the region late this week. Combined seas could approach
conditions hazardous to small craft for Chuuk. That said, even if
seas remain below 10 feet, inter-island travel could become
difficult at times across Chuuk later this week.

&&

.Prev discussion... /issued 539 PM ChST Mon Mar 10 2025/

Marianas Synopsis...
A dry trade-wind pattern, with just spotty showers, continues across
most of islands. A weak shear line is moving through the northern
Marianas Islands, north of Saipan, and bringing increased cloud cover
and some showers. Buoys and altimetry data are showing seas around 5
to 7 feet this evening.

Discussion...
Guam and Rota are expected to remain fairly dry for the rest of the
week and into the weekend as the current weather pattern is not
expected to change much. The weak shear line that is moving through
the northern Mariana Islands will continue to move southward and may
stall near Saipan later this week. This would increase cloudiness
and the potential rain over Saipan and Tinian Tuesday night or
Wednesday, with breezy conditions Wednesday night and Thursday. The
GFS shows the shear line fragmenting and lifting northwestward
Thursday night, while the ECMWF shows the shear line lingering near
Saipan into Saturday before falling apart and lifting northwest. If
the GFS is correct, then winds, showers, and cloud cover will
decrease across Saipan and Tinian Thursday night, otherwise, breezy
conditions, clouds and showers would linger across Saipan and Tinian
into Saturday. For now, the forecast more closely follows the GFS
because the shear line currently looks weak and somewhat fragmented
on satellite imagery.

Marine...
Moderate trades will prevail the next few days, becoming moderate to
fresh for the latter half of this week, with some stronger gusts at
Tinian and Saipan as a shear line stalls near or just north of
Saipan. Buoys and altimetry data are showing seas around 5 to 7 feet
this evening, and will increase over the next few days as a stronger
northerly swell enters the region around Wednesday. Winds and seas
may become hazardous to small craft across Saipan and Tinian waters
around Thursday. Right now models keep winds below 22 kt and seas
below 10 feet at Guam and Rota, but any southern shift in the shear
line or if north swell comes in stronger than expected, would cause
hazardous marine conditions to develop. Surf is likely to become
hazardous, 9 feet or greater, along north facing reefs, with a high
risk of rip currents as the north swell comes in on Wednesday. Swell
wrapping to adjacent reefs may also generate a high risk of rip
currents along east facing reefs, while causing the risk of rip
currents to become moderate along west facing reefs.

Eastern Micronesia...
A typical trade-wind pattern persists across the region with
relatively shallow clouds and showers pushing across the area along
moderate to fresh ENE trade winds. The next couple of days should see
little change, but convection will largely start to increase between
4N and 8N as the ITCZ starts to assert itself. Have largely broad-
brushed the ITCZ axis as a middle ground of the models as the GFS
depicts a very narrow band of showers compared to the ECMWF`s
looser-organized convection. Mid to long-range forecast POPs show an
increase from prior forecasts. Winds, though decreasing just a small
amount from the trade-wind surge that has dominated the past few
days, will stay moderate to fresh through the week.

Have extended the Small Craft Advisory (SCA) for Majuro based on
altimetry data still showing seas around 10 ft in the open waters
just north and east of the atoll. Models do show a downward trend, so
have only extended the SCA until 3 AM ChST Tuesday. Even when the
SCA is eventually ended, both winds and seas will still be elevated
across the region so boaters should anticipate increased chop when
navigating outside the protected lagoon waters. At Kosrae, the High
Surf Advisory continues for another couple of days along east facing
reefs.

For all 3 locations, a long-period northerly swell will spread into
the region around Thursday night-Friday. Have made a number of
modifications to the gridded wave data, largely sourcing the ECMWF-
wave for north swell heights and the GFS-wave for period. This
combination results in a 16-17 sec period north swell building into
the area as early as Thursday night with the period steadily
decreasing in the following days. Swell heights could range between 7
and 10 ft, certainly capable of producing hazardous surf exceeding 9
ft along north-facing reefs, but in conjunction with higher high
tides with the near-new moon phase coming up, there will be the
possibility for increased erosion. Latest PacIOOS shoreline impacts
guidance shows the possibility for minor coastal inundation -
something we will be watching closely in the coming days.

Western Micronesia...
Overall, a dry trade-wind pattern continues across the region this
afternoon. Weak troughs will bring periodic upticks in clouds and
showers over the next few days, until trade-wind convergence or an
ITCZ-like fragment develops across the region by late week, generating
unsettled weather across Chuuk and Palau, however, not quite
reaching as far north as Yap.

Buoy and altimetry data reveal combined seas are 5 to 7 feet across
the region, comprised of easterly trade swell and background north
swell. Building north-northeast swell in the latter half of the week
is expected to increase seas to 7 to 9 feet. This, along with
moderate to fresh trade winds over the next several days, would trend
towards choppier seas by late week. Latest wave models do not
indicate hazardous conditions for Yap and Palau, but sea heights near
Chuuk may be closer to hazardous levels of 10 feet around Friday
night and Saturday. There is some uncertainty to whether or not Small
Craft Advisory conditions would be met, so will continue to assess
subsequent model runs. Also, there is an increasing concern for
elevated surf when a stronger pulse of north-northeast swell arrives
Friday or Friday night. Wave models suggests that this swell may be
5 to 7 feet with a period up to 16 seconds, which would generate
hazardous surf for north-facing reefs. A High Surf Advisory may need
to be considered in the days leading up to this event.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk from Wednesday morning through late Thursday
     night for GUZ001.

     High Surf Advisory from 6 AM Wednesday to 6 AM ChST Friday for
     GUZ001.

MP...High Rip Current Risk from Wednesday morning through late Thursday
     night for MPZ001>003.

     High Surf Advisory from 6 AM Wednesday to 6 AM ChST Friday for
     MPZ001>003.

Marianas Waters...None.
&&

$$

Marianas Update: Doll
East/West Micronesia Update: Williams
Marianas: Schank
East Micronesia: B. Aydlett
West Micronesia: Cruz