874
FXPQ50 PGUM 140833
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
633 PM ChST Fri Mar 14 2025

.Marianas Synopsis...
Satellite and Doppler radar this afternoon reveal partly cloudy skies
and mainly isolated showers across the Marianas, becoming isolated
to scattered over Guam and Rota coastal waters. Patchy showers and
cloud cover are seen east of the Marianas, mainly upstream of Tinian
and Saipan, loosely aligned with a weak band of trade-wind
convergence. Surface observations and scatterometry indicate moderate
to fresh trade winds with occasional strong gusts across the
islands.

&&

.Discussion...
The shear line fragment over the northern CNMI has become indistinct,
the remnants of which have lifted northwest away from the area. The
Marianas still received scattered showers this morning from a shallow
trough crossing the area, and additional scattered showers look to be
in store tonight for Tinian and Saipan, as indicated by satellite
and model trends. The upstream environment is looking clearer for
Guam and Rota tonight, and this drier pattern should continue
tomorrow and Saturday night as models indicate a plume of drier air
with precipitable water values of 1.5 inches or less, moving across
Guam and Rota. A benign trade-wind pattern can be expected for the
rest of the forecast period, occasionally interrupted by a weak
passing trough or convergence boundary, especially around Tuesday and
Wednesday next week. Winds will remain moderate to fresh for the
next few days, becoming gentle to moderate at times as a col passes
over the northern CNMI, early to mid next week. The primary forecast
concern continues to be hazardous seas and surf for the next day or
so due to the passage of a significant long-period northeast swell.

&&

.Marine...
A significant northeast swell moving through the Marianas has
brought hazardous seas of 9 to 12 feet across marine zones, with the
highest seas expected over eastern Saipan and Tinian coastal waters.
The northeast swell is actually comprised of multiple sets of swell
from 30 to 40 degrees, with periods ranging 8 to 18 seconds and an
average of around 14 to 15 seconds. Much of the long-period swell
energy is focused well east of the Marianas, leading to an east-west
oriented gradient in sea heights. Tanapag buoy off the northwest
coast of Saipan has shown combined seas of 11 to 12 feet throughout
the day, while a drifting SOFAR buoy just east of Saipan waters is
measuring 12 to 13 foot seas. Meanwhile to the south and west,
combined seas are roughly 7 to 9 feet near Guam and Rota, as
indicated by Ipan and Ritidian buoys along with satellite altimetry,
illustrating the sharp gradient in sea heights with the incoming
swell and shadowing effect of the other islands. Combined seas over
the open ocean are expected to fall below hazardous levels of 10 feet
by Saturday night as the long-period swell energy gradually
dissipates.

&&

.Eastern Micronesia...
The ITCZ still extends across much of the region generating
scattered to numerous showers and isolated thunderstorms from the
southern Marshall Islands, south of Majuro, extending across Kosrae
and Pohnpei. Majuro remains on the drier north side of these showers
while Kosrae and Pohnpei remain in a wet pattern that should improve
somewhat by Sunday as the heavier showers of the ITCZ shift westward
into Chuuk and Yap States. However, another band of convergence is
expected to develop across the region, along the wind speed gradient
of a trade-wind surge. The strongest winds develop across the region
between 6N and 12N as pressure builds in the north central Pacific,
increasing the pressure gradient across the region late early next
week. This trade-wind surge will create breezy conditions and choppy
seas, especially for the Marshall Islands, but breezy conditions are
expected at times at Pohnpei and to a lesser extent at Kosrae since
it is located further south.

The large, long-period swell that been a large topic of discussion
the past few days has moved into the region. Pohnpei`s buoy earlier
in the day showed a significant change in conditions as the north
swell arrived. Sea heights nearly doubled, with seas over 7 feet
quickly climbing to about 14.4 feet within 5 hours, as the leading
edge of a significant northerly swell group, with periods ranging
from 12 to 20 seconds and averaging to around 16 seconds moved
through the region. The rapid and significant change in conditions,
especially compared to model guidance, highlights the fundamental
challenges in marine and surf forecasting. It is known that models
usually under-forecast distant and significant swells, especially
the leading edge of a swell, and though forecasters know of this bias
and try to correct for, the difference in observational data such as
the Pohnpei buoy and model data can be striking. Pohnpei`s buoy
shows that sea heights were 4 feet above the ECMWF model guidance
(which is usually one of the models with the lowest bias) and a
little over 2 feet above the correction usually used by forecasters
when trying to account for the typical low bias of the models.

Over the past 3 hours, the peak sea height has started to decrease
ever so slightly at Pohnpei from 14 feet to 13 feet, suggesting the
leading edge and peak of the northerly swell system is starting to
move south of the region. However, sea and surf conditions will be
slow to improve, with models typically being a little fast with the
decrease in the swell and impacts. With this in mind, the Small
Craft Advisory for Pohnpei and Kosrae has extended to Majuro and is
now out through Saturday night as seas around 13 feet this evening
slowly drop tonight and Saturday. However, seas are not expected to
drop below 10 feet until Sunday morning. The High Surf Advisory for
Pohnpei and Kosrae has also been extended to Majuro as this swell is
capable of producing surf up to 14 feet, though shadowing from the
northern islands may be keeping surf at Majuro slightly lower. For
more info on the rest of the Marshall Islands, see the Special
Weather Statement under the WMO header WWPQ81 PGUM. Since Majuro has
a higher threshold of 12 feet along north facing reefs, the surf
advisory is out through Saturday night for Majuro, and through Sunday
night at Pohnpei and Kosrae, with a threshold of 9 feet along north
facing reefs at these locations. East facing reefs of Kosrae are
also included in the High Surf Advisory due to their lower threshold
of 8 feet, compared to 12 feet for Pohnpei and Majuro, as the
combination of the east swell and wrap around from the north swell is
enough to push surf heights up to 10 feet along east reefs. Looking
towards next week, as the north swell continues to weaken, the
easterly swell and winds will become elevated as a trade-wind surge
develops across the region. This will likely cause the High Surf
Advisory for east facing reefs of Kosrae to be extended as surf
remains around 8 feet. Also, the trade-wind surge will create choppy
sea conditions with winds potentially reaching up to 25 knots and
seas around 9 to 10 feet, which may create another round of
hazardous marine conditions to small craft.

&&

.Western Micronesia...
A series of trade-wind troughs north of a near-equatorial trough
(NET) are generating patchy showers across far western Micronesia.
Scattered showers alongside one of the troughs are moving across Yap
Proper this evening and is expected to reach the main islands of
Palau around midnight. Farther east, the leading edge of the
Intertropical Convergence Zone (ITCZ), stretches across eastern and
southern Chuuk State and south of Yap State, where it collides with
the NET. Currently, satellite shows cloudy skies, numerous showers,
and isolated thunderstorms across Chuuk Lagoon and coastal waters.
The ITCZ is expected to shift north and west over the next few days,
maintaining numerous showers through Sunday night for Chuuk. Then,
the ITCZ shifts south, with isolated to scattered coverage expected
next week. Model guidance continues to show a broad trough breaking
away from the ITCZ pattern, becoming more developed as it approaches
the main islands of Yap and Palau Sunday night and Monday. Trailing
convergence may keep scattered showers around Palau over the weekend
and most of next week, meanwhile showers look to trend down to
isolated around the middle of next week for Yap.

A High Surf Advisory remains in effect along north-facing reefs of
Yap and Chuuk through Saturday afternoon. A High Surf Advisory will
go in effect Saturday morning through Saturday afternoon for Palau`s
north-facing reefs. A Small Craft Advisory remains in effect for
Chuuk through Saturday afternoon as well. Significant long-period
northerly swell has proved more impactful than what models has
anticipated, based on buoy data in eastern Micronesia and the
Marianas. Altimetry shows seas near 13 feet to the east of Chuuk. As
such, bumped up swell and surf heights for Chuuk. With this forecast
package, decided to be forward-leaning on swell heights, so will
reassess tomorrow if advisories need to be extended if we anticipate
swell would subside enough for seas and surf to drop below hazardous
levels Saturday night. Moderate to fresh east-northeast trades,
subside to light to moderate across Yap and Palau by Sunday, although
moderate to fresh trades prevail across Chuuk over the next several
days.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk through Sunday afternoon for GUZ001.

     High Surf Advisory until 4 PM ChST Sunday for GUZ001.

MP...High Rip Current Risk through Sunday afternoon for MPZ001>003.

     High Surf Advisory until 4 PM ChST Sunday for MPZ001>003.

Marianas Waters...Small Craft Advisory until 5 PM ChST Saturday for PMZ151>154.

&&

$$

Marianas: DeCou
East Micronesia: Schank
West Micronesia: Cruz