881
FXPQ50 PGUM 120755
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
555 PM ChST Wed Mar 12 2025

.Marianas Synopsis...
A dry trade-wind pattern continues across much of the Marianas,
producing mostly moderate winds with occasional fresh gusts. Doppler
Radar only shows a few small, isolated light showers crossing
Guam and Rota coastal waters. Himawari visible satellite reveals
scattered showers near and north of Saipan and Tinian, associated
with a weakening shear line stretching over the northern CNMI. Nearby
buoy data indicates 6 to 8 foot combined seas near the islands, with
altimetry peaking around 10 feet over the northern CNMI.

&&

.Discussion...
The shear line parked over the northern CNMI will continue to
influence the weather pattern over Saipan and Tinian, increasing
showers and cloud cover over the next day or so, and bringing
moderate to fresh wind speeds. The shear line is expected to
continue a weakening trend through the rest of the week as it
gradually lifts north of the region. To the south, dry weather
remains entrenched over Guam and Rota, with only isolated showers
expected through the forecast period. The main forecast concerns will
be in marine zones, with the possibility of hazardous seas developing
by the end of the week. The High Surf Advisory and High Rip Current
Risk were extended through Saturday afternoon, as the arrival of a
large, long-period northeast swell on Thursday is expected to
maintain these hazards through the end of the week.

&&

.Marine...
An elevated northerly swell, in combination with northeast trade
swell and shear line-driven wind waves, has brought near-hazardous 7
to 9 foot seas near Tinian and Saipan, and 6 to 8 foot seas near Guam
and Rota. These seas may increase another foot or two Thursday night
into Friday as a large, long-period north swell enters the region.
Much of the energy associated with this swell will be focused east of
the islands as the swell just grazes the Marianas, but seas may
become hazardous at times by the end of the week as combined sea
heights approach 10 feet. The effects of the incoming long-period
swell will mainly be felt over north and east portions of Saipan,
Tinian, and Rota coastal waters, most likely from late Thursday night
through Friday. Tinian and Saipan will see scattered showers
overnight as the shear line lingers nearby over the northern CNMI.

&&

.Eastern Micronesia...
Satellite imagery shows scattered showers and isolated thunderstorms
extending across most of the region between 4N and 8N, along the
ITCZ with some support of a weakening upper-level trough, with a
cut-off upper-level low south of Kosrae. Satellite Scatterometer data
shows moderate to fresh trades along the ITCZ, with some stronger
gusts near heavier showers and thunderstorms. The ITCZ will help to
favor a wet pattern within the region, mainly between 4N and 8N, with
the greatest potential for showers shifting towards Pohnpei while
decreasing at Majuro over the next few days as the ITCZ shifts
westward and loses the upper-level support of the upper-level trough
and cut-off low. Given this trend, the GFS shows 2 to 4 inches across
Pohnpei and Kosrae through Sunday, with 1 to 2 inches across Majuro
and the southern atolls, and less rainfall for the north Marshall
Islands. The GFS and ECMWF shows the ITCZ weakening early next week
with a potential resurgence around midweek, as a trade-wind surge
develops across the region around and north of 10N as an area of high
pressure builds in the north central Pacific, increasing the
pressure gradient across the region. This trade-wind surge will
create breezy conditions, especially for the Marshall Islands.

Satellite altimetry and buoy data near Kosrae this evening no longer
support surf up to 8 feet along east facing reefs, so the High Surf
Advisory for east facing reefs of Kosrae was cancelled. Combined seas
across the region are around 6 to 8 feet, currently driven primarily
by the easterly swell and winds across the region. However, a large,
long-period north swell will be entering the region starting
Thursday night first reaching Pohnpei and the northern Marshall
Islands and then spreading to Kosrae and Majuro around Friday. Since
surf will increase at Pohnpei first, a High Surf Advisory was issued
for north facing reefs from Thursday afternoon through Sunday as surf
is expected to build up to 9 feet Thursday night and likely peaking
around 11 feet around Friday or Friday night. Surf will also build
along north facing reefs of Kosrae with the potential of surf up to 9
feet, so a new High Surf Advisory will likely be issued for Kosrae
as well. During the peak of the the north swell, seas may build up to
10 feet starting Friday or Friday night which would also produce
hazardous conditions for small craft.

Majuro has a higher threshold of 12 feet along north facing reefs,
and there is still some uncertainty if surf will build that high,
especially if there is some shadowing from the atolls farther north,
which will see the largest and unimpeded swells. This north swell is
also well timed with the full moon phase and its higher tides so
there is the potential for minor coastal inundation along north
facing reefs and shores facing northward into the lagoons across the
Marshall Islands. Seas may also build near 10 feet, especially across
the northern Islands, making seas choppy and potentially hazardous
to small craft. More details can be found in the Special Weather
Statement under the WMO header WWPQ81 PGUM.

&&

.Western Micronesia...
A series of trade-wind troughs will be riding over a near-equatorial
trough, causing an alternating pattern of scattered and isolated
showers for Yap and the Republic of Palau. An attempt has been made
to time these showers, but it is just an attempt. It looks like the
first round of scattered showers for Koror, forecast for Thursday, is
going to arrive tonight instead.

Chuuk, meanwhile, will be influenced by the ITCZ from Eastern
Micronesia. Through Thursday night, showers will be varying between
scattered and isolated, similar to Yap and Koror. From Friday onward
though, they will be varying between scattered and numerous.

Winds will tend to be 10 to 15 knots with occasional gusts to 25
knots. This will keep marine conditions in the challenging part of
the spectrum, not quite hazardous, but definitely not benign either.
Within the Chuuk Waters, sustained winds will be more like 15 to 20
knots through Saturday. Seas at Koror will start at 6 to 8 feet then
diminish to between 4 and 6 feet by Sunday. Seas will be about a foot
higher in the Yap Waters, in the challenging area again. Seas at
Chuuk will start at 5 to 7 feet then grow to between 7 and 9 feet.
The GEFSwave does indicate up to an 80 percent chance of Small Craft
Advisory conditions for Chuuk from Thursday night through Saturday
night, so that bears watching, mainly from waves. Chuuk may be the
one to cross over from challenging to hazardous.

&&

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

     High Surf Advisory until 4 PM ChST Saturday for GUZ001.

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

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

Marianas Waters...None.
&&

$$

Marianas: DeCou
East Micronesia: Schank
West Micronesia: Stanko