323
FXPQ50 PGUM 131913
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
513 AM ChST Sun Dec 14 2025

.Marianas Update...
The Marianas are about to be back doored by a shear line. This means
the shear line had passed by us, still to the north, then moved east
of us, and only then did it move far enough south. Now, a day or two
later, a piece of the shear line broke off and now it`s coming
straight for us. Increased winds, increased showers, and added
thunderstorms for Guam and Rota. Increased waves and issued a Small
Craft Advisory, expected to be for today only.

&&

.Marine...
The shear line fragment will bring fresh to strong winds today with
near gale gusts. If you don`t have to do marine travel today, don`t.
Waves ware beginning to respond, and are currently 7 to 9 feet on the
buoys, and will probably edge a little higher later today. The
advisory is expected to end this afternoon, but could live on through
tonight.

&&

.Fire weather...
The drought index continues to increase so far. Much will depend on
the evolution of this shear line fragment. If it`s a dry heave, we
could reach high fire danger by Friday. If it turns out to be a super
soaker, we could be back in low. We`ll just have to check it tonight.

&&

.Eastern Micronesia Update...
Majuro and Kosrae are having some drier weather now. Pohnpei is still
in the mix of the trade winds converging into the near-equatorial
trough, but is moving toward the edge. This was reasonably well
covered in the inherited forecast, so no changes were needed.

&&

.Western Micronesia Update...
Western Micronesia is dominated by the near-equatorial trough, with
Chuuk also getting caught in the trade winds converging into it.
Being farther north, Yap will have the driest weather. Chuuk will
likely have the wettest with those converging trade winds. The
inherited forecast told this story fairly well, so no changes were
needed.

&&

.Prev discussion... /issued 659 PM ChST Sat Dec 13 2025/

Marianas Synopsis...
Satellite imagery shows partly to mostly cloudy skies over the
Marianas, while radar imagery shows a speckling of isolated showers
over the region. Altimetry shows about 6 foot seas while buoy data
shows 5 to 6 feet.

Discussion...
Overall, an ordinary trade-wind pattern reigns over the Marianas. A
slight uptick in showers are expected for Sunday night as a pocket of
moisture associated with a shear line fragment is anticipated to
move over Guam and Rota. Fairly pleasant, but breezier weather is
anticipated through most of next week.

Marine...
This is where today`s forecast challenge was at. First, a headline
was used to convey that the winds near Tinian and Saipan may approach
the 22 knot threshold for a Small Craft Advisory (SCA) for the
overnight hours. The GEFSwave probabilities suggests up to an 80
percent chance overnight, however, some uncertainty exists as this
is for a relatively short amount of time and recent shear-line
weather has been lackluster and timing is rather difficult to pin
down. The ASCAT pass later this evening may provide needed support or
verification in this scenario. For now, the forecast calls for
occasional gusts of 25 kt expected for Tinian and Saipan waters
through tonight.

Second, models have shown that the arrival time of the pulse of
northerly swell is yet again delayed. This is also reflected with
little change in the Ritidian buoy, a little over 6 foot sea
heights this afternoon. To reflect this, elevated swells have been
pushed back by about 12 hours. Regardless of if/when this pulse moves
through the region, seas are expected to be choppy and mariners
should take extra caution.

Lastly, a north swell pulse emanating from mid-latitude systems as
they push eastward from Japan, may move through the region near the
middle of next week. At this time, the magnitude, duration and time
of arrival are still in flux. Confidence with this pulse is low but
growing.

Eastern Micronesia...
The latest satellite data shows mostly cloudy skies with isolated
showers across all three forecast points. Scattered showers and
isolated thunderstorms are seen just south of Pohnpei. Latest buoy
data shows seas between 5 and 6 feet.

A fairly dry pattern remains across the region this afternoon with
the convection that is seen over the area is mostly south of Pohnpei
and Kosrae. Isolated showers this afternoon at Pohnpei look to
become scattered later this evening as the Near Equatorial Trough
(NET) meanders a little north and south with a trade-wind trough
just west of Kosrae approaches. This will maintain scattered showers
over the island through Monday. For Kosrae, the trough that was
generating scattered showers there is now just west of the island
and is the one currently approaching Pohnpei. However, another
trough is seen near Majuro and is expected to approach Kosrae Sunday
morning, bringing another round of low-end scattered showers to the
island through Sunday night. By Monday, a dry pattern looks to
prevail at both Kosrae and Pohnpei through the beginning of next
week. For Majuro, a dry trade-wind pattern looks to continue there
through Tuesday. By Wednesday, a weak convergence pattern sets up
from Majuro to Pohnpei, with weak trade-wind troughs traversing the
band of convergence. This is expected to bring a round of increased
showers and a slight chance of thunderstorms to the region Wednesday
into Thursday.

For the marine forecast, increasing trade winds will bring a
building northeast swell into the region. Meanwhile, a strong mid-
latitude system will produce another building north swell. Seas
currently look to remain below Small Craft Advisory levels, however,
hazardous surf conditions are possible along north facing reefs of
all three locations. Combined seas could reach as high as 9 feet at
all three locations near the end of next week. Mostly gentle to
moderate trade winds are expected through Tuesday, with moderate to
fresh winds expected by the middle of next week.

Western Micronesia...
Daytime ASCAT analysis shows a weak, elongated circulation within
the near-equatorial trough (NET), roughly near 2N150E in southern
Chuuk State. Overall convection along the NET is fragmented along
trade-wind troughs moving west of Palau and and Chuuk, and an area
of increased convergence along the eastern extent of the NET and the
colliding trade-wind pattern around southeastern Chuuk State.
Continue to expect periodic upticks in trade-wind showers, then
overall trade-wind convergence looks to increase north of the NET
axis, with a broad, NET disturbance possibly taking shape this
upcoming week. Initially, this is would begin to increase showers at
Chuuk as early as Sunday night and at Palau starting Tuesday. Yap
looks to be north of the pattern for the first half of next week,
then convection focused near Palau may extend into Yap waters in the
latter half of the week.

Gentle to moderate winds persists at Palau through early next week,
then could see fresh gusts around the middle of next week with the
increase of showers along the NET. For Yap and Chuuk, moderate to
fresh winds with strong gusts at times are expected through much of
the upcoming week. Altimetry data shows combined seas around 5 to 7
feet, comprised of mainly northeast trade swell, with secondary
north to northeast swell farther east near Chuuk. Seas look to
increase another foot over the next couple days in response to an
increase in northeast to east trade swell and choppier wind waves
near Yap and Chuuk. Pulses of long-period north swell emanating from
distant mid-latitude systems are expected to arrive at Chuuk
starting early next week, then to Yap and Palau around Tuesday. North
swell may reach 3 to 4 feet around the middle of next week, possibly
bringing some elevated surf heights along north-facing reefs and
potentially pushing combined seas up to 7 to 9 feet.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...Small Craft Advisory until 6 PM ChST this evening
                  for PMZ151>154.

&&

$$


Marianas/East and West Micronesia Update: Stanko
Marianas: Bowsher
East Micronesia: Kleeschulte
West Micronesia: Cruz