FROM: NWS GUM
AREA: NWS GUM County Warning Area
SENT: 2026-07-27T07:31:00+00:00
Area Forecast Discussion: NWS GUM
000
FXPQ50 PGUM 270731
AFDGUM
Area Forecast Discussion
National Weather Service Tiyan GU
531 PM ChST Mon Jul 27 2026
.Marianas Synopsis…
Light to moderate trades prevail across the region. Satellite imagery
and Doppler radar indicate clusters of showers entering eastern
Marianas waters ahead of remnants of 93W. Buoys show seas are less
than 3 feet.
&&
.Discussion…
The leading edge of a sharp surface trough to the east is beginning
to move into the Marianas coastal waters, with shower coverage to
increase considerably throughout the night, as well as thunderstorms
developing near midnight. Breezy conditions will ensue thereafter,
with on-and-off showers and some thunderstorms expected through at
least midweek, before improving in the second half of the week. As a
distant Tropical Depression Twelve (12W) passes the Marianas well to
the north-northeast, it will induce a more sustained monsoon flow
across the area coming into the weekend, so introduced higher PoP
trend toward the end of the forecast period.
&&
.Marine…
Combined seas of around 4 feet will increase by a foot briefly as
trade swell strengthens, to then taper down once again around
midweek, followed by the arrival from an easterly swell pulse
generated by distant TD 12W. A low risk of rip currents is expected
along all reefs of the Marianas for the next few days.
&&
.Tropical Systems…
Tropical Depression Twelve (12W), formerly known as NHC’s Invest
92C, is centered near 13N178E well east-northeast of Utirik in the
Marshall Islands. It is moving west at 16 mph, with maximum sustained
winds to 35 mph. This system is expected to remain on this trajectory
through Tuesday, to then turn to the northwest as it continues to
intensify. At this time, it is not likely to have any direct damaging
wind impacts to any of the warning points in the region as it crosses
between the Marshalls and Wake Island over the next several days.
For more information please refer to bulletins from the Joint
Typhoon Warning Center under WMO header WTPN31 PGTW and from the
National Weather Service under WMO header WTPQ31 PGUM.
&&
.Eastern Micronesia…
The main forecast challenge was rainfall probabilities with the west
to southwest monsoon flow, and any swell concerns with Tropical
Depression 12W. Overall, the satellite imagery and numerical
guidance are in good agreement with one another. 12W is forecast to
develop into a tropical storm tonight, with the latest forecast
guidance showing a turn to the northwest closer to when it reaches
172E, near the edge of the RMI chain, versus the guidance consensus
from 2 or 3 days ago, which delayed the turn until it reached 165E.
With this quicker turn to the northwest shown in today’s forecasts,
with better continuity in this observed over the last 36 to 48 hours,
swell concerns for Utirik are less. As such, the Special Weather
Statement was not reissued, as 12W showing convection starting to
wrap around the center in a decreased shear environment supporting
this decision.
With this in mind, attention returned to the west to southwest
monsoon flow as the “primary” concern. Given the model/satellite
trends, keeping rainfall coverage limited to scattered coverage
(30-50 percent probabilities) seemed like a good route to go, and
this was maintained in the forecast package.
As for marine conditions, Pohnpei and Kosrae will see winds increase
into the moderate to fresh range Tuesday, continuing through
Wednesday night, as southwesterly flow strengthens in relation to
TD 12W (which should be a tropical storm by then, thus the “inflow”
increase into the disturbance). After that, winds will become more
comparable to what Majuro will see, mainly in the gentle to moderate
range. As for seas, as 12W strengthens, the southeasterly swell will
increase at Pohnpei and Kosrae, which may cause some minor erosion
concerns at Kosrae for the latter half of the week. With this in
mind, a headline was inserted into the forecast at Kosrae, to
increase awareness on this potential.
&&
.Western Micronesia…
A disorganized, unsettled pattern continues with the western extent
of the monsoon trough anchored just north of the region. Convergent
moderate to fresh southwesterlies are producing scattered showers and
isolated thunderstorms across the Republic of Palau, wrapping into a
weak monsoon circulation located to the north near 10N135E. To the
northeast, satellite imagery shows patchy showers building near Yap
Proper late this afternoon, along the axis of the monsoon trough
within the eastern outskirts of the weak circulation. Far to the
east, moderate to deep convection is organized along two main areas
of convergence, located to the southwest and northeast of Chuuk, with
lighter, scattered showers seen near Weno. At least scattered
showers will be maintained in the forecast for all three locations
over the next few days as convergent west to southwest winds continue
across the region, and the monsoon trough axis remains just north of
Micronesia. Showers will become numerous and locally heavy at times,
and latest model guidance now shows a peak in showers near Palau
around Tuesday night and Wednesday, near Chuuk around Wednesday
night, and for Yap Proper, late in the week around Friday and
Saturday as stronger convergence lifts northward into the area.
Late in the week and this coming weekend, the weather pattern across
western Micronesia will be increasingly affected by the development
of Tropical Depression 12W, which looks to strengthen significantly
over the open ocean far northeast of the region. The current forecast
track and development would lead to indirect effects in the form of
stronger southwest monsoon winds across the region, a continuation of
showery, unsettled conditions, and increasing seas due to pulses of
long-period swell from the northeast, mainly for Chuuk. In the
meantime, benign marine conditions continue, with latest altimetry
and Yap buoy data showing combined seas between 2 and 4 feet near Yap
and Palau, and between 3 and 5 feet near Chuuk.
&&
.GUM WATCHES/WARNINGS/ADVISORIES…
GU…None.
MP…None.
Marianas Waters…None.
&&
$$
Marianas/Tropical: Montvila
East Micronesia: Doll
West Micronesia: DeCou
Alert Expiration: 2026-07-27T15:31:00+00:00
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