218
FXPQ50 PGUM 040759 AAC
AFDGUM

Area Forecast Discussion...UPDATED
National Weather Service Tiyan GU
559 PM ChST Wed Jun 4 2025

.Marianas Synopsis...
Satellite and radar observations show scattered showers and isolated
thunderstorms across Guam and Rota, and isolated showers across
Saipan and Tinian. Buoys indicate seas are 4 to 5 feet.

&&

.Discussion...
A taste of wet season has settled in across the Marianas and looks
to continue through the rest of the week. A slight lull is expected
tonight, with isolated showers and thunderstorms forecasted after
midnight. Shower activity looks to gradually increase starting
Thursday with numerous showers possible across Guam and Rota by
Friday night. There is some uncertainty into the weekend as the
better support for showers and thunderstorms looks to shift westward.

Overall, models are in general agreement with the increase in shower
activity across the Marianas towards the end of this week. However,
the GFS and ECMWF begin to diverge this weekend, with the GFS
developing an organized circulation in the Philippine Sea while the
ECMWF keeps broader activity. This would favor drier conditions with
the GFS scenario as the focus of convection shifts west, while the
ECMWF hangs onto some shower activity through the weekend due to a
more disorganized and broader circulation. With the MJO forecasted to
amplify into phase 6 and 7, this would lead some credence towards
the GFS solution. That said, those two phases also favor increased
precipitation across the Marianas. For now, will use a blend of the
GFS/GEFS and the ECMWF, which downtrends showers to scattered and
then isolated this weekend. Due to the influence of the MJO and a
TUTT forecasted to remain north of the Marianas through Saturday
night, PoPs may need to be increased over the next few days.

&&

.Marine...
Overall, little change was made to the marine forecast. Combined
seas of 3 to 5 feet will persist through the week before dropping a
foot this weekend. Sometime next week, a westerly swell could develop
for the western half of the coastal zones of the Marianas. This
hinges on the development of a monsoon trough across western
Micronesia later this week.

&&

.Eastern Micronesia...
The Inter-Tropical Convergence Zone (ITCZ) begins east of the Date
Line and extends west-southwest, passing south of Majuro and ending
south of Kosrae. Only isolated trade showers today from Pohnpei,
Kosrae and Majuro. An area of trade convergence to the north will dip
south and west and bring scattered showers into Pohnpei this evening
and Kosrae late tonight, while Majuro should stay isolated. In
addition, the ITCZ will drift north and west and further increase
shower coverage and bring isolated thunderstorms into Majuro and
Kosrae tomorrow and into Pohnpei by Friday. Showers look to become
isolated again for Majuro by the weekend as the ITCZ dissipates.
However, a weak circulation could develop southwest of Pohnpei and
Kosrae by the weekend, further enhancing the convergence and shower
potential. Both Pohnpei and Kosrae will see scattered showers and
isolated thunderstorms through the weekend, with PoPs bouncing
mostly between 30 and 50 percent. Early next week, all three forecast
points could see a chance of showers (probability of precipitation
30 percent) and a slight chance of thunderstorms as the ITCZ or trade
convergent zone builds into eastern Micronesia.

Gentle to moderate winds will prevail through Thursday. Winds look
to become light to gentle for Pohnpei and Kosrae and generally gentle
for Majuro by Friday, to continue into the weekend. Combined seas of
4 to 6 feet are expected through Thursday, becoming 2 to 4 feet by
Friday, as wind waves and trade swell subside. Winds and seas look to
rise early next week as the trade flow strengthens and a background
southerly swell increases.

&&

.Western Micronesia...
An unsettled weather pattern remains in store for Palau and Yap for
the rest of the week, as a tropical disturbance attempts to form and
consolidate well northwest of Yap and Palau, and a southwesterly
monsoon surge settles over the area.

Ensemble model guidance has been indicating a formation of a tropical
disturbance in western Micronesia, that would then trek northwest and
produce a monsoon surge along its southeastern peripheries as it
exits the region. Satellite imagery shows a general zone of
instability across Palau and Yap State, with a few minor westerly
surges in the monsoon observed just southwest of Palau. Additionally,
a tropical disturbance could form in western Yap State, that the
deterministic GFS and ensemble GEFS latch onto for further
development in the next 36 to 48 hours. While the feature remains
largely disorganized and its formation continues to look delayed on
satellite, the general trend in decreasing wind shear and buildup of
localized moisture will allow the development of significant showers
across Palau and Yap for much of the rest of the week once the
feature moves into a more convectively favorable area. Due to the
lackluster nature of the current stage of the monsoon and the
aforementioned potential disturbance, variability in areas of
convective development may produce periods of little to no weather
followed by sporadic heavy showers and thunderstorms. In addition,
high uncertainty remains regarding rainfall amounts, but the trend
continues to indicate around 5 inches at Palau, and around 6 to 8
inches at Yap over the next 72 hours. At this time, landslides
continue to be unlikely; however, if soils become saturated, any
additional rainfall may quickly become runoff and increase mudslide
risks, especially at Palau. Conditions are expected to improve
beginning early next week.

At Chuuk, patchy showers and some thunderstorms brought by gentle to
moderate trades are expected for the rest of the week, as a broad
tropical wave/disturbance moves in from the east. A short period of
drier weather may arrive by early next week.

While trade swells have weakened, localized wind waves and a
developing southwest swell will produce choppy seas in Palau and Yap
coastal waters over much of the week. Surf will continue to increase
along south and west facing reefs of Palau and Yap, but are not
likely to become hazardous at this time. For now, expect 3 to 5 foot
seas for the next few days, occasionally being a foot or two higher
throughout the week. Meanwhile at Chuuk, 4 to 5 foot seas may drop a
foot by the weekend.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.
&&

$$

Marianas: Williams
East Micronesia: Slagle
West Micronesia: Montvila