659
FXPQ50 PGUM 160740
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
540 PM ChST Tue Jul 16 2024

.Marianas Synopsis...
Heavy showers and isolated thunderstorms are seen skirting near and
southeast of Guam as a surface trough approaches the Marianas.
Additional showers will arrive from the southeast along the southern
edge of an outflow boundary, with the arrival of gusty, convergent
winds overnight through the next few days.

&&

.Discussion...
A Hydrologic Outlook for heavy rainfall has been issued for the
latter half of this week as a wet pattern sets up across the
Marianas. An area of surface troughing and deep convection near 6.5N,
160E looks to move northwest and develop over the area by Thursday as
the monsoon trough pushes north, greatly increasing rainfall
potential for several days. This potential may be enhanced by the
positioning and interaction with a Tropical Upper Tropospheric Trough
(TUTT). Locally heavy rainfall will be likely through Friday, with
diminished rainfall chances continuing through the weekend. The GFS
model aggressively wants to develop the passing disturbance as a
strengthening circulation come Sunday as it shifts northwest of the
Marianas, which we will be keeping a close eye on.

&&

.Marine/Surf...
Moderate to fresh winds are expected this week with occasional near
gale gusts near showers starting Thursday. Seas of 4 to 6 feet for
tonight will become 5 to 8 feet and choppy at times with an arriving
westerly swell. A trough approaching from the southeast will cause
showers and isolated thunderstorms, especially late Wednesday night
through Friday.

A moderate risk of rip currents will continue along east facing
reefs. A moderate risk of rip currents may develop along south and
west facing reefs later this week. There will also be a moderate risk
of lightning through Sunday night. Surf along west and south facing
reefs may build 1 to 3 feet higher later this week with southwest
swell.

&&

.Hydrology...
An ESF/Hydrologic Outlook was issued for heavy rain threat on
Thursday and Friday. A surface troughing pattern will be arriving
from the southeast and developing over the Mariana Islands this week.
Computer model guidance indicates 3 to 5 inches of rainfall should
be expected on Thursday and Friday, with locally higher amounts
likely. Rain showers will likely linger through Saturday into Sunday
with additional rainfall accumulations.

&&

.Eastern Micronesia...
Active weather pattern across eastern Micronesia over the next few
days with an ITCZ meandering and transitioning at times to more of a
zone of trade convergence. In addition, a trade-wind surge continues
to push into the region and enhance convergence. A disturbance along
the leading edge of the surge is currently pushing through Kosrae
and into Pohnpei and bringing heavy downpours, thunderstorms and
gusty winds. Scattered to numerous showers and isolated thunderstorms
expected through at least Thursday. The trade convergence zone looks
to fragment into a series of troughs by Friday into the weekend.
This pattern will still support scattered showers and isolated
thunderstorms across the region, but showers will start to cluster
more around troughs passing through the convergence zone.

The trade-wind surge is bringing gentle to moderate winds for
Pohnpei and Kosrae, with moderate to fresh winds for Majuro. Fresh to
strong gusts possible within the surge and with heavy showers. Gusts
up to near-gale possible for Majuro waters. The higher wind waves and
elevated trade swell and also creating seas of 7 to 10 feet, and
have issued a Small Craft Advisory through Wednesday afternoon. Lower
seas in the 5 to 7 foot range for Pohnpei and Kosrae as the trade
swell is diminished west of the Marshall Islands, although high
enough to warrant a High Surf Advisory along east reefs of Kosrae
through Wednesday afternoon. The gradient will weaken and the trade-
wind surge and swell will diminish by late week. Seas look to lower
to 3 to 5 feet for Pohnpei and Kosrae and 4 to 6 feet for Majuro,
with winds becoming light to gentle for Pohnpei and Kosrae and
becoming gentle to moderate for Majuro.

&&

.Western Micronesia...
The monsoon trough passes through JTWC`s Invest 91W near the
Phillipines, centered around 9N128E, and continues east-southeast
across the main islands of Palau and southern Yap State, ending near
Chuuk. Unsettled weather across the region is induced by monsoon
west-southwesterly flow, that stretches across Palau, colliding with
the trade-wind pattern across Yap and Chuuk. Scatterometry data
reveals a pocket of diffluent flow that lead to suppressed shower
activity over Yap today, however this lull is short-lived with the
overall active pattern in the area. The gradual north to northwest
shift of the monsoon trough is expected to maintain showery monsoon
weather with gusty winds and heavy showers at times through Thursday
for Palau and Yap. Farther west near Chuuk, a disturbance within the
ITCZ is expected to reach the island Wednesday and Wednesday night,
bringing heavier convection to the area. For the remainder of the
week, an ITCZ pattern remains across eastern and central Micronesia,
possibly fragmenting late in the week. Even so, trade-wind troughs
propagating from eastern Micronesia are still expected to bring
periods of showery weather.

Altimetry and scatterometry data show seas of 3 to 5 feet and light
to moderate winds. However, localized effects of heavier showers
will still lead to choppy seas at times across Palau and Yap over
the next few days, and possibly for Chuuk sometime Wednesday with a
passing disturbance. Southwest to west winds and swell in far western
Micronesia will further develop, allowing for seas to build up to 6
feet around Wednesday or Thursday, especially for Palau, while seas
near Chuuk look to stay between 4 and 6 feet.

&&

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

&&

$$

Marianas: Simpson/DeCou
Micronesia: Cruz/Slagle