Flash Flood Warning 4, NWS Guam

Area Forecast Discussion
National Weather Service Tiyan GU
827 AM ChST Tue Sep 1 2020

.Marianas Synopsis...Light winds and isolated showers prevail over
the Marianas this morning.

.Discussion...
A few changes made to the forecast. Earlier forecast had scattered
showers developing over the Marianas last night. Satellite and radar
imagery did not show this. The earlier forecast had some of the
scattered showers coming from a band of showers coming from north of
the local area. This band did indeed develop but remained north of
the area. This band of showers are south of Tropical Storm 11W
and latest models keep them north of the area. Models do show
low-level convergence south of a weak disturbance bringing in
scattered showers during the afternoon and ending by midnight. There
will be enough instability to generate a few thunderstorms over the
area today and this evening. Weak high pressure will build over the
Marianas Wednesday and keep showers isolated the second half of the
week. Increasing low-level moisture will increase clouds and the
chance for showers and thunderstorms by the weekend.

&&
.Marine...
Ritidian...Ipan...and Tanapag buoys indicated seas were between 5 to
6 feet. Ritidian and Tanapag show a west swell 5 feet with a period
of 12 seconds. Expect surf on west facing reefs between 5 to 7 feet
and 3 to 5 feet on north facing reefs. Swell has increased during the
morning but do not anticipate it getting higher. The seas and swells
will persist through tonight before falling slightly on Wednesday.
The rip current risk is moderate. Models suggest seas and swell will
not change too much Wednesday through Friday but may increase over
the weekend.

&&

.Tropical systems...
Tropical Depression 11W formed just north of the Marianas just after
midnight. 11W quickly became a tropical storm later in the morning
and is expected to continue to intensify the next few days. The
system is relatively small and a high pressure ridge to the north is
preventing it from moving north, shunting it southwest. 11W will
eventually get stronger and the ridge weaker and it will finally move
northwestward in a few days.

&&

.Eastern Micronesia...
Trade-wind convergence is forming near Majuro and will bring showers
and thunderstorms to the eastern edge of Micronesia. This
disturbance will be the main weather maker producing scattered
showers and isolated thunderstorms for the next few day as it moves
westward. The weather will worsen tonight for Majuro; then by
Wednesday both Kosrae to Pohnpei will have scattered showers and
isolated thunderstorms lasting through Friday. After the trade-wind
disturbance moves past the region high pressure will stabilize the
weather pattern leading to calmer conditions for next weekend.

Combined seas will stay in the 3 to 5 foot range for Pohnpei and
Kosrae while reaching 6 feet at Majuro by mid-week.

&&

.Western Micronesia...
A broad circulation located west of Koror and is centered near
8N133E. This disturbance is not producing much convection but is
causing cloudy conditions across much of the region. Further east
Chuuk is located on the back end of a surface trough.

At Chuuk conditions will be improving today and by this evening will
be calm with mostly clear skies as high pressure moves into the
region. These fair conditions will continue until Wednesday night
when the trade-wind disturbance near Majuro reaches west Micronesia.
The trough currently west of Chuuk is expected to fade back into the
atmosphere before reaching the rest of west Micronesia. Without the
effects of the trough the weather will remain relatively quiet.
Isolated showers and thunderstorms will be possible for the rest of
the week but chances for anyone point remains low at around 20%.

&&

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

Ziobro/Slaughter