Flash Flood in Guam, September 8, 2023

Area Forecast Discussion
National Weather Service Tiyan GU
647 AM ChST Fri Sep 8 2023

.Marianas Synopsis...
Satellite and radar indicating scattered to numerous showers with
thunderstorms in the forecast area, especially in the Guam and Rota
coastal waters. Ipan buoy and latest altimetry data indicating seas 4
to 6 feet.

&&

.Discussion...
A weak tropical circulation continues to drift west of the Marianas
this morning and will meander west of the islands into the weekend,
before nudging northward by early next week. Although no significant
tropical development is expected, a monsoon-like pattern develops
south and east of the circulation and brings increased moisture flux
over the Marianas. In addition, a Tropical Upper-Tropospheric Trough
(TUTT) cell continues to pass overhead and will work to the northwest
of the region, enhancing instability and upper divergence through
tomorrow. Scattered to numerous showers, at times heavy, with the
potential for thunderstorms will continue through the weekend and
possibly into early next week.

&&

.Marine...
Winds will become gentle to moderate from the south to southwest
today and continue through Sunday, then becoming light to gentle from
the west to southwest early next week. Some fresh to strong gusts
will be possible under heavier showers and thunderstorms. Combined
seas 4 to 6 feet through Saturday night, lowering by about a foot
Sunday into early next week.

&&

.Hydrology...
Tropical disturbance JTWC's Invest 90W near 14N144E will continue to
meander to the north and west of the Marianas. A band of heavy
showers and embedded thunderstorms along its eastern periphery will
impact the islands today, especially Guam and Rota. A monsoon like
pattern will then likely set up to the south and east of 90W, with
enhanced moisture flux and convergence over the Marianas through the
weekend and possibly into early next week. Expect scattered to
numerous showers, at times heavy, and potential of thunderstorms
today through the weekend. The previous forecast's thoughts of two
to four inches with locally higher amounts through the weekend seems
to be on track.

&&

.Tropical systems...
Invest 90W is currently centered near 14N144E, roughly 50 miles west
of Guam, meandering slowly to the west towards the Philippine Sea.
The trends in the latest runs have strengthened the winds slightly,
but show this remaining as a weak circulation that doesn't undergo
any significant development as it meanders off to the west into the
weekend, before drifting north by early next week. Aside from a few
stronger wind gusts associated with heavier showers and
thunderstorms, the main concern with this feature for Guam and the
CNMI will be periods of locally heavy rainfall potentially
developing.

&&

.Eastern Micronesia & Chuuk...
A relatively dry trade-wind pattern persists across the region with
a weak high pressure ridge stretching west-southwestward across
Pohnpei and Chuuk. Brief periods of scattered showers will be
associated with minor troughs traversing the region through the
weekend. In the upcoming week, models indicate more robust trade-
wind troughs developing, leading to increased showers and
thunderstorm potentials beginning Monday night or Tuesday for eastern
Micronesia, and Wednesday night for Chuuk.

Combined seas are expected to be between 3 to 5 feet across the
region, and up to 6 feet at Majuro. Winds are overall light to
gentle, with an area of gentle to moderate winds for Kosrae and
Majuro through Saturday.

&&

.Western Micronesia for Yap & Palau...
The main weather features in the region, JTWC's Invest 90W (sub-low)
and its monsoon tail, remain perched over the Marianas and western
Micronesia this morning. Scatterometry data indicates gentle to
moderate westerly flow across Yap and Palau, feeding into 90W,
centered over the western waters of the Marianas. Based on trends in
satellite and model guidance, scattered to numerous showers build
into the region today, however brief lulls in activity are still
possible as convection shuffles around and become spotty at times.
As 90W gradually lifts northward, monsoon southwesterlies persists,
and conditions trend wetter this upcoming week as convergence
increases across the islands.

Combined seas of 3 to 5 feet and gentle to moderate southwest winds
are expected over the next several days. Occasional gusts to 25 kt
are still possible in heavier showers and thunderstorms, which will
cause wind waves up to 6 feet.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...

GU...None.
MP...None.
Marianas Waters...None.

&&

Marianas: Slagle
Hydrology: Slagle
Tropical: Slagle/Doll
Micronesia: Cruz