Flash Flooding in Guam, August 31, 2023

Area Forecast Discussion
National Weather Service Tiyan GU
845 AM ChST Thu Aug 31 2023

.Marianas Synopsis...
Satellite shows scattered showers and isolated thunderstorms
associated with the monsoon trough continuing to move through the
Marianas Waters. The Ipan buoy reveals an east-northeast swell of 3
to 4 feet. Computer wave model guidance suggests another swell from
the west of 5 to 7 feet, resulting in combined seas of 6 to 8 feet.

&&

.Discussion...
The proximity of the monsoon trough will continue to cause scattered
showers and isolated thunderstorms through at least Monday. Rainfall
accumulations should be gradually diminishing over this time period.
After Sunday or Monday, we will transition to an isolated showers
regime, which could last the rest of the week.

Through Saturday, winds will be occasionally fresh, then they will
become just gentle to moderate, then light to gentle. This could
allow island convection to become an issue for Guam at least,
though.

&&

.Marine...
Combined seas of 6 to 8 feet will continue through Saturday, then
begin to diminish. The seas could reach 3 to 5 feet by Wednesday.
After that it looks like a week of benign marine conditions is on
tap.

&&

.Tropical systems...
Tropical Storm Haikui (10W) is near 21.4N and 133.8E, moving west-
northwest at 9 mph with maximum sustained winds of 60 mph. It will
likely exit Micronesia Friday, and do nothing more for Micronesia
then put out some more west to northwest swell.

Tropical Storm Kirogi (11W) is near 14.6N and 154.3E, moving north at
7 mph with maximum sustained winds of 50 mph. Kirogi will be with us
for at least 2 more days, possibly more, as it turns west and passes
north of the Marianas. It will also have no direct effects, and will
just generate an east swell turning northeast then north.

&&

.Eastern Micronesia...
Overall the large scale-pattern described in the previous forecast
discussion has not changed much, but some minor details have caused
some short-term changes. Well north of the region 11W has become
Tropical Storm Kirogi and continues to move north away from the
region. A minor ridge is building near Pohnpei but the main axis is
just west of Pohnpei and the westerly flow around this axis is
converging with the easterly trade winds between Pohnpei and Kosrae.
This has caused a large area of showers and thunderstorms to develop
just south of Pohnpei and extending south towards the EQ to the east
of Kapingamarangi. The trade disturbance/circulation near the Date
Line has opened into a trough and joins the series of trade-wind
troughs extending into the Marshall Islands.

Pohnpei: due to the proximity of the large area of convection
mentioned above, kept low-end scattered showers and isolated
thunderstorms in the forecast today, but still expect showers to
become isolated tonight and Friday. The series of trade-wind troughs
will move through during the weekend, bringing on-and-off showers and
some thunderstorms, before another drier cycle ensues in the first
half of next week.

Kosrae: As the trade-wind troughs near the Marshall islands continue
to move west, expect periods of low-end scattered showers and some
thunderstorms to move across through Saturday. Starting around Sunday
a period of drier weather is expected before the next series of
trough is expected to push into the region, south of Majuro around
Tuesday or Wednesday.

Majuro: A series of trade-wind troughs are currently pushing through
the Marshall Islands now and extend back towards the Date Line.
Expected periods of scattered showers and isolated thunderstorms
today through Friday night, followed by isolated showers starting
around Friday and possibly into the first half of next week. Models
do show the next series of trough pushing south of Majuro next week,
so we will see if this trend holds or if scattered showers need to be
added to the forecast for next week.

Satellite altimetry still shows combined seas of 5 to 7 feet across
the region, with the highest seas near Pohnpei. Seas are expected to
diminish by 2 to 3 feet by the end of the week, as the westerly
swell in the region diminishes and the background easterly trade-
wind swell becomes the primary swell once again.

&&

.Western Micronesia...
The region continues to experience southwest to westerly flow as the
monsoon trough and embedded tropical systems continue to extend
northern West Pacific. A tropical high continues to remain over
Palau and eastern Yap and western Chuuk States.

Palau: The tropical high will keep weather pleasant at Palau today,
though models still show back-building convergence zone developing
between the high and monsoon trough which will allow slightly more
showers to trickle in tonight. Models are not overly supportive of
showers for Friday and Saturday, so kept showers isolated until
Sunday when models support a wetter pattern as the ridge weakens and
support for showers increases again.

Yap: Still at the tail-end of the monsoonal influx into other
tropical disturbances to the northeast. Expected to see isolated
showers today due to the influence of the tropical high to the
south, then on- and-off showers and some thunderstorms into next week
as convergence increase between the monsoon trough to the north and
the tropical high to the south.

Chuuk: As 11W, now Tropical Storm Kirogi, continues to move north the
inflow into the storm will also move north, though the proximity of
this flow will keep Chuuk under a wetter pattern with periods of
scattered showers and isolated thunderstorms for the next couple of
days. Looking towards the weekend, the easterly trade-winds will
start to push back into Chuuk and this is expected to create a zone
of convergent flow, creating periods of scattered showers. Once the
trade-winds push into Chuuk expect some drier weather to start next
week.

Combined seas of 4 to 6 feet will persist through the weekend, before
tapering down by a foot or two thereafter as the westerly swell
weakens.

&&

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

Marianas: Stanko
Micronesia: Schank