397
FXPQ50 PGUM 110819
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
619 PM ChST Wed Sep 11 2024

.Marianas Synopsis...
Satellite and radar observations show Tropical Depression Bebinca
lifting to the northwest away from the Marianas. Northerly moving
rain bands are seen across Guam and Rota. Combined seas remain
between 9 and 12 feet.

&&

.Discussion...
Earlier today the Joint Typhoon Warning Center downgraded Bebinca
into a tropical depression as it exited the Marianas. In the wake of
TD Bebinca, the monsoon surge has set up across the region. This
feature has brought persistent windy conditions of 25 to 35 mph with
gusts to 45 mph to the Marianas this afternoon. Winds are expected to
taper off overnight, but gusts to 40 mph will remain possible
overnight, especially near heavier showers. A wind advisory remains
in effect until 7 PM. Heavy rain will continue across the Marianas
through the week. See the hydrology section for more details.

The forecast becomes quite complicated towards the weekend. The
models do agree with a lull on Saturday wind wise, but keep the
possibility for heavy rainfall in place across the Marianas. On
Sunday, the ECMWF and EPS begin to diverge in comparison to the
other models. The ECMWF forms a circulation near the Philippines on
Sunday, leaving the Marianas in a "weak" monsoonal flow with heavy
rain remaining possible. Winds would remain around 15 to 20 mph with
this scenario. The GFS, GEFS, CMC, GEPS (CMC ensembles), ICON, and
AIFS all seem to be near agreement with a circulation developing far
closer to the Marianas. This scenario would reinforce the monsoon
surge with westerly winds potentially approaching 35 mph. This would
require extensions or re-issuances of multiple headlines. For now,
blended the GEFS with the ECMWF to find a middle ground. This
forecast may trend away from the ECMWF and EPS if they continue to be
the sole outliers. Those with interests in the Marianas should
continue to monitor forecasts for future updates.

&&

.Marine...
A Small Craft Advisory and a High Surf Advisory are in effect until
at least 7 AM Friday. Large breaking waves of 9 to 12 feet will
remain possible through Thursday night. Winds of 20 to 25 knots with
gusts to 35 knots will also be possible during this period,
especially near heavier showers. Both advisories may need to be
extended into the weekend depending on how quickly, or slowly, the
swell decreases towards the end of the week before winds, swell, and
surf increase again around Sunday.

&&

.Hydrology...
The Flash Flood Watch has been extended until Saturday morning for
the Marianas. 4 to 8 inches of rain have fallen already and an
additional 4 to 6 inches of rain will be possible through Friday
night. The monsoonal surge that is filtering in behind now Tropical
Depression Bebinca will allow for continued heavy rain for the
Marianas. The flash flood watch may need to be extended through the
weekend depending on if the forecast trends towards the GFS solution.

Localized flooding will be possible in low-lying and poor drainage
areas, while flash flooding could also be possible in areas of
heaviest rainfall. This situation will continue to be monitored
closely and additional Flash Flood Warnings may be issued if
conditions warrant.

&&

.Tropical systems...
Bebinca, briefly a tropical storm with 40 mph winds this morning, was
downgraded to a 35 mph depression early this afternoon based on a
scatterometer pass which captured the center and the stronger swath
of winds in the north-east-south half of Bebinca, showing winds less
than 40 mph. All TS warnings were cancelled at 2pm with the large
distance between the islands and Bebinca. As Bebinca continues WNW
into the PI Sea, gradual intensification is still expected to resume.

With Bebinca moving away, regional focus centers on the monsoon tail
pattern bringing strong winds and hazardous seas to West Micronesia
and the Marianas, and, what some models are hinting at, could be a
second round of tropical cyclone genesis in the region.

&&

.Eastern Micronesia...
Latest satellite imagery shows a westward moving surface trough
and convection pushing out of the Marshalls and into Kosrae and
Pohnpei states with most convection between 5N and 10N. Surface winds
are generally between 10 and 15 kt, as noted in midday scatterometer
passes. This trough will continue west with drier conditions
following in behind. Over the next couple of days, low-level flow
west of Kosrae will become more variable as small convective
disturbances stretch eastward from the southwesterly monsoon flow
west of Chuuk. Some of this convective activity will be aided by an
upper-level trough dipping down to near 12-13N. While some convective
flare ups will be possible, this activity will most likely be north
of 10N, north of Pohnpei. Later in the week, heading into the
weekend, a new surface trough is still progged to move into the
Marshalls and then gradually move into Kosrae and Pohnpei states by
Sunday or Monday.

Combined seas will stay between 5 and 6 feet the next several days
with no marine hazards anticipated.

&&

.Western Micronesia...
High Surf and Small Craft Advisories and a Special Weather Statement
remain in place for both Yap and Palau.

Yap and Palau are experiencing a monsoon surge. The surge is
anticipated to linger through the week and begin to diminish this
weekend as TD Bebinca moves deeper into the northern Philippine Sea.
Rainfall totals are anticipated to be 6 to 10 inches over this time
period with some areas getting a bit more. The risk of mudslides in
Palau is expected to increase due to the over-saturation of the
ground. Winds have been a growing concern and will be nearing gale
warning territory which is defined as frequent gusts or sustained
winds of 34 knots or 39 mph. It is anticipated to get near this mark,
however, confidence is low on if it will meet or exceed this
threshold.

Chuuk can expect to see scattered showers this week with intermittent
periods of isolated showers. This is due to Chuuk still being in the
intake flow of TD14 and passing troughs from east Micronesia. Showers
may linger through the week but will be to a lighter than what is at
Yap and Palau. Showers at Weno appear to taper off around the
weekend.

Combined seas between Yap and Chuuk are seen to be around 5 to 6
feet, currently. As this monsoon surge progress this week, seas are
anticipated to reach 9 to 13 feet near Yap and Palau, and winds are
expected to be fresh to strong with gusts up to gale force. Chuuk is
expected to be much more relaxed as seas near Chuuk are expected to
be 4 to 6 ft. Winds near Chuuk are expected to be light to gentle.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Flood Watch through Saturday morning for GUZ001.

     High Surf Advisory until 7 AM ChST Friday for GUZ001.

     Wind Advisory until 7 PM ChST this evening for GUZ001.

MP...Flood Watch through Saturday morning for MPZ001>003.

     High Surf Advisory until 7 AM ChST Friday for MPZ001>003.

     Wind Advisory until 7 PM ChST this evening for MPZ001>003.

     Small Craft Advisory until 7 AM ChST Friday for MPZ003.

Marianas Waters...Small Craft Advisory until 7 AM ChST Friday for PMZ151>154.

&&

$$

Marianas: Williams
Tropical: W. Aydlett
East Micronesia: W. Aydlett
West Micronesia: Bowsher