271
FXPQ50 PGUM 200906
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
706 PM ChST Mon May 20 2024

.Marianas Synopsis...
Satellite observations show few to scattered clouds with isolated
showers across the Marianas. Latest altimetry data shows seas in the
6 to 9 foot range just east of the Marianas.

&&

.Discussion...
Slight changes were made to the current forecast package. Uncertainty
still exists in how far north the convergent flow will spread into
the Marianas, which is limiting the northward expansion of chance
(50%) POPs. For now, kept chance POPs in for southern Guam and
across Guam`s southern coastal waters. The GFS in particular has been
too quick to bring the moisture surge into the Marianas. Went with a
blend of the GFS and ECMWF with the previous forecast to maintain
some consistency. Scattered showers will be possible overnight
tonight through the middle of the week as the tropical disturbance
(Invest 93W) pivots to the west-northwest.

&&

.Marine...
There has been a steady decrease in the size and strength of the
trade-wind surge east of the Marianas. ASCAT data shows predominately
moderate trades across the Marianas. Trades could bump into the
moderate to fresh range on Tuesday into Wednesday. Combined seas of
6 to 9 feet are expected the next few days. There still may be a need
for a Small Craft Advisory for either Tuesday or Wednesday, but for
now, combined seas and winds look to remain below criteria.

A high risk for rip currents for east facing reefs will persist
through Wednesday. A moderate risk for rip currents will continue for
north facing reefs as well.

&&

.Fire weather...
The KBDI is currently at 561, which is in the high category. There
will be a chance for wetting rains beginning Tuesday that should
limit the ability for fires to spread rapidly. If drier conditions
persist, fire weather concerns could return by the end of the week.

&&

.Tropical systems...
Invest 93W, centered near 3N143E, has drifted a bit eastward over
the last 24 hours. 93W remains broad and very disorganized and looks
to remain fairly stationary for the next few days. Latest model
guidance shows 93W moving to the west-northwest, approaching Palau
Tuesday. This is appears to be too fast and Thursday or Friday looks
more reasonable. The main concern with 93W revolves around a robust
trade- wind trough approaching the region. This trough, currently
between Kosrae and Pohnpei, looks to move into western Micronesia
and begin to interact with 93W around Wednesday or Thursday. If the
trough remains as strong as it is now, this could be the kicker that
93W needs to begin development into a tropical cyclone. This is not
expected at this time, but is a possibility. This system will
continue to be watched carefully over the next few days. Residents
of western Chuuk State, all of Yap State and Palau should continue
to monitor forecasts for any changes in expected conditions.

&&

.Eastern Micronesia...
The Inter-Tropical Convergence Zone (ITCZ) has pushed north of 10N
and is weakening. There is also a robust trade-wind trough oriented
to southwest to northeast stretching from the equator near 160E
across Kosrae to intersect the ITCZ just north of Kwajalein near
10N168E. This trough is generating numerous showers and isolated
thunderstorms across Kwajalein to Kosrae then southward to the
equator. Convection at Kosrae looks to decrease to overnight, with
showers becoming scattered later this evening, then isolated by
Tuesday morning. Meanwhile, the trough will approach Pohnpei through
the night, increasing showers from scattered to numerous near
midnight. The trough looks to pass Pohnpei during the morning on
Tuesday, with decreasing convection there through the day. A dry
pattern is seen across Majuro, which is east of the trough, and the
atoll looks to remain dry for the next several days. This dry
pattern will move into Kosrae by Tuesday morning, then spread into
Pohnpei by Tuesday afternoon. The dry pattern looks to break late in
the week as another, fairly wide, band of trade-wind convergence
develops across the region. Nothing spectacular about this band of
convergence, it looks to be fairly weak and is expected to produce
POPs (Probability of Precipitation) around 30 percent across the
area, beginning Thursday night at Majuro and then Friday night at
Pohnpei and Kosrae. Latest guidance doesn`t really show any
significant trough traversing the convergence band, but if one does
develop later in the week, it would increase POPs over the region.

Altimetry shows seas between 6 and 8 feet east of Majuro, with seas
between 4 and 6 feet west of the atoll. Seas look to slowly subside
at Majuro, falling to between 4 and 6 feet Thursday. Seas in the 4
to 6 foot range at Kosrae and Pohnpei look to increase a bit near
the end of the week to between 5 and 7 feet, then subside again over
the weekend. Gentle to moderate trade winds will prevail through the
week.

&&

.Western Micronesia...
The Near-Equatorial Trough and Invest 93W remain the focus across
western Micronesia. Over the past several days, model guidance has
gone through numerous changes in the expectations on development and
movement of Invest 93W. Latest GFS and ECMWF, along with GFS
ensembles, show little development of 93W until well northwest of
Palau over the Philippine Sea. However, models have been moving 93W
through Palau way too fast for the past several days. Therefore,
model guidance was delayed by about 48 hours on expected movement.
This would delay the increase in convection over both Yap and Palau
until at least Thursday. Based on the very low confidence in the
forecast for the area, decided to introduce 50 percent POPs for Yap
and Palau on Thursday, continuing at 50 percent through the weekend.
As the actual movement and any possible intensification become more
clear, the forecast will be corrected to account for any changes.

For Chuuk, the slower movement of 93W will also affect how long
scattered showers and isolated thunderstorms will remain over Weno.
Based on the uncertainty of the movement of 93W, maintained
scattered showers and isolated thunderstorms over Weno through
Thursday, with POPs decreasing to around 20 percent at the end of
the week. For the weekend, a broad, weak band of convergence
developing over eastern Micronesia looks to drift into Chuuk State,
bringing increased convection to Weno.

Winds will be mostly gentle to moderate for Yap and Chuuk through
the week, with short periods of fresh winds possible. Light to
gentle winds will continue at palau through the week. Seas of 3 to 5
feet are seen on altimetry for Palau, with 4 to 7 feet at Chuuk and
Yap. Seas are expected to remain at these levels through the week.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...High Rip Current Risk through Wednesday afternoon for GUZ001.

MP...High Rip Current Risk through Wednesday afternoon for MPZ001>003.

Marianas Waters...None.
&&

$$

Marianas: Williams
Micronesia: Kleeschulte