961
FXPQ50 PGUM 202045 AAA
AFDGUM

Area Forecast Discussion...UPDATED
National Weather Service Tiyan GU
645 AM ChST Tue May 21 2024

Updated the forecast for Western Micronesia and the center of Invest
93W in the Tropical.

.Marianas Synopsis...
Satellite imagery showing partly cloudy skies and isolated trade
showers. Latest altimetry data is indicating seas of 6 to 9 feet to
the east of the coastal waters.

&&

.Discussion...
A tropical disturbance (Invest 93W) well south of the Marianas,
continues to drift west over the coming days. Moisture and convergent
flow along the northern flank of 93W looks to remain a bit further
south then originally through. Have lowered shower potential and only
bring scattered showers and isolated thunderstorms into Guam through
Wednesday. Mainly isolated showers expected for the CNMI, even with
a trade trough/surge moving through the Marianas. Another disturbance
passing to the south on Thursday and Friday, along with a couple of
trade troughs, will bring an slightly elevated chance of showers,
mainly for Guam which will be closer to the convergent flow along the
north side of the disturbance.

&&

.Marine/Surf...
A trade-wind surge and an elevated trade swell will move through the
Marianas through Wednesday night. Winds will be moderate to fresh,
with occasional strong gusts, and combined seas of 6 to 9 feet
expected. Winds and seas may approach Small Craft criteria through
Wednesday afternoon, but for now look to remain just below
thresholds. Scattered showers and isolated thunderstorms will be
possible at times over the Guam coastal water, especially the
southern waters. Winds and seas diminish Thursday onward.

A high risk for rip currents will persist through Wednesday along
east facing reefs, with moderate risk along north facing reefs.
Surf looks to lower beginning Wednesday night as the trade swell
diminishes, with the rip risk possibly decreasing along north and
east facing reefs.

&&

.Fire Weather...
Only a trace of rain fell yesterday at the Guam airport, and the
KBDI rose to 569; well into the high category. Shower potential
looks be a bit higher through the week, and a chance for a wetting
rain will be possible, especially through Wednesday. Gusty winds
expected through Wednesday as well, but with the elevated clouds
and moisture, the fire threat looks to remain below Red Flag
criteria.

&&

.Tropical Systems...
Invest 93W, centered near 3N142E, still remains broad and
disorganized. Latest model guidance shows 93W moving to the west-
northwest and slowly approaching Palau and Yap over the coming days.
A robust trade trough, currently near 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, but this is unlikely to occur before passing through Yap and
Palau by late this week. This system will continue to be watched
carefully over the next few days. Residents of Yap State and the
Republic of Palau should continue to monitor forecasts for any
changes in expected conditions.

&&

.Eastern Micronesia...
A robust trough that is located between Pohnpei and Kosrae continues
to march westward. This is expected to bring numerous AM showers to
Pohnpei. These showers are anticipated to diminish in the afternoon
to evening hours. Once this trough passes, a drier pattern will begin
to set up for the rest of the work week.

For Majuro and Kosrae, a fairly dry day is expected with isolated
showers that may pop up as weak convergence may ignite some quick-
moving and spotty convection.

The Inter-Tropical Convergence Zone (ITCZ), has continued to farther
degrade and has became hard to distinguish from the increased trade-
winds converging in the area.

Altimetry shows combined seas of 6 to 9 feet east of Majuro, 3 to 5
feet near Kosrae, and 6 to 8 feet near Pohnpei. Winds are expected to
be gentle to moderate this week.

&&

.Western Micronesia...
Update:
Based on recent satellite trend and latest model guidance, a trade
trough and surge on the northern periphery of Invest 93W is pushing
west, with showers and thunderstorms increasing and approaching Yap.
Decided to update the forecast and bring scattered showers and
isolated thunderstorms for Yap starting today and for Palau starting
tonight.

Previous Discussion:
Satellite imagery and the ASCAT show that model guidance were
lacking. Luckily, the forecast was very robust, so not a lot was
needed to be changed. Currently, the main concern is how a incoming
robust trough will interact with Invest 93W later in the week, if it
interacts at all.

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
(Probability of Precipitation) 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 6 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 Guz005.

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

Marianas Waters...None.

&&

$$

Marianas: Slagle
Tropical: Slagle
Eastern Micronesia: Bowsher
Western Micronesia: Slagle/Bowsher/Kleeschulte