NWS Guam wrote its last area forecast discussion before the first event at Apr 30, 7:42 AM UTC, 6 hours 18 minutes earlier.
Area forecast discussion, Apr 30, 7:42 AM UTC, 6 hours 18 minutes before
Area Forecast Discussion
National Weather Service Tiyan GU
542 PM ChST Wed Apr 30 2025
.Marianas Synopsis...
Gentle to moderate trades prevail in the region. Satellite imagery
and Doppler Radar indicate seasonal trades across the Marianas.
The buoys indicate seas are 4 to 6 feet.
&&
.Discussion...
Little change has been made to the forecast. Drier seasonal trades
are expected to continue across the Marianas for much of the forecast
period, as most of the convection will be limited to other sectors of
Micronesia to the south and west as various disturbances make their
way across Guam's AOR. An extension of a surface trough may bring
low-end scattered showers to Guam Friday and Friday night.
&&
.Marine...
Combined seas of 4 to 6 feet are expected the next several days,
tapering down a foot during the weekend. A moderate risk of rip
currents is expected along north and east facing reefs through
Friday night. Rip current risk may drop to low along north facing
reefs this weekend, as trade swell weakens slightly.
&&
.Fire weather...
So far today, 0.24 inches of rain has fallen since midnight, marking
the first wetting rain in quite a while. This means that the KBDI
will drop considerably overnight. However, more wetting rains are not
likely for at least the next several days, so the KBDI may continue
to creep upwards. The KBDI is currently at 707 (extreme category),
but will fall into the high category overnight. However, winds are
expected to remain gentle to moderate, as well as RH remaining above
60 percent, so conditions critical to fire weather are not expected
for at least the next few days.
&&
.Tropical systems...
JTWC's Invest 99W is still a broad area of rotation just west-
southwest of Koror, Palau, located near 6N133E. Today's satellite
scatterometer data does not show a well defined circulation with 99W
but this disturbance is associated with locally heavy showers and
thunderstorms, that are now more concentrated around Palau and
extending up toward Yap Proper, that could produce gusty winds and
choppy seas. The Joint Typhoon Warning center rates 99W as a "sub-
low" for development, which means significant development into a
tropical cyclone is unlikely over the next 24 hours. Farther east in
Yap State, is Invest 90W, a weak circulation near 4N144E, close to
where the broad area of mid-level vorticity was noted yesterday.
Similar to 99W, this system is disorganized but currently showers and
thunderstorms around 90W cover a much larger area compared to 99W,
likely due to the support of the mid-level vorticity in the area.
JTWC also rates 90W's potential over the next 24 hours as "sub- low".
Model trends continue to support one, possibly two, defined
circulations developing within the NET and shifting northwest through
the end of the week, with significant development not occurring
until after moving northwest of Palau and Yap. The likely locations
for these circulations are associated with Invests 99W and 90W. This
slow and disorganized development will lead to widespread variability
in conditions across Palau and Yap State, however the potential for
locally heavy showers, gusty winds and choppy seas still exists
across the region. More details can be found in the Special Weather
Statement issued for Palau and Yap under WMO header WWPQ82 PGUM.
&&
.Eastern Micronesia...
The broad area of trade-wind convergence has weakened a bit in the
last 24 hours. Although, upper-level diffluence is still supporting
an area of showers between Pohnpei and Kosrae which will maintain
scattered showers through the evening for Kosrae and build into
Pohnpei around midnight. Model guidance still shows trade-wind
convergence further fragmenting over the next day or so, allowing
periods of isolated showers, but also hinting at trade showers
building back in near Pohnpei and Kosrae over the weekend. Farther
east, visible satellite imagery is showing upper-level clouds moving
across Majuro from deeper convection to the south, however, only
isolated showers are expected over the atoll tonight. These
relatively quiet conditions are expected over the next several days,
with scattered showers possible around the middle of next week as
another trade-wind convergence zone rebuilds across the region.
Combined seas are expected to be between 5 and 7 feet over the next
several days. Moderate to fresh winds across the region subside to
light to moderate late week and over the weekend.
&&
.Western Micronesia...
At Chuuk State, scattered showers and isolated thunderstorms are
developing across the outer islands with showers to the southeast of
Chuuk Lagoon slowly lifting northwest, and should reach the lagoon
tonight. Scattered showers and isolated thunderstorms will continue
to linger across the state over the next few days, driven by weak
trade convergence and passing trade wind troughs, but models continue
to support a drying trend late this weekend into next week.
Otherwise, moderate to fresh winds tonight are expected to become
gentle to moderate Thursday and continue through the weekend.
The forecast for Palau and Yap remains complex due to the broad and
disorganized pattern across the region, driven by the NET and the two
embedded tropical disturbances 99W and 90W. Invest 99W is producing
scattered to numerous locally heavy showers and isolated
thunderstorms, over Palau and western Yap State this evening, while
90W is producing scattered to numerous locally heavy showers and
isolated thunderstorms across eastern Yap State. The ECMWF continues
to differ from the GFS and Canadian, with the ECMWF still favoring
one distinct circulation near 99W while the other models support two
distinct circulations developing within the NET, with 90W becoming
the more dominate circulation. These differences create a lot of
variability in the location and intensity of showers and
thunderstorms, which also influences the potential wind direction,
location of locally stronger gusts and choppy seas. Models are
consistent that Palau and Yap will experience a wet pattern through
the rest of the week, though significant development is still favored
once the circulation(s) move northwest of Palau and Yap. More
details can be found in the Special Weather Statement issued for
Palau and Yap under WMO header WWPQ82 PGUM
Buoy and altimetry data show that seas are mainly 3 to 5 feet across
most of the region. ASCAT data also shows gentle to moderate winds
across the region, but there are pockets of gusty winds, associated
with locally heavy showers and thunderstorms which are likely
creating choppy sea conditions at times. Given the large uncertainty
and variability in the models, the overall model trends keep seas
around 3 to 5 feet or increasing slightly to 4 to 6 feet late this
week, but this does not take into account choppy sea conditions
caused by gusts near heavier showers and thunderstorms. Mariners
should continue to monitor the latest forecast for any changes to
marine conditions as the pattern changes through the week.
&&
.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.
&&
Marianas: Montvila
East Micronesia: Cruz
West Micronesia/Tropical: Schank