NWS Guam wrote its last area forecast discussion before the first event at Apr 28, 8:00 PM UTC, 1 hour earlier.
Area forecast discussion, Apr 28, 8:00 PM UTC, 1 hour before
Area Forecast Discussion
National Weather Service Tiyan GU
600 AM ChST Tue Apr 29 2025
.Marianas Synopsis...
Satellite and radar show widely scattered showers from a weak
disturbance overnight departing the Guam Waters, which largely missed
the CNMI. The buoys reveal combined seas of 3 to 5 feet which are
composed of a trade-wind swell and wind waves,
&&
.Discussion...
Trade-wind swell could rise a foot over the next day or two, but that
will be barely noticeable. Another possible widely scattered shower
event Friday night through Saturday night, but once again nothing to
write home about. Wet season is still brewing to the south, but it's
still most likely about 2 to 4 weeks away for the Marianas.
&&
.Tropical systems...
JTWC Invest 99W is currently located at 6.2N and 131.7E. The latest
ASCAT data continues to show a poorly organized surface trough.
Significant development remains very low for the next 24 hours.
Invest 99W will continue to bring scattered to numerous showers to
Palau and Yap the next few days.
&&
.Eastern Micronesia...
A few changes were made to the forecast package. Satellite
observations show numerous showers southeast of Kosrae with
scattered showers near Kosrae and Majuro. Isolated showers were
introduced for Pohnpei this morning with little shower activity
present overnight and in the early morning on satellite. Trade-wind
convergence should lead to increased convection near Pohnpei with
scattered showers and isolated thunderstorms this afternoon. For
Majuro, shower coverage was increased for tonight and Wednesday to
scattered. Continued shower activity can be seen up stream where
trade-wind convergence is seen. PWAT values of 2.00 to 2.25 inches
persist all the way to the dateline. The forecast leans closer to
the GFS, which shows the continued shower activity while the ECMWF
favors dry conditions. The forecast still reflects a general drying
trend towards the latter half of the week.
No changes to the marine forecast were made.
&&
.Western Micronesia...
Only minor changes were made with this forecast package. Numerous
showers are seen just off the eastern coastal waters of Palau, while
scattered showers are developing across Chuuk. Little shower activity
is seen across Yap, but with proximity to Invest 99W and a mid-level
circulation east of Palau, the forecast kept low end scattered
showers (30%) for the afternoon. Latest satellite trends show
increasing showers south of Yap on the north side of the mid- level
circulation.
&&
.Prev discussion... /issued 631 PM ChST Mon Apr 28 2025/
Marianas Synopsis...
Gentle to moderate trades prevail across the region. Satellite imagery
and Doppler Radar indicate some patchy showers heading toward Guam
waters. The buoys indicate seas are around 4 to 5 feet.
Discussion...
A batch of low-end scattered showers are expected to cross over Guam
through Tuesday as well as sometime during the weekend, meanwhile the
rest of the Marianas shall continue to observe mostly dry conditions
for much of the week.
Marine...
Combined seas of 4 to 5 feet are will persist for much of the week,
as the trade swell now remains to be the only significant swell this
week. A moderate risk of rip currents is expected along north and
east facing reefs for at least the rest of the week, with little
change in surf heights during this time.
Fire weather...
So far today, 0.06 inches of rain has fallen since midnight. Patchy
showers will bring higher rainfall amounts tonight and Tuesday to
Guam, but not likely to be wetting rains, so the KBDI may continue to
creep upwards, which is at 691 (high category). Even so, winds are
expected to remain gentle to moderate, so conditions critical to fire
weather are not expected for at least the next few days.
Tropical systems...
JTWC's Invest 99W is centered near 6N131E, associated with an area
of vorticity in the 850-700mb layer that extends from 6N131E and
towards Palau, following the Near-Equatorial Trough (NET).
Scatterometer data shows sharp trough, possibly a weak surface
circulation within the NET near the center of Invest 99W, but this
feature is still very disorganized so development within the next 24
hours is still very low. Invest 99W is expected to linger west of
Palau the next few of days before shifting northwest towards the
Philippines late this week.
Further east, near 3N147E and well southwest of Chuuk Lagoon,
satellite water vapor imagery shows another area of mid-level
vorticity within the NET, in the 850-700mb layer, but scatterometer
data earlier in the day did not show a surface circulation near this
location either. The GFS and Canadian Ensemble support this area of
vorticity shifting towards Palau and Yap later this week, with a
surface circulation slowly developing as the feature shifts west-
northwest. The ECMWF and its ensemble members continue to favor the
current area of vorticity associated with 99W, but are starting to
show depict this vorticity feature southwest of Chuuk in the latest
model runs, but show little support that a circulation will develop
until after it passes northwest of Palau and Yap.
Eastern Micronesia...
Pohnpei and Kosrae:
An active pattern continues as the "ribbon" of highest moisture is
currently centered between 3N and 8N, with several surface troughs
delineating the leading edge of somewhat better convergence in the
trade flow. The increased lift from these features and short distance
between them will keep at least scattered showers in much of the
forecast for Pohnpei and Kosrae. This pattern looks to hold through
the end of the week.
Majuro:
You too will see scattered showers and isolated thunderstorms through
Tuesday before the moisture thins out a bit and the convergence axis
starts to slide to your south to southwest by Wednesday.
Marine:
Several outflow boundaries are noted on this afternoon's visible
satellite imagery, acting as a foci for new convective development.
Observations near Majuro have shown near 30 mph gusts. As such, we
attempted to try to add in isolated gust wording to both the land and
marine zones to account for this. However, temporal duration and
aerial coverage precludes any need for a headline.
Otherwise, look for seas to remain in the 4 to 7 foot range this
week, with the highest seas east of the Majuro atoll. Gentle to
moderate winds will prevail for Pohnpei and Kosrae this week. At
Majuro, moderate to fresh winds tonight will become gentle to
moderate Tuesday night, continuing for the remainder of the week.
Western Micronesia...
The NET continues to produce a large area of disturbed weather across
the region, with locally heavy showers found across the southern
portions of the Republic of Palau and the outer islands of Yap and
Chuuk States. As of this evening the main islands of Palau, Yap and
Chuuk are north of the heavier weather. As stated in the Tropical
system section above, there are currently two main areas of vorticity
in the NET, the first associated with 99W near 6N131E, and the
second is a new area southwest of Chuuk near 3N147E. Currently both
features are disorganized, so while both will help to support a large
area of showers and thunderstorms across the region over the next
few days, it is making it difficult to pinpoint the exact location
and timing of these showers or the potential for gusty winds. The
ECMWF continue to favor the location near 99W for any development of
a circulation within the NET, which would favor more of a southeast
flow and potentially limit the threat of gusty winds around Yap and
Palau, but the GFS and now the Canadian Ensemble has the main
circulation developing further east, closer the the area of vorticity
well southwest of Chuuk, which would lead to more variable winds
around Palau along with heavier showers and gusty winds later this
week, potentially extending up to Yap Proper, depending on the path
that any potential circulation takes. A circulation developing in Yap
State would also strengthen the trade convergence over Chuuk, which
would also lead to an increase in potential for showers and gusty
winds near heavier showers and thunderstorms. Given the model
spread, the forecast is a blend of both possibilities mentioning the
potential for gusts at Palau later tonight and again Thursday, while
introducing the potential for gusts at Chuuk Wednesday. Yap Proper
currently sits more on the northern edge of the NET so limited the
mention of gusty winds at this time, but still expecting a wet
pattern across the State.
Seas are currently around 3 to 5 feet and models keep seas near this
range through the week, driven primarily by the northeast-to-east
swell and local winds. The unsettled weather pattern across the
region may produce locally gusty conditions, which could lead to
choppy sea conditions not captured by the models, and then there
always is the potential for lightning within heavier showers as they
develop into thunderstorms. Mariners should continue to monitor the
latest forecast for any changes to marine conditions and practice
caution, especially if planning any inter-island travel.
&&
.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.
&&
Marianas: Stanko
East/West Micronesia: Williams