633
FXPQ50 PGUM 052014
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
614 AM ChST Sun Oct 6 2024

.Marianas Update...
A lot has changed over the past 12 hours. Showers have diminished
over much of the Marianas. Invest 99W has mostly moved out of the
coastal waters of Saipan and into the coastal waters of Alamagan,
Pagan and Agrihan. Winds and seas in the CNMI are expected to
diminish as today progresses since most of the sea height was due to
wind waves.

&&

.Tropical systems...
Invest 99W has been upgraded to a TCFA, meaning that 99W is expected
to become a tropical depression within the next 24 hours. Seas and
winds are the worst on the northern half of this system. 99W has
moved in a due north directions along the eastern coasts of the
Marianas. The islands of Alamagan, Pagan and Agrihan can expect to
see effects of this system. Damaging winds are not expected for these
islands at this point unless 99W strengthens quicker than expected.

&&

.Eastern Micronesia Update...
Based on recent model and satellite trends, lowered the clouds and
showers around Pohnpei today, but increased the clouds tonight as a
cloud field is seen a little farther east. Increased the showers at
Kosrae as they are in the middle of a shower area on satellite, then
lowered the showers tonight while retaining some of the clouds. Also
increased the clouds at Majuro, as they have some passing through
right now and another field just a bit to the east. Shower potential
seems limited though, so left the showers at isolated. The extended
forecasts looked good, so blended these short term edits into the
previous grids.

Winds of 5 to 10 knots and seas of 3 to 5 feet will prevail across
Eastern Micronesia through at least Thursday. This will result in a
continuation of the benign marine conditions that have been enjoyed
for a while now.

&&

.Western Micronesia Update...
Reduced the clouds today at Koror based on recent satellite trends.
Went ahead and added thunderstorms based on proximity of GLD hits to
the north and GDI in the model data. Retained scattered showers and
isolated thunderstorms at Yap, however reduced the PoP by 10 percent
today as satellite shows the thicker stuff could pass to the south.
Increase the clouds tonight at Chuuk as it looks like showers could
brush just to the south of them. From then on, blended these short
term tweaks into the previous grids.

Winds of 5 to 10 knots and seas of 3 to 5 feet at Chuuk and 2 to 4
feet for Yap and Koror will continue through Thursday. Similar to
Eastern Micronesia, this will maintain the benign marine conditions
that have been enjoyed for some time already.

&&

.Prev discussion... /issued 616 PM ChST Sat Oct 5 2024/

Marianas Synopsis...
Light winds and wet soils have been producing island showers over
Guam, meanwhile fast-moving rainbands produced by passing 99W are
about to cross Tinian and Saipan, as indicated by Doppler radar.
Ritidian buoy indicates seas are 4 to 5 feet near Guam.

Discussion...
Tropical disturbance Invest 99W remains the primary weather feature
bringing unsettled weather to the Marianas the next few days. During
its passage northward across Saipan`s eastern waters, slight shifts
in the storm motion are expected to bring windy conditions to Tinian
and Saipan today. These winds are expected to shift from the east-
southeast to the southwest around midnight, with gusts to 40 mph
remaining possible during passage. The compactness, albeit largely
disorganized nature of this system continues to impact risk
assessment, as global model guidance continues to struggle
initializing with the smaller system size. As such, short-term
forecast relies on radar & satellite analysis, in addition to the
higher-res model guidance. The mid to long-term forecast remains
largely unchanged; once 99W exits to the north, this will bring back
monsoonal flow, with a monsoon surge along with it. At this time,
this monsoon surge is not expected to be as significant as the one
experienced just under a month ago, but a wet pattern is still in
store for the region, with some heavier showers and associated gusts
through at least the middle of the week. The general public should
continue to follow the development of 99W and associated weather, as
the forecast may change considerably.

Marine...
Passing tropical disturbance Invest 99W is expected to cross eastern
Saipan waters tonight, bringing gale conditions to eastern waters
during passage. Due to its very disorganized nature, there is a
chance for the disturbance to shift westward, bringing the gale-force
wind field closer to the main islands near midnight. Conditions will
improve quickly after 99W exits to the north as early as Sunday
afternoon. Choppy seas are to be expected through at least Sunday
night. Combined seas of 5 to 7 feet are expected to linger through at
least the first half of next week. A moderate risk of rip currents
will continue along north and east facing reefs, with a low risk
along south and west facing reefs. A moderate risk of lightning is
expected through at least Monday night. Surf and rip risk look to
rise along south and west facing reefs around the middle of next
week, as a southwest swell sets up in the wake of 99W.

Tropical systems...
A tropical disturbance, known as JTWC`s Invest 99W, is centered
southeast of Saipan near 14N147E. JTWC has labeled 99W as a "MEDIUM",
which means that it is likely to become a significant tropical
cyclone, but this is expected to occur beyond 24 hours from now.
Latest satellite imagery and Doppler radar indicate a consolidating
low to mid level circulation, with increasing convection near the
center and along the northern peripheries heading for Tinian and
Saipan. As 99W continues its trek north-northwest (mainly along
eastern Saipan waters), it will enter a much more favorable
environment for development, with Sea Surface Temperatures (SSTs) in
the mid to high 80s, along with weak vertical wind shear. For more
information on Invest 99W, see bulletins issued by the Joint Typhoon
Warning Center under WMO header ABPW10 PGTW.

Eastern Micronesia...
Dry conditions prevail across Majuro and Pohnpei this afternoon. For
Kosrae, moderate convergence to the south of an east-west oriented
trough is generating scattered showers and isolated thunderstorms
across the island. This area of showers is expected to move out of
Kosrae and into Pohnpei Sunday morning, leaving a dry pattern over
Kosrae beginning Sunday morning. The showers look to move out of
Pohnpei by Sunday evening. A dry pattern will then prevail across
eastern Micronesia for the next couple of days. Models continue to
show the development of an Inter-Tropical Convergence Zone (ITCZ)
over the Marshalls by Monday night, then spreading westward to Kosrae
by Tuesday, and Pohnpei by Wednesday. This will bring increased
showers and a risk of thunderstorms to all three locations through
most of next week. Kosrae will see a drier pattern first, near the
end of the week, as the ITCZ drifts a bit northward before beginning
to dissipate.

Benign marine conditions will continue through the end of next week,
with combined seas of 5 feet or less and mostly light to gentle
winds. Kosrae, however, should see occasional moderate winds Sunday
night into Monday.

Western Micronesia...
Fairly quiet conditions prevail across western Micronesia. Satellite
shows isolated showers and partly cloudy skies for much of the
region. Light to gentle monsoon flow prevails over Palau and Yap,
north of a broad high pressure ridge and south of a weak monsoon
trough which extends eastward near the Marianas. Scattered showers
are seen west of Yap within the weakly convergent westerlies. Yap may
see an increase in showers for tomorrow through Monday with a slight
strengthening in the southwest monsoon flow, along with increased
low-level convergence and moisture. Otherwise, showers will remain
mainly isolated through the coming week as a broad surface ridge
maintains its hold on the region. Late next week, Chuuk may see a
boost in showers as a series of weak troughs begin to meander across
central Micronesia and ridging weakens.

Benign marine conditions continue. Light to gentle winds are
expected, occasionally becoming moderate near Yap tonight and
Sunday. Altimetry data shows 3 to 5 foot seas between Yap and Chuuk,
and Babeldaob buoy data shows 2 to 4 foot seas over Palau`s eastern
waters. 3 to 5 foot seas near Chuuk may increase another foot through
early next week as a larger northeast swell moves in.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.

&&

$$

Marianas/Tropical: Bowsher
Eastern Micronesia: Kleeschulte/Stanko
Western Micronesia: DeCou/Stanko