948
FXPQ50 PGUM 150846
AFDGUM

Area Forecast Discussion
National Weather Service Tiyan GU
646 PM ChST Thu Aug 15 2024

.Marianas Synopsis...
Satellite imagery shows an overcast stratus and cirrus deck across
the Marianas. Showers have remained isolated today with a few showers
passing through the Northern Marianas. Ritidian and Tanapag buoys
show seas between 3 and 5 feet.

&&

.KEY MESSAGES...

* Heavy rain, flooding, and gusty winds will be possible this
  weekend.

* Uncertainty in timing and location of the heaviest rains still
  exist and will be worked out with future forecasts.

* A Special Weather Statement has been issued and gives more detail
  on the upcoming heavy rain event.

.Discussion...
Satellite observations have shown robust convection to the east of
the Marianas persisting throughout the day. This region of
convection is where a Tropical Upper Tropospheric Trough (TUTT) is
interacting with an Upper Level Anticyclone (ULAC) and a trade-wind
trough. These features will generally move west-northwestward into
the Marianas and will line up with an enhanced area of surface
convergence that extends southward from Typhoon Ampil. These features
should interact to produce an atmosphere favorable for numerous
showers and isolated thunderstorms this weekend. Very heavy rainfall
of 3 to 5 inches with locally heavier amounts will be possible for
the Northern Marianas. For Guam there is quite a bit more uncertainty
in the rainfall forecast, but locally heavy rain will still be
possible. The GFS and ECMWF has flipped their position on the
location of the heavy rain. The GFS keeps the bulk of the rain over
the northeastern coastal waters of Saipan, while the high resolution
ECMWF has the axis of heavy rain over Tinian and Saipan. The updated
forecast package leans closer to hi-res ECMWF, which is more aligned
with the ECMWF ensembles. Stay tuned as future forecasts continue to
refine the timing and location of heavy rain.

&&

.Marine...
Combined seas of 3 to 5 feet could build to 4 to 6 feet early next
week. Winds will generally be gentle around 10 knots, however,
gustier winds of 20 knots will be possible near showers this weekend.
A moderate risk of rip currents will continue to be expected along
east facing reefs this weekend and into next week.


&&

.Eastern Micronesia...
Relatively quiet weather prevails over this region. ASCAT Analysis
indicates weak surface troughing over the Marshall Islands, while
weak surface ridging is between Pohnpei and Kosrae. A few lightning
strikes were noted recently just north and northeast of Majuro, and a
few lightning strikes were southeast of Kosrae. Models indicate the
surface trough will continue to move westward and cause a few showers
for Kosrae and Pohnpei in the coming days. Otherwise, just spotty to
patchy showers are expected for Eastern Micronesia the next few days.

The southeast swell from south of the Equator persists, especially
for Kosrae, and Pohnpei and Majuro to a lesser extent. Utwe/Kosrae
Buoy, just off the southeast coast of Kosrae, shows combined seas
around 5 feet, with an occasional bigger wave, mainly from the
southeast. This buoy should be monitored in case some waves arrive
much bigger than model data suggests.

&&

.Western Micronesia...
After tonight`s showers, Palau and Yap are expected to see a much
more pleasant weather regime through the weekend, followed by a
series of trade-wind troughs bringing on-and-off showers during the
first half of next week. Meanwhile, Chuuk will observe dryer,
diffusive trades through Friday, before a broad surface trough moves
in to bring patchy showers and some thunderstorms during the weekend.
Shortly after, a wetter trade-wind pattern will ensue.

Combined seas of 2 to 4 feet near Palau and Yap, in addition to seas
of 3 to 5 feet near Chuuk, are expected to persist into next week.

&&

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

$$

Marianas: Williams
Micronesia: Simpson/Montvila