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FXPQ60 PGUM 210818

Area Forecast Discussion
National Weather Service Tiyan GU
615 PM ChST Fri Jul 21 2017

.Marianas Synopsis...
Satellite shows quite a bit of high clouds over the area while
radar shows only isolated showers nearby. A developing disturbance
is near Koror and a Special Weather Statement was issued for that
area. VAD gradient winds are east-southeast 12 to 15 kt. Ipan
Buoy shows combined seas around 4 feet, and Saipan and Ritidian
Buoys show 2 to 3 ft.


Refreshed wind and wave grids based on latest GFS Model. Isolated
thunderstorms are still expected mainly Sunday through Monday. High
cloudiness was more than expected today, but mid-levels will remain
relatively dry through Saturday. Models show increasing moisture
arriving from the southeast on Sunday along with light southeast
surface winds by Monday, turning south-to-variable at times about
Tuesday or Wednesday. Although a little drier weather is expected
Wednesday, the GFS model shows active weather to our southwest and
west, and we could have southwest winds later in the week.


Introduced a small southwest swell on Tuesday for secondary wave
grids based on WW3 Guidance, which is itself based on developments
of the disturbance near Palau, and other things. There is already
southwest monsoon flow over Palau, and I want to account for a
possible first episode of southwest waves affecting our Marianas
shorelines this week.


.Eastern Micronesia and Chuuk...
Trade-wind convergence will generate scattered showers and isolated
thunderstorms across Kosrae and Majuro tonight. A weak surface trough
will develop across eastern Micronesia and bring scattered showers
and isolated thunderstorms to Pohnpei and Chuuk Saturday and Saturday
night. Another weak surface trough will bring scattered showers to
Kosrae Monday. A combination of trade-wind convergence and weak
surface troughs will keep skies mostly cloudy and produce on and
off showers and isolated thunderstorms across eastern Micronesia
through the remainder of the forecast.


.Western Micronesia...
The main weather producer over western Micronesia is a developing
circulation southeast of Palau near 6N136E. Models show that the
development of the circulation will be gradual through the weekend.
Scatterometer data from last night and this morning shows a band of
20 to 30 knot winds west and south of the circulation center.
Satellite imagery shows areas of scattered to numerous showers with
isolated thunderstorms associated with this circulation. GFS shows
the center passing just south of Palau Saturday while ECMWF has it
passing over Palau. Scattered to numerous showers with isolated to
scattered thunderstorms are likely though Monday. Winds of 10 to 20
knots are expected tonight and Saturday but will likely increase
to 15 to 25 knots with higher gust Sunday night and Monday. These are
the first estimates on this system and things may change with time.
On a positive note...the 24-hour pressure change over Palau is
increasing pressure. If the circulation was rapidly developing
there should be falling pressures and this is not the case. Even
if the circulation does not intensify rapidly it will bring
periods of heavy rain. There had been reports of flooding on Palau
on Thursday and this was before the circulation really began to
develop. No matter what intensity the circulation reaches it will
still bring periods of heavy rain to Palau and more flooding is
likely as the ground is already saturated. If the rainfall is
persistent than there will be the possibility of mudslides in some
areas. The circulation will also cause an increase in seas of up
to 10 feet by Sunday night. A southwest swell will gradually
build through the weekend. The southwest swell will increase surf
on west, southwest, and south facing reefs. Surf could briefly
become hazardous sometime on Sunday or Monday. As the circulation
pulls northwest of Palau the weather will gradually improve by

Convergence will develop northeast of the circulation center and
bring scattered showers and isolated thunderstorms to Yap Saturday
through Sunday night. The increased pressure gradient will also cause
winds to increase to 10 to 20 knots Saturday night and remain that
high through next Wednesday.


Marianas Waters...None.


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