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FXPQ60 PGUM 240727

Area Forecast Discussion
National Weather Service Tiyan GU
527 PM CHST TUE MAY 24 2016

A surface ridge extends southwestward from northeast of Wake
Island at 25N175E to near the southern Philippines. It is
separating the Mariana Islands from a stationary front near 21N
and converging trade winds south of 10N. A trade-wind disturbance
approaching southeast of Guam is creating some spotty showers
southeast of Guam waters this evening.


Latest sounding data from Guam this afternoon hint a slightly
increase in low-level moisture to near 550mb. This should help to
generate a few showers near Guam and Rota as the disturbance
passes south of Guam from late tonight to Wednesday evening. There
is also a very remote chance of a thunderstorm or two during this
period. Otherwise, fair weather will remain over all the islands
thru Saturday.

Both ASCAT and RapidSCAT satellite imagery this afternoon depict
west to west-southwest monsoon winds from the Indian Ocean
eastward to Indo-China. A surface trough extending east-
northeastward from Indo-China to southeast of Iwo To Island near
24N134E is beginning to show monsoonal characteristics. All models
are suggesting this feature will transform into a monsoon trough
by this weekend. The GFS model suite in particular, develops this
feature into a robust monsoon trough over the Philippine Sea and
brings in converging southwest monsoonal winds over the Mariana
Islands early next week. While both the ECMWF and NAVGEM keep it
as a modest monsoon trough and maintain the current surface ridge
over the Marianas. Decided to go with latter two and prolong
gentle east-southeast winds and fair weather for the forecast
zones thru this weekend.


A gentle trade-wind swell will continue to produce seas of 3 to 4
feet across the Mariana Waters thru this weekend.


.Fire weather...
A few passing showers tonight and Wednesday along with gentle
trade winds should prevent red flag conditions over the next
several days.


.Eastern Micronesia...
A band of trade-wind convergence persists from Chuuk State across
Pohnpei and Kosrae States to the central Marshall Islands, mainly
between 4N and 8N this afternoon. A thunderstorm developed near
Chuuk earlier today. Also...a trade-wind disturbance currently
southeast of Majuro is firing a few thunderstorms. GFS Model
indicates this disturbance will move slowly west and then toward
the west-northwest. It should progressively trigger scattered
showers across Kosrae State the next couple of days and farther
westward across Pohnpei and Chuuk States by this weekend. Showers
could be fairly widespread at times across Eastern Micronesia but
rainfall amounts will be variable.


.Western Micronesia...
Reasoning for the Chuuk forecast is included in Eastern Micronesia
section above.

Weather is very unstable across far Western Micronesia and models
do not initialize all that well today. ASCAT analysis shows a
surface trough extends from southeast of Koror to south of Guam.
Showers and thunderstorms are developing in several areas from
Koror eastward across Yap to just west of Chuuk. The upper-level
wind pattern also remains favorable for shower development. Patchy
areas of showers will affect both Yap and Koror the next few days.
Model guidance indicates upper levels will become less conducive
for showers by this weekend with mainly just isolated showers by
then. On a larger scale...the weather over Southeast Asia seems to
be in transition as monsoon winds develop across the Philippine
and Taiwan areas. This larger-scale transition may eventually
affect weather conditions over far Western Micronesia.


.GUM Watches/Warnings/Advisories...
Marianas Waters...None.


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