Area Forecast Discussion
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801
FXPQ60 PGUM 241015
AFDPQ

Area Forecast Discussion
National Weather Service Tiyan GU
813 PM ChST Sun Sep 24 2017

.Update...

Radar and satellite trends show only isolated showers over the
Marianas. Nixed the scattered showers from the forecast, but
kept the slight chance of thunderstorms.

&&

.Prev discussion... /issued 440 PM CHST Sun Sep 24 2017/

Marianas Synopsis...
A surface trough west of the Marianas continues to exit away from the
area, but showers and thunderstorms persist across the region and
satellite imagery shows more developing to the southeast. A surface
ridge is just to the east, but just beyond, another surface trough is
found near 154N and slowly moving westward. Deep convection well
north of Chuuk and Pohnpei is being enhanced by an elongated upper-
level low centered near 17N159E.

Discussion...
Based on satellite imagery and model guidance, went ahead and
increased shower coverage for tonight as scattered showers are
expected to linger for much of the night before the low-level ridge
enters the area. Upper-level winds will remain favorable for some
deep convection Monday and Monday night so have maintained a slight
chance of thunderstorms then. Afterwards, clouds and showers will
increase again on Tuesday as the surface trough near 154E approaches
the Marianas. The upper-level low southwest of Wake Island will also
drift west helping the development of showers and thunderstorms. The
bulk of this system will stay south of 10N, but models have been
showing an increase in moisture for several runs with the ECMWF
showing the best opportunity for showers in the area.

By late week, winds will decrease, becoming variable at times. Models
show much less moisture, so lessened showers and cloud cover seems
reasonable, however, weak winds could leave the door open for an
outside chance of island-effect convection in the afternoons, more so
at Guam. By next week, clouds will increase again as another surface
trough reaches the area.

Marine...
Buoys shows seas between 4 and 6 feet, a slight increase from
earlier. This is also reflected in latest altimitry data in showing
that an area of larger seas between the Marianas and Wake Island has
been moving westward with the increased winds the past couple of
days. A small 12-second northeast swell was also evident in the
Tanapag buoy. With yesterday`s surface trough moving west and
winds weakening, seas will decrease about 1 foot Monday night,
allowing for surf to fall. This will bring the risk of rip
currents down to low by Tuesday.

Eastern Micronesia...
Fair weather prevailed across much of eastern Micronesia this
afternoon. The surface analysis depicted a surface trough stretching
from 4n141e to 7N180. Scattered showers and isolated
thunderstorms could be found along the trough axis southeast and
east of Majuro. This trough will move westward over the next few
days. Isolated showers and thunderstorms at Majuro tonight will
give way to scattered showers and isolated thunderstorms Monday
and Monday night as the trough treks west. The trough will bring
scattered showers and isolated thunderstorms to Kosrae Monday
night and Tuesday. The trough will bring an increased chance of
showers and thunderstorms at Pohnpei Monday through Tuesday night,
but will weaken by the time it gets there causing only isolated
showers and thunderstorms.

Western Micronesia...
The circulation that brought wet weather to Palau and Yap Saturday
has moved west of the area. Low-level convergence east of the
circulation will produce mostly cloudy skies and isolated
thunderstorms at Palau and Yap tonight and part of Monday. A surface
trough will quickly move west and isolated showers and thunderstorms
will continue at Palau and Yap Monday night ahead of the trough . As
the trough axis gets closer to these two islands scattered showers
and isolated thunderstorms will push across Yap Tuesday and Tuesday
night and Tuesday night and Wednesday at Palau. High pressure will
bring an end to the scattered showers, and partly cloudy skies and
isolated showers will prevail at Yap Wednesday through Friday and
Thursday through Friday at Palau.

Chuuk is on the edge of a ridge of high pressure and only isolated
showers are expected there tonight through Monday night. The
trough that is over eastern Micronesia this afternoon, will move
west and bring only mostly cloudy skies to Chuuk Tuesday through
Wednesday. An area of low-level convergence will develop near Chuuk
and produce mostly cloudy skies and isolated showers and
thunderstorms at that location Thursday and Friday.

&&

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

$$

Ziobro/W. Aydlett



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