Area Forecast Discussion
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761
FXPQ60 PGUM 200813
AFDPQ

Area Forecast Discussion
National Weather Service Tiyan GU
613 PM ChST Wed Sep 20 2017

.Marianas Synopsis...
Andersen Air Force Base Doppler weather radar shows isolated
showers moving through the Marianas waters. The buoys indicate sea
surface temperatures of 87 degrees and a north swell of 2 feet
with trade-wind waves around 3 feet. The VAD wind profile reveals
south wind 5 to 10 knots through the lowest 3 thousand feet of the
air. Well above this are east winds of 10 to 15 knots between 20
and 30 thousand feet.

&&

.Discussion...
A trough west of the Marianas is resulting in south winds in the
Marianas waters with northeast winds in the Yap sounding this
morning. There used to be a circulation here, seems to be more of
a trough now though. The majority of the convection seems to be
associated more with the upper-level low a little west of Guam.
This and the accompanying upper-level trough will keep isolated
thunderstorms in the forecast most of the forecast period. Most
likely it is the trough currently closing in on Kosrae that the
GFS then bring up toward Guam over the weekend, contributing
scattered showers in addition to the isolated thunderstorms. Looks
like the most recent runs have kept it south of us though, which
is starting to look good on satellite trends. Will watch it
through Thursday at least before chancing a removal of the
scattered showers. The lowered PoP from the mid shift is starting
to look like it was a good idea. For the most part...a rather laid
back trade-wind regime for the Marianas. Seas of 3 to 5 feet could
diminish to between 2 and 4 feet next week. No hazards are
foreseen at this time.

&&

.Eastern Micronesia...
A surface trough between Majuro and Kosrae will begin to affect
Kosrae later tonight as it moves slowly away from Majuro. Portions
of this trough are forecast to bring scattered showers to Pohnpei
Thursday night and Friday before yielding to a slight break in
the ITCZ. Satellite imagery shows another pulse of showers are
likely to reach Majuro late tonight, before a slight break is
expected. Although models show little significant rain reaching
Majuro over the next several days, a wind analysis puts them
within the ITCZ so periodic showers can always be expected.
Affects from the trough should pass through Kosrae by Friday night
and by the weekend for Pohnpei. The GFS and the ECMWF are
basically in agreement with this forecast. Little change in the
marine forecast from the previous forecast period.

&&

.Western Micronesia...
Chuuk is expected to remain with the surface ridge until late
Thursday or Friday with the approach of the trough to their east.
Have given Chuuk scattered showers on Friday but this is dependent up
the amplitude of the trough reaching this far north as the GFS has
it passing most to the south while ECMWF has it passing though the
atoll.

An unusual situation today with the GFS and ECMWF mostly agreeing
over the western half of the AOR. The weak low west of Guam has
already dissipated and melded into a large low pressure area farther
to the west of Guam and well to the northwest of Yap. Both models
show this system moving on to the northwest with ridging moving in
to these two sites.  This should give drier weather for Yap
beginning tonight and for Koror on Friday. Timing and strength of
the next trough affecting Yap and Koror is a bit of a stretch at
this point so have elected to give a more climatological outlook.
Scattered showers for Palau on Saturday and Sunday came from the GFS
but with low confidence as this portion is not in agreement with the
ECMWF which delays its approach.  Again nothing significant is
expected in the marine zones.

&&

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

$$

Stanko/Edson



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