Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 171352

Area Forecast Discussion
National Weather Service Tiyan GU
1152 PM ChST Tue Sep 17 2019

An upper-level low and associated trough have triggered isolated
thunderstorms northwest of Guam and might do the same across the
CNMI later tonight. Therefore, decided to update the forecast by
adding isolated thunderstorms for the rest of tonight.


.Prev discussion... /issued 642 PM CHST Tue Sep 17 2019/

Marianas Synopsis...
Mostly dry conditions and partly cloudy skies prevail this evening.
An upper-level low centered roughly 200 miles east of Saipan is
sparking scattered thunderstorms just east of 150E while a disturbed
area well to the northwest of the Marianas continues to move farther
from the islands.

Wednesday should start out relatively dry but the chance of showers
and thunderstorms will increase as the day progresses as a broad
circulation drifts into the region from the east. The two features
at play are a broad, weak circulation near 12N160E and an upper-level
low east of Saipan that may help to increase the chance of scattered
showers and thunderstorms Wednesday night and Thursday. Both the
ECMWF and GFS bring this circulation to the west through the Marianas
over the weekend without much development until it moves well west of
the islands. However, any slow down or slightly more development than
anticipated could bring in a renewed chance of monsoon-type rain
after a brief dry period late Friday and Saturday.

The long-lived west swell continues to wind down. Surf along west
facing reefs is expected to drop below hazardous levels Wednesday.
Combined seas will gradually fall through the end of the week. The
GFS-based WW3 advertises another pulse of west swell will arrive this
weekend but heights should remain below advisory levels.

Eastern Micronesia...
A prominent surface ridge remains south of Chuuk, Pohnpei, Kosrae and
Majuro, keeping fair weather in place across much of the region south
of 6N. A tropical disturbance remains north of 10N, currently located
near Enewetak near 12N160E and moving slowly west. Visible satellite
animation shows numerous showers and isolated thunderstorms over and
north of Pohnpei, to northeast of Chuuk. Showers and thunderstorms
will continue through tonight at Pohnpei and taper off on Wednesday.
Expect isolated showers and thunderstorms at Chuuk through Wednesday
night as the disturbance heads west. For now, much of the associated
weather should keep just north of Weno.

Looking ahead, models show the monsoon trough keeping generally in
place along or just north of 10N into the weekend. Another
disturbance will increase the chance of showers and thunderstorms
this weekend at Majuro, Kosrae and Pohnpei.

A monsoon swell along with weak trade-wind swell will maintain seas
of 4 to 6 feet across Chuuk, Pohnpei and Kosrae States and the
Marshall Islands thru Thursday. Afterward, the declining monsoonal
swell should allow seas to drop about one foot.

Western Micronesia...
A surface ridge remains in place over Koror and just south of Yap
allowing pleasant weather to continue across Palau. However, showers
and thunderstorms have been in place over Yap within the convergent
flow there just south of the monsoon that extends northward over the
Philippine Sea. Scattered showers at Yap will dissipate later this
evening allowing better weather to return by Wednesday. A weak
disturbance currently north of eastern Micronesia will approach the
region by Friday, bringing more showers and a better chance of
thunderstorms to Yap on Friday and Koror on Saturday.

A moderate southwest swell will keep seas at 3 to 5 feet near Yap
thru the end of the week...and between 2 and 4 feet at Koror.


GU...High Rip Current Risk until 6 AM CHST Wednesday for GUZ001>004.

     High Surf Advisory until 6 AM CHST Wednesday for GUZ001>004.

Marianas Waters...None.



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