Area Forecast Discussion
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FXPQ60 PGUM 210722

Area Forecast Discussion
National Weather Service Tiyan GU
520 PM ChST Sat Jan 21 2017

Latest PGUA WSR-88D and visible satellite images show isolated
showers over the Marianas this afternoon, with scattered showers
and isolated thunderstorms to the east and south of Guam coastal
waters. Partly cloudy skies with moderate to fresh trade winds
prevail over the area.


Some minor changes were made to the short term forecast for timing
of onset of scattered showers for the Marianas Sunday. Overall,
the forecast and reasoning remain unchanged. The shear line, now a
fragment near Yap, is now drifting north toward Guam. Scattered
showers and a few thunderstorms are seen on radar to the east and
south of the Guam waters. These showers will drift northward over
night, bringing scattered showers to guam and Rota Sunday morning,
then Tinian and Saipan in the afternoon. The scattered showers will
be short lived, as the shear line is already diffuse and is
continuing to dissipate, though the shower activity will remain
active. Models are still indicating that another shear line, now
located to the north of the CNMI, will affect the Marianas in the
coming week. This shear line will likely bring another round of
scattered showers and strong winds to the islands for the middle of
the week.


Latest buoy observations show seas between 5 and 7 feet across the
Marianas coastal waters this afternoon. Seas will slowly build over
the next couple of days and could reach a peak between 9 and 11 feet
on Tuesday, as another shear line approaches the islands. A strong
low pressure system to the north will produce a north swell over the
next few days, building to between 4 and 6 feet by midweek.


.Eastern Micronesia...
Moderate to fresh trade winds will continue across the region from
Chuuk to the Marshall Islands through Sunday, before another surge
in the trade winds brings strong winds back to all four locations
Sunday night into Monday.

Fairly dry and quiet conditions are now in place from Chuuk to
Majuro. A weak surface trough just east of Pohnpei is kicking off
scattered showers south to southwest of Pohnpei. This feature will
continue to move off to the west over the next few days, with most of
the activity remaining to the south of Chuuk. Farther east, a trade-
wind convergence zone is seen on the latest ASCAT data to the east
of the date line is expected to spread westward across Eastern
Micronesia over the coming days, bringing an overall unsettled
pattern back to the region through next week. The bulk of the
showers will remain to the south of Majuro and Chuuk, though more
clouds and a risk of thunderstorms will return both locations early
in the week. Pohnpei and Kosrae will likely see a return of scattered
showers around midweek.

Trade-wind swell will remain large enough to prolong hazardous surf
on Kosrae for the next several days. It will also maintain seas
hazardous for small craft operation from Chuuk State eastward to the
Marshall Islands through the week.


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

A shear line fragment near Yap produced showers and thunderstorms
over Yap and Koror waters today. This convection is on the decline
this afternoon as the shear line fragment drifts northward over the
next day or two. Scattered showers will subside through the evening,
becoming isolated by midnight, although an overall unsettled pattern
will continue through monday morning. Later in the week, the GFS is
trying to show a disturbance developing to the south of Yap and
Koror,while the ECMWF shows only a trough moving through the area
during the same time frame. This scenario will be monitored closely
over the next couple of days, as the two scenarios will result in
much different weather patterns. For now, maintained the inherited
forecast for increasing clouds and isolated showers and thunderstorms
around midweek.

Northeast swell has subsided sufficiently to allow surf to fall below
hazardous levels at Yap and Koror. However, trade-wind swell will
remain large enough to sustain hazardous seas for small craft next


Marianas Waters...None.


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