Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 132151

Area Forecast Discussion
National Weather Service Tiyan GU
751 AM ChST Fri May 14 2021

.Marianas Synopsis...
Partly cloudy skies, isolated showers, and moderate to fresh trade
winds made for a quiet start this Friday morning. Infrared satellite
imagery shows isolated showers associated with a surface trough to
the southeast of the Marianas. Latest buoy and altimetry data show
seas between 5 and 7 feet, mainly a mix of trade-wind swell and wind


Basic scenario for the forecast remains the same...looking forward
to pleasant weather through most of the next week. Near-term, the
surface trough southeast of Guam will continue westward. The CIMSS
MIMIC TPW imagery shows increased atmospheric moisture associated
with the trough, exceeding 2 inches south of 13N with drier
conditions farther north. Because of this, have increased showers for
Guam and Rota Saturday and Saturday night, maintaining drier
conditions for Saipan and Tinian. However, for late this weekend into
next week, the shallow-level moisture, shown daily in the model
time-heights, and the drier air farther east in MIMIC TPW imagery,
suggest a continued drier trade-wind pattern in weather conditions
across the Marianas. For late next week, the current forecast
maintains an increase in showers, similar to prior forecasts.
However, latest model support for this rain has waned slightly,
therefore, future shifts may need to consider some forecast
adjustments if models continue this trend.


Wave models show little change in sea conditions through next week
with seas staying between 5 and 7 feet. Overnight scatterometer data
showed solid 15-20 kt winds across the region with a patch of 20-25
kt winds near 148E, ahead of a westward-moving surface trough in
Chuuk State. The strongest of these winds will pass south of Guam
today, but winds will come close to the 22 kt threshold of becoming
hazardous for small craft. Drier conditions are expected for next
week with moderate to fresh trades.


.Eastern Micronesia...
Moderate to fresh trade winds dominate the entire region. Two trade-
wind disturbances, one is between Chuuk and Pohnpei near 155E and
another one is near the Date Line at 178E. A modest upper-level trough
extends northeastward from west of Chuuk at 8N145E, passing east of
Wake Island at 20N170E to beyond 25N at 180. This feature should sustain
marginal divergent flow above the region thru Saturday night. Then
another stronger upper-level trough progressing eastward from the
Philippine Sea will further enhance divergent flow by late Saturday
evening or early Sunday morning.

Patchy showers and a few thunderstorms will continue near Chuuk thru
this evening as the first trade disturbance moves across the area.
Residual convergent winds east of this disturbance will also support
similar weather near Pohnpei thru tonight. On the other hand, these
same convergent winds should take advantage of increasing divergence
aloft to trigger sporadic deeper convection near Chuuk Saturday thru
Sunday. There is the potential for substantial rainfall of 1 to 3
inches by Sunday night. To the east, slightly drier trades should
promote relatively quieter weather near Kosrae thru Saturday, and
Majuro until Saturday. As the second trade disturbance steadily
tracks westward toward the Marshall Islands, anticipate increasing
shower and thunderstorm activity near Majuro by Saturday, and Kosrae
by late Saturday evening. This same pattern will also reach Pohnpei
and Chuuk later this weekend. For next week, a trade-wind convergent
zone trailing eastward from the second disturbance will prolong
showery conditions across Chuuk, Pohnpei and Majuro. Kosrae might be
too far south of the main convergent area, so nicer weather is

Moderate trade-wind swell and wind waves are going to keep seas at 4
to 7 feet across Chuuk, Pohnpei and Kosrae into next week. Seas near
Majuro will be higher at 6 to 8 feet thru Saturday night, then
subside to between 5 and 7 feet next week.


.Western Micronesia...
A broad surface trough is between Koror and Yap near 135E this
morning. The leading edge of moderate to fresh trade winds is
approaching Yap water near 140E. An upper-level trough stretches
northeastward from a low northwest of Koror at 12N132E across the
Philippine Sea to beyond 25N at 150E.

The broad surface trough is going to spark a few showers and
thunderstorms near Koror thru tonight, and near Yap today. By this
evening, drier trades within the core of the trade surge should
provide more stable conditions near Yap. For Koror, surging trades
will increase convergence behind the surface trough and trigger more
wet weather this weekend. A trade-wind disturbance currently near
Chuuk at 155E is expected to reach Yap later this weekend, and Koror
early next week. In conjunction with the upper trough passing thru
far western Micronesia, this disturbance will introduce showery
conditions to Yap and prolong similar weather for Koror thru Tuesday
next week.

Increasing swell and wind waves generated by the surging trades will
cause seas to remain between 4 and 6 feet near Koror thru tonight,
and Yap into next week. Seas near Koror should gradually decrease to
between 3 and 4 feet next week. In addition, the leading edge of the
trade surge will cause frequent wind gusts to 25 knots across
Yap waters today; which will be hazardous for small craft.


Marianas Waters...None.


Marianas: Aydlett/Cruz
Micronesia: Chan is the U.S. government's official web portal to all federal, state and local government web resources and services.