Area Forecast Discussion
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FXPQ60 PGUM 220744

Area Forecast Discussion
National Weather Service Tiyan GU
342 PM ChST Tue Aug 22 2017

.Marianas Synopsis...
A surface trough just east of the Marianas generated showers and
thunderstorms earlier today about 100 miles to the east but
satellite...and that convection has been slowly
dissipating this afternoon. An upper-level trough stretching from
the Northern Marianas toward Chuuk State has also helped trigger
the deep convection in the region.


Decided to make a few adjustments in the first half of the forecast
by increasing showers and bringing thunderstorms back into the
forecast tonight. With the trough moving through, there could be just
enough instability to produce scattered showers late tonight into
early Wednesday morning with gradual clearing through the day. Models
maintain a conducive pattern for isolated convection the next few
days, so have kept isolated thunderstorms for the remainder of the
week. Another surface trough is expected to pass through the area
late in the week so have increased showers to scattered around
Friday. Timing will be a little uncertain for the moment, but late
week does look to be somewhat wetter than the preceding days. Drier
weather is expected later in the weekend and early next week.

Something worth noting in the model outlooks is the difference
between the GFS and ECMWF. The ECMWF keeps the monsoon present over
the Philippine Sea the next couple of days and then extends it
eastward across the Marianas over the weekend before lifting it
northward early next week. Although it shows gentle winds, the
directions would be from the south to southwest versus east, and we
could see more unsettled weather. On the other hand, the GFS has the
monsoon trough retreating westward in the wake of TS Hato, now near
Luzon, and easterly winds coming back into the area. It is possible
GFS could be eroding the near-equatorial trough across central and
eastern Micronesia too quickly. Subsequent surface analyses and model
guidance and initialization will be useful to determine which model
performs best.


Little change is expected in sea conditions through the weekend as
wave models show similar conditions into early next week. Have
slightly lowered east-facing surf heights based on recent surf
observations reporting max heights of 3 to 4 feet, and altimetry and
buoy data confirming model forecasts of seas between 2 and 3 feet.


.Eastern Micronesia...
Lots of rain today for Majuro as a result of the passing of a weakly
defined trough that is expected to move off in the direction of
Kwajalein by tomorrow morning. Isolated, to possibly scattered,
showers may linger over Majuro until tomorrow afternoon before drier
air is predicted to move in by the models. Kosrae shows drier
conditions prevailing for the next couple of days as it remains
within a buffer ridge south of the trough to the north. Look for more
climatological conditions to come back in by Friday. Pohnpei is now
within the east-west trough axis so expect isolated to some periods
of scattered showers to come in with the diurnal cycle. Nothing
significant shows up for any these locations from the models.

Seas are quiet and expected to be no greater than 3 to 4 ft for
Majuro and Kosrae and 2 to 4 ft for Pohnpei.


.Western Micronesia...
Latest satellite imagery and synoptic data show Yap and  Koror along
the edge of the monsoon flow moving into the circulation to the
northwest...Tropical Storm Hato...and a mostly dry buffer ridge to
the south and east extending back as far as Chuuk. Once the current
weather moves on...models have mostly isolated showers for Yap and
Koror. I have partly cloudy and isolated for these two beginning
during the day on Wednesday...but there is a possibility of a slight
delay if things don`t move off to the west as quickly as expected.
Chuuk is along the southeast side of a rather vigorous trough
located between them and Pohnpei. The models are not handling this
very well. In this case I`ve delayed the drier conditions for Chuuk
for not earlier than Thursday. The weak circulation mentioned in the
SIMGUM northwest of Pohnpei is not expected to directly affect Chuuk.

Couple of changes to the seas forecast for all three islands. Wave
Watch III models and altimetry show very calm conditions. All three
islands are in weak surface wind flow with no major synoptic feature
expected to affect them during the week. Hence, combined seas are
expected to stay below 3 ft or less for all locations.


Marianas Waters...None.


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