Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 022129

Area Forecast Discussion
National Weather Service Tiyan GU
729 AM ChST Wed Mar 3 2021

.Marianas Synopsis...
Satellite imagery shows partly cloudy skies over the Marianas and
surrounding waters. The trough of yesterday is off to the west now,
another minor wrinkle is near 154E. The deep tropical moisture
remains trapped south of 12N on our side of 180. The buoys reveal
6 to 8 foot combined seas, composed of a decaying north swell and a
combination of east trade-wind waves and swells.


The Marianas remain entrenched in a dry season trade-wind regime. The
GFS predicts 0.75 inches or less for the next 10 days, which is below
normal rainfall even for dry season. Winds for the foreseeable future
look like gentle to moderate trades, with occasional fresh values
possible. Waves will remain at 6 to 8 feet for a few days, then begin
to diminish Monday. By next Friday, they could diminish all the way
to between 3 and 5 feet.

Winds are expected to remain below red flag criteria, so no fire
weather watches are anticipated. However, the dry weather continues,
and the Keetch-Byram drought index is now 696 today and could exceed
700 Thursday, which would technically be extreme fire danger. Whether
that will affect our wind criteria for red flag events in the future
will be a subject for discussion, at this point it is not expected


.Eastern Micronesia...

The leading edge of better trade wind convergence is approaching
Kosrae this morning, and extends to the dateline, where another
pocket of enhanced convergence is noted, with the latest
scatterometer data showing 20 to 25 knots of wind at the surface.
This is also along the leading edge of higher precipitable water
values (PWATs), as they climb above 2.25 inches per the latest
CIMMS MIMIC imagery.

The overall theme is for these features to continue westward, all
the way to western Micronesia through the weekend while also
back-building to the dateline. In fact, the latest Galvez-
Davidson Index (KDI) increases from it`s current values of 20 to
30 that stretches from Pohnpei to the 35 to 45 range
by Thursday to Saturday while extending from Palau to the dateline.
To put that in perspective, the current 20 to 30 values implies
isolated to scattered shallow convection with isolated
thunderstorms possible, where as 35 to 45 implies scattered
thunderstorms with the potential to produce heavy rainfall. Timing
the arrival of this was the main forecast challenge, and our
latest thinking was reflected in the forecast. GFS PWAT forecasts
show it climbing into the 2.5 to 3 inch range which also supports
this notion, especially for Pohnpei and Majuro, as Kosrae will be
on the edge of the higher moisture/convective potential.


.Western Micronesia...

The latest MIMIC satellite data shows the higher moisture wedge
of 2.5 inch PWATs sinking south of Koror with a brief wedge of dry
air moving in from the northeast. However, another pocket of
higher moisture values is approaching Yap from the northeast,
along the tail end of a surface trough that`ll remain north of
Palau and Yap, as it continues to depart the western Marianas.
This argues for maybe a shower continuing in the short term for
these locations, with lower moisture values farther east at Chuuk,
but sufficient for an isolated shower there as well.

The better moisture potential arrives at Chuuk starting Thursday,
and at palau and Yap for the weekend, as an ITCZ becomes better
established as described above. Here too, PWATs of 2.5 to 3 inches
and KDIs of 35 to 45 all point to a wetter end to the week, with
area cross sections showing the same as well given the amount of
lift and moisture depth seen in the forecast profiles.


Marianas Waters...None.



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