Area Forecast Discussion
Issued by NWS Tiyan, Guam

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FXPQ50 PGUM 070900

Area Forecast Discussion
National Weather Service Tiyan GU
700 PM ChST Sun Mar 7 2021

.Marianas Synopsis...
Partly cloudy skies with spotty light showers and moderate to fresh
trade winds prevail across the Marianas this evening. Buoys show
seas between 6 and 8 feet.


A dry pattern remains in place across the region this evening,
though a more moist pattern is expected over the next week. Beginning
Monday night, a band of convergence drifting north from Micronesia
will interact with an approaching trade-wind trough to generate
scattered showers over Guam and Rota beginning Monday night. Showers
could spread farther north, though this is not likely as the band
of convergence continues to weaken over the next few days. Therefore,
expect Tinian and Saipan to remain dry. By Wednesday, dry conditions
will return to Guam and Rota.

Then, on Thursday, models indicate that a weak shear line will
approach the region from the north. At the same time, a series of
weak trade-wind troughs and associated convergence will move through
the Marianas. This is expected to bring increasing showers to all
the islands, though scattered showers may begin over Guam and Rota
Thursday, then spread northward over Tinian and Saipan. The GFS is a
bit more aggressive with this scenario, indicating the possibility of
numerous showers, though the ECMWF shows the increasing activity as
well. Rainfall amounts could be as high as 1 to 2 inches for Guam and
Rota (including Monday night thru Tuesday night) and as high as 1
inch for Tinian and Saipan. By Friday night, a drier pattern is
expected to move back into the region, though the GFS is now trending
toward a more moist pattern into next week.


Latest buoy observations show seas between 6 and 8 feet. Seas are
expected to slowly subside, falling to between 5 and 7 feet Monday.
Latest WW3 guidance is trending towards even lower seas around
midweek. Moderate to fresh trade winds will subside to moderate


.Fire weather...
Dry conditions combined with winds between 15 and 20 mph will result
in an increased fire hazard through at least Monday afternoon. RHs
dropped to as low as 46% this afternoon while sustained winds of 20
mph with gusts to 30 mph, a fuel stick reading of 10 and a KBDI
resulted in a Red Flag Warning being issued through early evening.
Winds will decrease overnight, though a Fire Weather Watch will
remain in effect. The KBDI will likely increase slightly again on
Monday, with similar conditions expected. Another Red Flag Warning
is possible late Monday morning or early afternoon. Increasing
showers may bring a little relief to the fire danger Monday night
through Tuesday night as increasing showers are expected over Guam.


.Eastern Micronesia...
The ITCZ continues its weakening process this evening, with just a
few isolated showers noted in the latest analysis from Pohnpei
eastward to Majuro. Looking at the latest scatterometer plots, no
pockets of enhanced convergence (speed or directional) is
apparent, with the surface analysis showing a lack of a trigger to
foster more convective development at any of these locations as
well in the short term. Thus, look for mainly isolated showers to be
the most we can hope for through tomorrow night.

That said, there is a surface trough crossing the dateline and the
streamlines do indicate better convergence along and behind this
feature. This has allowed for convection to be more widespread
east of the dateline, and these features will trek westward for
the Wednesday through Friday time frame, with cross sections
showing increasing moisture and lift to foster better shower
coverage. Given the amount of vorticity and omega seen in the data,
a few thunderstorms will likely develop as well, with the latest
Galvez-Davidson Indices (KDI) climbing back into the 35 to 45 range,
indicative of scattered convection with pockets of locally heavy
rainfall possible.


.Western Micronesia...
Palau and western Yap state:
The main forecast challenge is: will heavy rainfall develop for Palau
and Yap as a tropical circulation, currently 575 miles southeast of
Yap, or at 3N144E, continues its slow but steady west northwest
movement. This, along with the leading edge of some trade wind
convergence to the east of Koror, roughly near 137E, and an
approaching surface trough exiting Chuuk state will all interact to
allow for good convergence in the lower to mid-levels of the
atmosphere. Given good moisture depth, plenty of lift seen in the
cross sections, with strong surface convergence, and you have a
pretty decent setup where some fairly robust showers and
thunderstorms will likely develop. The latest CIMMS MIMIC water vapor
imagery also suggests precipitable water values climbing to near 3
inches as the speed convergence near 137E passes overhead. Then with
the incoming surface trough behind it, and you`ll probably have an
extended period of potentially heavy rainfall.

The previous shift has this well-covered as mentioned in the
Special Weather Statement issued earlier this morning. The only
real change to the previous forecaster`s thoughts were to bump up
the timing of heavier rainfall for the Republic of Palau and
western Yap State, with it now expected to begin sometime Monday
afternoon versus Monday night. Otherwise, a period of heavy
rainfall and some gusty winds are likely for the Monday through
Wednesday time frame, before things settle down a bit.

Eastern Yap state and Chuuk:
Look for a mainly dry pattern with isolated showers developing
from time to time to be the main weather theme through tomorrow
evening, before some increase in rainfall potential is expected
late Monday night into Wednesday, as another weak surface trough
tries to develop/move overhead. This allows for rapid moistening
throughout the column with a quick burst of moderately strong lift
seen, mainly Tuesday night into Wednesday. This looks to be a
"quick-hitter" though, so no prolonged heavy rain threat is
anticipated. Drier air then moves in for the end of the week.


GU...Red Flag Warning until 7 PM ChST this evening for GUZ001.

     Fire Weather Watch through Monday afternoon for GUZ001.

Marianas Waters...None.


Marianas: Kleeschulte
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