High Surf in Guam, December 25 to 30, 2022

Area Forecast Discussion
National Weather Service Tiyan GU
721 PM ChST Sat Dec 24 2022

.Marianas Synopsis...
Partly cloudy skies with spotty showers and gentle to moderate winds
prevail across the Marianas this evening. Buoys show seas between 4
and 5 feet while altimetry shows seas up to 8 feet east of the
islands.

&&

.Discussion...
Much of the area is looking drier this evening than previously
expected. Latest MIMIC Precipitable Water (PWat) data shows PWats
remain in the 2 to 2.25 inch range with Galvez-Davison Index (GDI)
remaining in the upper 30s to low 40s. These values indicate that
yet another round of showers and thunderstorms are still possible.
CIMMS MIMIC does show increasing PWats coming in from the north.
However, convection looks to be weaker than previously indicated.
Regardless, satellite shows a fairly clear pattern over the
islands, with an approaching shear line seen over the northern CNMI.
Satellite shows this shear line extending northeast from near 2N130E
to near 19N147E, then continuing as a decaying stationary front
moving very slowly south. This shear line/front will continue to
approach the Marianas over the next couple of days. Meanwhile, IR
satellite shows a mid-level circulation to the east that will bring
things together, resulting in yet another round of showers tonight.
Otherwise, region near midnight. As the shear line moves into the
area, winds will gradually increase, with gusts of 30 to 35 mph
likely by the middle of next week. A drier pattern is then expected
later next week.

&&

.Marine...
Himawari satellite data shows a strong shear line to the north.
As it approaches the region, winds will gradually increase,
possibly reaching levels hazardous to small craft early next week.
Preliminary indications are that fresh to strong winds will require
yet another Small Craft Advisory around the middle of next week.
However, this could change if the shear line weakens or if,
obviously, it remains north of Saipan. This scenario, though, is not
likely to occur. Combined seas between 5 and 8 feet are likely, as
indicated by altimetry and buoy observations. A larger north swell,
developed from near-gale to gale force winds off of Japan, will
affect the region Christmas evening into next week. This would
yield a high risk of rip currents along north facing reef and a High
Surf Advisory as well.

&&

.Eastern Micronesia...
The peak in convective activity across the region is occurring along
a trade-wind trough at the leading edge of an overall weak ITCZ
pattern. This feature created moderate to strong convection over
Pohnpei and Kosrae earlier today, but has lost most of it convective
vigor this afternoon. The remnants of this disturbance will move
through Pohnpei tonight and showers perk up again once it progress
over Chuuk Christmas day and into the evening. Drier air and weak
convergence behind this feature provide a break in heavy weather for
the first half of next week, oscillating back and forth between
isolated and low-end scattered showers. The latter half of the week,
the ITCZ strengthens, mainly over Kosrae and Majuro Wednesday night
and tilting northward to include Pohnpei by Friday. The bulk of ITCZ
showers look to stay south of Chuuk next week, but can still
occasionally scrape by and give Chuuk intermittent scattered
showers.

High pressure passing to the north of the area will increase the
pressure gradient and this will cause an increase in winds. Breezy
conditions are possible Wed-Thu for Chuuk, Wed-Fri for Pohnpei, and
Thu-Fri at Kosrae.

A High Surf Advisory remains in effect for east-facing reefs of
Kosrae through early Sunday morning. The Coastal Flood Advisory has
been canceled for Majuro since model projections have not depicted
an overwhelming signal the last few runs and as we transition away
from the new moon cycle, conditions are expected to trend down.
Nonetheless, minor coastal inundation of about a foot or less is
still possible over the next few days. Other marine hazardous to
account for will be building long-period north swell generating
hazardous surf along north-facing reefs in the latter half of next
week and an increase in pressure gradient that may bring breezy
conditions at Pohnpei Thursday and Friday, and Thursday at Kosrae.

&&

.Western Micronesia...
A monsoon-like trough now runs alongside the shearline, about 350
miles to the north. Mainly isolated showers are seen across the
islands, but a subtle trough within the monsoon flow will cause
conditions to deteriorate late tonight through Sunday. The shearline
looks to sink south a little further, but not cross 10N. Convergence
between monsoon southwesterlies and the shearline will maintain
scattered showers in forecast through Sunday night. Unsettled weather
continues through the first half of the week due to a circulation
developing at the end of the shearline (northwest of Palau) and a
near-equatorial trough developing to the south. As the circulation
lifts north, a diffluent pattern is induced Thursday, however, this
is short-lived since trade convergence north of the near-equatorial
trough builds in towards the end of the week.

A high surf advisory along north-facing reefs was issued for early
Sunday morning through Tuesday due to swell generated from strong
winds north of the shear line. This in combination with King Tides
and La Nina effects, increases the concern for coastal flooding. An
addition of a headline may be warranted in the next few forecast
cycles.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...None.
MP...None.
Marianas Waters...None.
&&

Marianas: Kleeschulte
Micronesia: Bukunt/Cruz