Flash Flood and Flooding in Guam, January 8 to 11, 2023

Area Forecast Discussion
National Weather Service Tiyan GU
946 AM ChST Sun Jan 8 2023

.Marianas Synopsis...
Guam Doppler weather radar shows isolated showers moving through the
Marianas Waters, with more near Guam, and a band of scattered showers
south of the Guam Waters. The buoys reveal combined seas of 7 to 9
feet, composed of a 5 to 6 foot east swell and a 6 to 8 foot north
swell.

&&

.Discussion...
The trade-wind trough to the east and the shear line coming in from
the north do seem to be in prime position to have some non-linear
interactions between them. This could produce some pockets of heavy
rain and thunderstorms, with little or no ability to predict in
advance where those will erupt. Even with this potential, the forecast
seems like it might currently be a little heavy handed with regard to
rain. And it now looks like it could be underdone with regard to surf.
So, we might be seeing some tweaks today, general forecast philosophy
is unchanged though, just the details are a bit fluid and sliding
around.

The main body of rainfall is a little slower than was previously
forecast, so inserted a 6 hour period this morning of isolated
showers. Based on night time visible from overnight, went partly
cloudy for the skies. For Rota, Tinian and Saipan, this has worked
out well. For Guam, it's been a bit dodgy. Clouds and showers will
still increase this afternoon though, with the possibility of some
thunderstorms being added also.

The interesting part could be Thursday and Friday, where we currently
have a pretty wet forecast out. The 12Z model data didn't seem all
that wet for Guam, although it was wetter for the Northern Marianas.
This will be something to watch as the next day or two wears on.

&&

.Marine...
Based on the higher winds staying north, the higher seas staying
north, and a lack of anything this morning, canceled the small craft
advisories for Guam and Rota Waters, while maintaining them for the
Tinian and Saipan Waters. Status quo on all other advisories, however
it seems like the surf advisory might need to be extended, possibly
into Thursday or Friday.

&&

.Eastern Micronesia and Chuuk...
Moderate trade winds at Chuuk, Pohnpei and Kosrae increase to fresh
levels across the Marshalls. Showery weather is seen on satellite
around Majuro this morning as a portion of the ITCZ interacts with
upper-level divergence. Another shower complex is seen on satellite
near Pohnpei this morning.

Little change to the previous forecast. Episodes of trade-wind showers
are still expected the next several days, with the brunt of the
shower activity north of Kosrae and across the Marshalls between 5N
and 8N. Another gusty trade disturbance is slated to transit Majuro
Monday evening as supported by the latest model guidance. While
Majuro should see steady trade winds through the week, the rest of
the main islands will likely see trade winds relax to gentle levels.
This slackened trade flow may lead to a drier week as a whole for
Kosrae, Pohnpei and Chuuk. A wetter regime is modeled for early next
week as convergent trade flow rebounds.

Overnight altimetry showed seas of 5 feet across Chuuk, with a
gradual increase towards the Marshalls to between 7 and 9 feet. The
main source is background trade swell and wind waves, while distant
northerly swell continuing to trickle in from the ongoing busy storm
pattern far to the north. The overall relaxation of the trade winds
this week should allow wind waves and seas to ease a couple of feet.
By this weekend, a sizable north swell arrives as supported by GFS
and ECMWF guidance. Surf may approach hazardous levels for north
facing reefs this weekend.

&&

.Western Micronesia for Palau and Yap...
Yap and Palau reside on the north side of the near-equatorial trough,
with a collection of disorganized trade-wind troughs impinging on the
trough, inciting areas of showers and the odd thunderstorm. A
shearline is also evident N of the region, with fresh winds and larger
seas.

Forecast is largely the same as previous. The east-to-west oriented
near-equatorial trough will pivot and lift northeastward the next
few days. This should shut down the trade winds and introduce
southwesterly flow for Palau. Periodic showers are likely as a
"troughy" pattern persists through much of the week. Yap will sit on
the eastern extent of this synoptic feature, with trades becoming
variable or possibly southwesterly around midweek. It'll be that tug
of war between the monsoon-like flow advancing from the west and the
large-scale trade winds attempting to encroach from the east. If Yap
ends up in the weak flow between these two large-scale features, a
drier forecast may ensue. A ridge is likely to edge next weekend to
promote a drier pattern.

Northeast swell from the shearline will build the next few days, with
two pulses shown in the swell data; one centered on Monday/Monday
night and the second pulse late this week. The second pulse may bring
hazardous surf to north and northeast facing reefs for Friday into
the weekend. In addition to the dominant northeast swell, short-period
southwesterly swell is slated to build across Palau around midweek.

&&

.GUM WATCHES/WARNINGS/ADVISORIES...
GU...Flood Watch through Monday afternoon for GUZ001.

     High Rip Current Risk from Monday morning through late Monday
     night for GUZ001.

     High Surf Advisory from 6 AM Monday to 6 AM ChST Tuesday for
     GUZ001.

MP...Flood Watch through Monday afternoon for MPZ001>003.

     High Rip Current Risk from Monday morning through late Monday
     night for MPZ001>003.

     High Surf Advisory from 6 AM Monday to 6 AM ChST Tuesday for
     MPZ001>003.

Marianas Waters...Small Craft Advisory from 6 PM this evening to 6 AM
     ChST Tuesday for PMZ153-154.

&&

Marianas: Stanko
Micronesia: Bukunt