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908
FXCA62 TJSJ 030902
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
502 AM AST Fri Oct 3 2025

.KEY MESSAGES...

* A strong, long-period northerly swell will create hazardous
  marine and beach conditions across the islands through at least
  early next week.

* Showers and thunderstorms in the afternoon over will enhance the
  flooding and lightning threat for the next few days. If thunder
  roars, stay indoors.

* Occasional passing showers are expected across the U.S. Virgin
  Islands tonight through early Saturday.

&&

.SHORT TERM...Today through Sunday...

Tranquil conditions prevailed during the night hours across Puerto
Rico and the Virgin Islands. Some showers managed to form in the
local waters, but otherwise, skies were mostly clear.

A series of deep-layered longwave troughs are maintaining very weak
winds across the islands. In fact, the steering flow is expected to
be from the northeast at 4 to 8 kts today, from the southeast at 4
to 10 knots on Saturday, and also from the southeast on Sunday, but
a little stronger, with speeds around 10 kts. There is also a weak
low to mid level ridge centered west of Puerto Rico. This feature
will keep moisture below normal for today. Even though the
atmosphere is not prime for widespread significant rainfall, diurnal
heating will still trigger showers and isolated thunderstorms across
the Cordillera Central in Puerto Rico. Since winds are light, some
localized urban flood could develop, while isolated lightning
strikes cannot be ruled out. In the vicinity of the Virgin Islands,
some showers are expected to develop too, although mostly staying
over the local waters.

On Saturday, an upper level low will move over Hispaniola, then
advancing toward the local islands by Sunday. This feature will
induce some showers over the local waters, potentially reaching the
Virgin Islands and portions of eastern Puerto Rico. It is worth
mentioning that moisture at the mid levels will remain below normal,
although it will increase a little at the surface. Nevertheless,
these conditions should still support afternoon showers and isolated
thunderstorms each afternoon. On Saturday, the focus of the rain
should be along the Cordillera Central. On Sunday, the western
interior and northwestern portions of Puerto Rico are favored. These
areas will likely experience localized urban and small stream flood.
Thunderstorms are not expected to be too widespread, but some
lightning strikes are still anticipated each afternoon.

&&

.LONG TERM...Monday through Friday...

The long-term forecast remains on track, with a wet and unstable
pattern for the beginning of the workweek. Recent model guidance
indicates an upper-level cutoff low over the region, which is
expected to gradually move south and migrate westward by the end of
the period. As mentioned in previous discussions, the deterministic
guidance from the GFS and ECMWF keeps suggesting an increase in
moisture content, with PWAT values likely to range between 2.0 - 2.2
inches (up to or above climatological values). Additionally,
ensemble members continue to suggest a wetter and unstable pattern
across the CWA on Monday and Tuesday, with Monday through early
Tuesday being the wettest period. A difference between the previous
solution runs is related to instability, as 500 mb temperatures look
slightly warmer (between -6.5 to -7.0 degrees Celsius).
Nevertheless, the latest Galvez-Davison Index (GDI) solution keeps
suggesting the potential of isolated to scattered thunderstorms
across the CWA, but not a widespread scenario. Another factor that
may influence the weather pattern is the low-level wind speed,
expected to increase and remain E-SE during that period. The most
likely scenario remains the same, with an increase in shower and
thunderstorm activity across the windward sections of Puerto Rico
and the U.S. Virgin Islands throughout Monday and early Tuesday. The
deeper convection should be expected in the afternoon, particularly
over the mountain ranges and western/northwestern Puerto Rico. Given
the expected conditions, the flood threat will remain limited to
elevated, and a limited lightning threat is also expected across the
aforementioned areas. The gradual improvement of weather conditions
is still anticipated by Wednesday, as a drier air mass filtering
into the region will promote stability aloft. Nevertheless, this
pattern may increase the heat threat, becoming elevated to
significant on Wednesday and Thursday.

The National Hurricane Center is monitoring a tropical wave expected
to move off the coast of Africa in the next few days, with a 20%
chance of cyclonic formation for the next 7 days. The latest model
solutions continue to suggest an increase in moisture content by the
end of the forecast period due to this tropical wave approaching the
Caribbean Basin, but the variability between them continues high in
terms of trajectory and intensity. Hence, the uncertainty remains
high for Fridays forecast.

&&

.AVIATION...

(06Z TAFS)

Mainly VFR conditions are expected to
prevail. SHRA is expected after 17Z along the Cordillera Central,
causing mountain obscuration. VCTS are also expected for TJPS
after 17Z, with brief periods of reduced VIS and low ceilings.
SHRA in the vicinity of the USVI should cause little to no impact
to operations. Winds are light, coming from the NE and below 10
knots.

&&

.MARINE...

A col near the region will promote a light to gentle variable wind
flow across the islands for the next few days. A long- period north
to northwesterly swell arriving into the region will spread across
the local Atlantic waters and passages today into the weekend,
deteriorating marine and coastal conditions through early next week.
Small Craft Advisories are in effect for the Atlantic offshore and
coastal waters, including the Mona and Anegada Passages.

&&

.BEACH FORECAST...

A strong, long-period northerly swell arriving into the region
will create hazardous beach conditions through at least early next
week. Hence, the high rip current risk is in effect for exposed
coasts to the Atlantic, including western to northeastern Puerto
Rico, Culebra, Vieques, St. Thomas and St. John through at least
Monday night. As mentioned in previous discussions, the potential
of breaking waves exceeding High Surf Criteria is very high,
resulting in localized beach/dune erosion and dangerous swimming
conditions. Therefore, a High Surf Advisory is in effect for the
exposed beaches to the Atlantic as well, from northwestern to
northeastern Puerto Rico, Culebra, and St. Thomas and St. John
through Monday morning. Citizens and visitors are encouraged to
continue monitoring the beach forecast and heed the advice of the
flag warning system. Beachgoers, do not risk your life, is better
to stay out of the water!

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Monday night for PRZ001-002-
     005-008-010-012-013.

     High Surf Advisory until 6 AM AST Monday for PRZ001-002-005-008-
     010-012.

     High Rip Current Risk through Saturday afternoon for PRZ011.

VI...High Rip Current Risk through late Monday night for VIZ001.

     High Surf Advisory until 6 AM AST Monday for VIZ001.

AM...Small Craft Advisory until 6 PM AST Monday for AMZ711-712-716-
     741-742.

     Small Craft Advisory from noon today to 6 PM AST Monday for
     AMZ723.

     Small Craft Advisory until 6 AM AST Saturday for AMZ745.

&&

$$

SHORT TERM/AVIATION...ERG
LONG TERM...MNG
MARINE/BEACH FORECAST...ERG/MNG

NWS SJU Office Area Forecast Discussion