313
FXCA62 TJSJ 031821
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
221 PM AST Fri Oct 3 2025

.KEY MESSAGES...

* A strong, long-period northerly swell will continue to produce
  hazardous marine and beach conditions across the islands through
  at least early next week. Small Craft Advisories, Rip Current
  Statements, and High Surf Advisories remain in effect.

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

* Across the U.S. Virgin Islands, typical showery pattern during
  the overnight and morning hours.

* Light winds and warm temperatures are anticipated over the next
  few days, stay hydrated!

&&

.SHORT TERM...Tonight through Sunday...

The day began calm, clear, and pleasant, with cloudiness increasing
by late morning. Showers first developed across the U.S. Virgin
Islands and eastern Puerto Rico before spreading inland toward the
Cordillera Central and the southern hills, where very localized
rainfall produced up to about one-half inch. Highs reached the low
to mid 90s, and heat indices climbed into the upper 100s across
western and northern Puerto Rico. Winds stayed light and variable,
with conditions largely sea-breeze dominated during the afternoon
peak period.

Weather conditions are expected to be fair and tranquil tonight,
with mostly clear skies. Winds will stay light under the lingering
surface trough, and no land hazard risks are anticipated.
Temperatures will remain on the warm side, with overnight lows
generally in the upper 70s, and a few coastal locations holding near
80F. Later tonight, the mid-level ridge over Hispaniola and the
upper-level low to the west will begin to separate, leaving the
region marginally unstable heading into the weekend.

Highs will peak well above normal through the weekend, while warm
overnight lows provide little relief. Winds will slowly increase,
helping to provide some direction for showers and thunderstorms
instead of allowing them to remain nearly stationary. Even so,
conditions will continue to be sea-breeze dominated, favoring
afternoon activity. With instability remaining marginal and moisture
increasing to above 2 inches by late Sunday, afternoon thunderstorms
are expected, capable of producing heavy downpours. On Saturday,
flooding risk will favor the interior-central to western Puerto
Rico, while on Sunday, with more southeast flow, storms will favor
the north and northwestern Puerto Rico. Along with localized
flooding and dangerous lightning, the strongest storms could also
bring gusty winds. No tropical threat is expected during this
period, but attention will turn to what may develop next week over
the Atlantic.

.LONG TERM...Monday through Friday...
.PREV DISCUSSION... /issued 502 AM AST Fri Oct 3 2025/

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...
(18Z 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 will continue to produce hazardous marine
conditions across the Atlantic waters and local Caribbean passages
through at least early next week. As a result, Small Craft Advisories
are in effect through at least 6 PM AST Monday due to seas around
6 to 10 feet expected. Inexperienced mariners, especially those
operating smaller vessels, should avoid navigating in hazardous
conditions.

&&

.BEACH FORECAST...

A strong, long-period northerly swell will continue to produce
hazardous beach conditions through at least Monday night along
Atlantic exposed coasts, including western to northeastern Puerto
Rico, Culebra, Vieques, St. Thomas, and St. John. As a result,
there is a high risk of life-threatening rip currents in the
mentioned areas.

Additionally, there is a high potential for breaking waves exceeding
High Surf Criteria, which may result in localized beach and dune
erosion, dangerous swimming conditions, and minor coastal flooding
in the most vulnerable areas. Therefore, a High Surf Advisory is
in effect for Atlantic-exposed beaches through Monday morning.
Based on the latest data and models, the peak of this event is
expected this afternoon and evening, coinciding with high tide.
Residents and visitors are urged to monitor the beach forecast and
follow the beach flag warning system. Beachgoers, do not risk
your life, it`s better to stay out of the water throughout the
upcoming weekend!

&&

.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-
     723-741-742.

     Small Craft Advisory until 6 AM AST Saturday for AMZ745.

&&

$$

SHORT TERM/AVIATION...ICP
LONG TERM...MNG
MARINE/BEACH FORECAST...YZR