204
FXCA62 TJSJ 160932
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
532 AM AST Thu Oct 16 2025

.KEY MESSAGES...

* For Puerto Rico, limited to elevated/moderate flood risk, with
  the heaviest along northeastern PR. Therefore, ponding of water
  of road and poorly drained areas is forecast.

* For the U.S. Virgin Islands, shower and thunderstorm activity is
  expected to increase through Saturday.

* A gradual improvement in weather conditions is forecast by late
  Monday as a drier airmass moves across the islands.

* A northerly swell will lead to deteriorating marine and coastal
  conditions by the end of the weekend into next week. Stay
  updated for updates regarding the issuance of statement or
  advisories.


&&

.SHORT TERM...Today through Saturday...

During the night, GOES-16 satellite data and the Geostationary
Lightning Mapper indicated strong thunderstorms with frequent
lightning associated with an advancing frontal boundary over the
northwestern Atlantic waters. These storms later moved across the
northwestern municipalities of Puerto Rico. Several Special Marine
Warnings were issued for the affected regional waters, and a Marine
Weather Statement remained in effect for the offshore Atlantic
waters through the morning hours. In addition, Flood Advisories were
issued for western/northwestern and southwestern municipalities due
to periods of heavy rainfall that caused minor flooding and ponding
of water on roadways. The storms also produced gusty winds and
frequent lightning, with several stations reporting wind gusts in
the upper 20s to low 30s mph. A weather station in Aguada even
recorded a peak wind gust of 40 mph. For the remainder of the
morning hours, as the frontal boundary continues to move southward,
additional showers and isolated thunderstorms are expected across
northern sections of Puerto Rico, but likely will remain over the
water for the most part.

A polar trough and its associated frontal boundary will remain the
dominant features influencing local weather through the end of the
week and into the weekend. The pre-frontal trough, currently located
across the region, along with the frontal boundary situated just
north of the area, is expected to stall and linger through Saturday,
maintaining a moist and unstable environment. Model guidance
indicates that precipitable water values (PWAT) will remain above
climatological normals, fluctuating between 1.8 and 2.2 inches.
These elevated moisture levels will support periods of widespread
showers and isolated thunderstorms. Upper-level instability is
expected to persist as 250 mb height fields remain near or below
normal, while 500 mb temperatures hover around -7 to -8C, further
enhancing convective potential.

During the afternoon, a prevailing southerly component will favor
moisture convergence across the northern and northeastern portions
of Puerto Rico. Scattered to numerous showers and thunderstorms are
expected, particularly over the interior and northern/northeastern
municipalities, with activity gradually diminishing into the
evening. However, the extent and intensity of afternoon convection
will depend on cloud coverage; if cloudiness persists through the
morning and early afternoon, it could limit surface heating and
hinder convective development. Localized flooding, ponding of water
on roadways, and reduced visibility in heavy rain remain possible.

On Friday and Saturday, the frontal boundary will remain nearly
stationary as upper-level troughing deepens. Persistent moisture and
continued instability will support another round of showers and
thunderstorms, particularly across the interior and north-central
areas. Model guidance indicates elevated relative humidity in the
700-500 mb and 850-700 mb layers, along with relatively steep lapse
rates within these levels, maintaining favorable conditions for
convection.

A limited to elevated flooding threat will persist through the short-
term period, with the interior, northern, and northeastern portions
of Puerto Rico, as well as the northern U.S. Virgin Islands,
potentially experiencing the brunt of the activity. Urban and small
stream flooding will remain possible, along with frequent lightning
and gusty winds accompanying the strongest thunderstorms. Although
periods of unsettled weather are expected, activity will not be
constant throughout the period, and intervals of calmer conditions
are also anticipated. Residents are encouraged to stay tuned for
updates regarding any advisories, warnings, or statements that may
become necessary.

Temperatures at 925 mb remain above normal today, indicating
continued warm and humid conditions, particularly across coastal and
urban areas. However, extensive cloud cover could limit surface
heating, which in turn may reduce the overall heat threat. Weather
conditions will continue to be monitored, and if significant cloud
clearing occurs during the morning hours, Heat Advisories may be
required and will be issued accordingly. A downward trend in the
heat threat is expected through the weekend as the frontal boundary
lingers and cloud cover persists.

.LONG TERM...Sunday through Wednesday...

Wet and unstable weather conditions are expected to persist through at
least Monday as the remnants of a frontal boundary and its
associated moisture continue to influence the local area. This
pattern will result in increased moisture and instability,
supporting periods of showers and thunderstorm activity across the
region. The latest model guidance indicates deep-layer moisture
extending from the surface to the mid-levels, maintaining a moist
environment favorable for convection. As a result, frequent
showers and thunderstorms are expected, with activity affecting
mainly the windward and coastal sections during the nighttime and
early morning hours, and developing across the interior and
western portions of Puerto Rico during the afternoon hours. Given
the abundant moisture and instability, lightning and localized
flooding threats will range from limited to elevated, particularly
across portions of Puerto Rico through Monday.

By late Monday, a gradual improvement in weather conditions is expected
as drier air filters into the region. For Tuesday and Wednesday,
weather conditions should become more stable overall, with
moisture levels fluctuating around seasonal to below values for
this time of the year. Despite the drying trend, residual moisture
and local effects combined with daytime heating will still
promote isolated to scattered afternoon showers, mainly over the
interior and western Puerto Rico. The flooding threat during this
period should remain limited to none.

However, by the later part of the workweek, model guidance shows a
gradual increase in moisture and shower activity as a tropical
wave or potential tropical system moves south of the region. Until
then, fair weather and limited convection are expected, with
isolated afternoon showers possible over the western interior due
to local and diurnal effects.

We urge citizens and visitors to continue to monitor the forecast
for any significant changes.

&&

.AVIATION...
(06Z TAFS)

A stalled frontal boundary over and N of the area could result at
times in BKN cigs with -SHRA/ISOLD TSRA across the region this
morning, where TEMPO MVFR conds are possible. After 18Z, SHRA/ISOLD
TSRA are expected to develop over the interior and move toward the N
and NE coastal terminals due to diurnal heating and local effects,
leading to brief MVFR conds and MTN OBSC. SFC winds LGT/VRB less
than 10 kt, except near TSRA/SHRA where gusty/vrb winds may occur.

&&

.MARINE...

The approaching frontal boundary will continue to induce a pre-
frontal trough for the rest of the workweek, increasing shower and
thunderstorm activity across the regional waters and passages.
This will also increase wind speed from time to time with the
heaviest thunderstorm activity. Pulses of a long period easterly
swell will reach the local exposed waters by Thursday into Friday.
Pulses of another long period northerly swell associated to low
pressure system over the northwestern Atlantic waters will reach
the Offshore and local Atlantic waters and passages late in the
weekend into the upcoming workweek.

&&

.BEACH FORECAST...

Low rip current risk will prevail today. However, a moderate rip
current risk will return by Friday and coastal conditions will
gradually deteriorate as pulses of another long period northerly
swell increases the threat of life-threatening rip currents along
the west, north, and eastern beaches of the islands. High surf
conditions and minor coastal flooding are likely on Sunday and
Monday as the northerly swell builds seas between 6 and 8 feet
around 12 to 14 seconds, which translates to breaking waves
between 10 and 15 feet.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION...CVB
LONG TERM/MARINE/BEACH FORECAST...MMC