835
FXCA62 TJSJ 151812
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
212 PM AST Wed Oct 15 2025

.KEY MESSAGES...

* A wetter and unsettled weather pattern is forecast from Thursday
  and through the weekend due to an approaching front and a mid-to
  upper-level trough.

* Light southerly winds and high humidity could promote an elevated
  heat threat briefly on Thursday before the onset of afternoon
  showers across most coastal counties. However, early morning cloud
  cover from the approaching front could limit the threat.

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

* A northerly swell will lead to deteriorating marine and coastal
  conditions by the end of the week into next week.

&&

.SHORT TERM...Tonight through Friday...

Mostly clear skies prevailed across the region during the morning
hours, with just a few clouds observed across eastern sections and
local waters, but no rainfall activity was detected. As the day
progressed, heat indices climbed, reaching up to 110 degrees along
the north-central and southwestern sections of Puerto Rico. At 12
PM, satellite imagery showed an increase in cloudiness and
showers along the Cordillera Central and some northern sections of
Puerto Rico. Winds were from the south up to 15 mph with
occasional gusts.

A wetter and more unsettled weather pattern is forecast to develop
later tonight and persist through the rest of the short term. In
the mid to upper levels, a deep-layered trough will continue to
sink across the islands, increasing instability throughout the
column as suggested by the 250 MB heights and 500 MB temperatures,
which will range from -8 to -7 degrees Celsius. Combined with
this instability, at the surface, the approach of a frontal
boundary will result in increased precipitable water values across
the islands. According to global model guidance, PWAT values will
range near normal, around 1.90 to 2 inches each day. Given the
expected conditions, residents can anticipate an increase in
showers and thunderstorms across the islands. The focus of the
activity will also be influenced by the presence of the surface
frontal boundary, as surface winds weaken due to the prefrontal
trough on Thursday, becoming westerly on Friday. As a result,
showers will be concentrated across the north and northeastern
areas, including portions of the southern coastal plains.
According to the Glvez-Davison Index (GDI), isolated to scattered
thunderstorms are expected on Thursday afternoon along the north
coast, becoming more widespread on Friday across the interior and
central-eastern Puerto Rico.

Under this pattern, cloudiness is forecast each day during the
period; however, the prevailing surface wind flow will maintain
warm conditions across the area, leading to elevated heat indices
along the northern, coastal, and urban zones. Therefore, a limited
to elevated heat threat will continue on Thursday and Friday.
Residents and visitors are advised to follow health department
recommendations to prevent symptoms of heat exhaustion.

.LONG TERM...Saturday through Wednesday... from previous
discussion issued 520 AM AST Wed Oct 15 2025...

As mentioned in previous discussion the wet and unstable weather
conditions are forecast to persist through the weekend, with
gradual improvement early in the workweek. On Saturday, winds
should remain light, shifting from the southwest under the
influence of a surface high pressure system over the central
Atlantic and a broad polar trough. This pattern will continue to
pool the remnants of a frontal boundary and abundant tropical
moisture over the region. Based on the latest model guidance
Precipitable Water (PWAT) values will remain seasonal to above
normals (2.00  2.30 inches) for this time of the year.
Additionally, the polar trough reflected at the mid to upper
levels may deepen enough to cool 500 mb temperatures (-7 to -8
degrees Celsius), enhancing instability and supporting deep
convection. The Galvez-Davison Index (GDI) also suggests potential
for isolated to scattered thunderstorms through at least Sunday.
As a result, showers and thunderstorms are expected to remain
frequent, affecting mainly windward sections during nighttime and
early morning hours, while afternoon convection develops across
the interior and western mountains of Puerto Rico. Given the
enough moisture and unstable conditions, lightning and localized
flooding threats will range from limited to elevated through the
weekend.

By late Monday, conditions should begin to improve as a drier air
mass filters into the region. Model guidance indicates a decrease
in PWAT values to around 1.7  1.9 inches, though variability
remains high among ensemble members due to a tropical wave
projected to pass south of the forecast area. Despite this,
daytime heating, local effects, and residual moisture will still
promote afternoon shower and thunderstorm development, mainly
across the western and northwestern portions of Puerto Rico,
though the flooding threat should remain limited.

Tuesday and Wednesday are expected to feature fewer showers and
more stable conditions overall, with moisture levels fluctuating
around seasonal values. Isolated convection may still occur each
afternoon over the western interior due to local effects, but
widespread activity is not anticipated.

&&

.AVIATION...

(18Z TAFS)
SHRA/TSRA expected to linger over NNE thru 15/23z, causing TEMPO
MVFR to IFR conds at TJSJ. Elsewhere across the PR terminals,
mostly VCTS. USVI SHRA on/off, more at night, particularly at TIST
and overnight across TJBQ/TJSJ due to an approaching front from
the NW. The 15/12z TJSJ sounding indicated south winds up to 6 kt
blo 7000ft. Light S-SSW winds will prevail through the fcst
period.

&&

.MARINE...

A frontal boundary approaching the region from the Atlantic waters
will induce a pre-frontal trough over the northeastern Caribbean
from Thursday into the weekend. This will promote light south to
southwesterly winds through Saturday, and an increase in showers and
thunderstorms across the regional waters and passages. Pulses of a
long period northerly swell will reach the Atlantic waters and
passages from Friday into early next week. Hazardous seas building
up to 8 feet are expected late Sunday into Monday, triggering Small
Craft Advisory conditions across the Atlantic waters and Caribbean
Passages. Moderate to fresh trades return from Sunday onward.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents along the north and eastern
coast of Puerto Rico. However, by Friday afternoon into early next
week, pulses of a long period northerly swell will increase 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...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...LIS
LONG TERM....MIDNIGHT CREW
AVIATION, MARINE, BEACH FORECAST...DS