737
FXCA62 TJSJ 150920
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
520 AM AST Wed Oct 15 2025

.KEY MESSAGES...

* There is a limited to elevated flooding risk across the
  northern and eastern half of Puerto Rico. Flooding in urban
  areas, roads, small streams, and washes possible, plan for water
  ponding on roads and in poorly drained areas.

* An elevated/moderate heat threat is expected once again across
  the urban and coastal municipalities or Puerto Rico and the US
  Virgin Islands.

* Across the US Virgin Islands, there is a low chance of localized
  urban and small streams flooding, regardless people should plan
  accordingly.

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


&&

.SHORT TERM...Today through Friday...

Isolated to scattered trade wind showers moved inland from the
surrounding waters overnight, mainly affecting eastern Puerto Rico,
Vieques, Culebra, and the U.S. Virgin Islands. However, the highest
rainfall since 8 PM occurred over western Puerto Rico, where
lingering afternoon and evening showers and thunderstorms produced
around three quarters of an inch of rain from Las Marias to
Mayaguez. Overnight lows remained similar to previous nights,
ranging from the upper 60s in higher elevations to the lower 80s in
the warmest coastal spots, although only one or two coastal
stations, mainly in Saint Thomas, reported temperatures in the 80s.
The steering flow began to veer overnight, but winds remained
generally light to calm and variable over land areas, with only
brief gusts near passing showers.

A deep layer trough and an approaching pre frontal trough will begin
increasing instability and moisture today. PWAT will rise through
the day and exceed typical values, while mid level 500 mb
temperatures begin to cool, gradually improving support for deeper
convection. Low level winds will veer from the south to southwest,
steering showers and isolated thunderstorms toward the north and
northeast during the afternoon, increasing flood risk in the north
central to northeastern sections, including the San Juan metro area.
Despite higher rain chances and the proximity of the frontal and pre
frontal trough, elevated heat risk will continue due to very warm
low level temperatures, humid southerly flow, and weak surface
winds. Hazards today include elevated heat, limited to elevated
localized flooding, and limited lightning.

A much wetter and more unstable pattern will develop as the trough
deepens and moisture increases in multiple waves, while mid level
500 mb temperatures cool to below normal, creating favorable
conditions for deep convection. Low level winds will remain light
and southwesterly, slowing storm motion and allowing showers and
thunderstorms to persist. Overnight activity will favor the southern
and western coasts, while each afternoon storms will again push
toward eastern and northeastern Puerto Rico, increasing the flood
threat for the San Juan metro each day. Heat will remain a daily
concern due to very warm low level temperatures and humid southerly
flow. Hazards will remain in the same order, with heat as the main
concern, followed by flooding and lightning, and both heat and
flooding trending higher late in the week. The greatest impacts are
expected late Thursday through Friday as moisture, instability, and
slow moving storms peak.


.LONG TERM...Saturday through Wednesday..

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...
(06Z TAF)

Var WX. Early trade SHRA mainly USVI. Aftn SHRA/TSRA shifts NNE.
PR terminals (esp TJSJ) SHRA/TSRA likely 15/1622Z with brief
MVFR/IFR VIS/CIG in heavier cells. TJBQ also impacted. USVI SHRA
on/off, more at night. Most activity fades aft sunset, but may
linger TJSJ/TJBQ. Winds LGT/VRB overnight, bcmg S 812 kt with
strg sea-breeze var, then LGT/VRB aft 22Z. Greatest ops impacts:
TJSJ 15/1622Z.

&&

.MARINE...

Winds will continue to become light to gentle from the south-
southwest from today onwards. The approaching frontal boundary
will induce a pre-frontal trough for the second half of the
workweek, increasing shower and thunderstorm activity across the
regional waters and passages. Pulses of a long period easterly swell
will reach the local exposed waters Thursday into Friday. Pulses of a
long period northerly swell will reach the Atlantic waters and
passages from Friday into early next week.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents along the north and
eastern coast of Puerto Rico, with a low risk across the same
areas tomorrow. However, a long period northerly swell will
increase the threat of life- threatening rip currents along the
northern and eastern coast of Puerto Rico, and the US Virgin
Islands to moderate on Friday once again.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for PRZ001>005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

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