586
FXCA62 TJSJ 310923
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
523 AM AST Sun Aug 31 2025

.KEY MESSAGES...

* Today, an Extreme Heat Warning will be in effect from 10 AM
  through 5 PM for the coastal and urban areas of Puerto Rico,
  Vieques, Culebra and the US Virgin Islands.

* The hot spell is expected to continue during the next couple of
  days for the US Virgin Islands and Puerto Rico, with a moderate
  to high chance of issuing an Extreme Heat Warning or Heat
  Advisory each day.

* Diurnal heating, local orographic effects and sea breeze
  convergence will promote strong afternoon thunderstorm
  development each day across portions of PR and possibly downwind
  from the USVI. In today`s case, a tropical wave moving south of
  the islands will also aid in their formation.

* The risk of rip currents will remain low through at least
  midweek.

&&

.SHORT TERM...Today through Tuesday...

A calm and mostly clear night prevailed across the region, with only
a few showers developing over the surrounding waters and some
filtering over northeastern Puerto Rico. Overnight minimum
temperatures remained in the mid-to-upper 70s across urban and
coastal areas, while cooler readings in the mid-to-upper 60s were
observed in mountainous and rural areas. Winds, similar to previous
nights, were light and variable at times.

For today, hot and humid conditions are expected once again. During
the afternoon, showers and thunderstorms are forecast to develop,
particularly across the interior and western municipalities of
Puerto Rico. As light wind flow will prevail, this activity may
become more widespread, with stronger thunderstorms possible, and
some of the activity could linger into the evening hours, even after
sunset. This pattern is consistent with recent days, as residual
moisture from the tropical wave that continues to move south of
Puerto Rico and the U.S. Virgin Islands keeps to interacting with
very warm surface and ocean temperatures.

Model guidance indicates precipitable water values between 1.70 and
1.80 inches, with 500 mb temperatures ranging from -8 to -7 degrees
Celsius. These conditions will support instability and vertical
growth of convection, increasing the potential for robust
thunderstorms capable of producing frequent lightning, gusty winds,
and heavy rainfall. Residents and visitors are strongly encouraged
to seek shelter immediately if thunder is heard or if rainfall and
lightning activity are occurring nearby.

Additionally, from 10 AM to 5 PM today, an Extreme Heat Warning is
in effect for all low-lying, urban, and coastal areas of Puerto
Rico, as well as Saint Thomas, Saint John, and Saint Croix.
Therefore, we anticipate a significant increase in the risk of heat-
related illness, and we urge residents and visitors to stay well-
hydrated, avoid strenuous outdoor activities during the peak heating
hours, never leave children or pets unattended in vehicles, and wear
light, loose-fitting, and light-colored clothing.

Looking ahead, the arrival of additional moisture by the beginning
of the upcoming workweek is expected to sustain a similar weather
pattern, characterized by excessive heat during the daytime,
followed by strong afternoon convection across the interior and
western sections of Puerto Rico.

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

A seasonal diurnal pattern is forecast during the second half of
the workweek, as patches of moisture are steered towards the
islands under east to east- southeast steering flow and weak mid
to upper-level troughiness moves over the area. Precipitable water
(PWAT) values will be at normal values for this time of the year,
with afternoon convective activity reaching values above 2
inches. Current model guidance suggests widespread PWAT values
reaching 2 inches during the weekend. A tropical wave will reach
the islands on Saturday and its moisture will linger during the
weekend. Current model guidance also suggests, an upper level low
will also moving northeast of the islands late Saturday and
Sunday. Overnight and morning passing showers will continue to
affect the eastern region. Along with the above mentioned
features, diurnal heating, orographic effects, east to east-
southeast steering flow and sea breeze convergence will promote
afternoon showers and t-storm activity, mainly over the interior
to W-NW PR, as well as downwind of El Yunque and the local
islands. Localized heavy downpours, lightning, and flooding risks
will be possible with the afternoon activity. Per the August 31st
2 AM AST Tropical Weather Outlook issued by the National Hurricane
Center, a tropical wave is forecast to emerge off the west coast
of Africa tomorrow, Monday, and has a low (30%) formation chance
in the next 7 days. With 925 mb temperatures remaining above
normal, up to elevated to possibly significant heat risk will
remain during the period. Elevated to significant heat risks
prompt the issuance of Heat Advisories and Extreme Heat Warnings,
respectively. An elevated heat risk affects most individuals
sensitive to heat, especially those without effective cooling
and/or adequate hydration. A significant heat risk affects anyone
without effective cooling and/or adequate hydration.

&&

.AVIATION...
(06Z TAFs)


VFR conditions for most of the period across all TAF sites. However,
VCTS/SHRA/TSRA associated with afternoon convection could lead to
brief periods of MVFR conditions between 31/17-23Z, particularly at
TJBQ, TJPS, and TJSJ. However, some rainfall activity could last
after sunset. Light E winds will dominate, with some variation due
to the sea breezes increasing after 31/13Z to around 315 kts, with
occasional higher gusts, particularly with the heaviest thunderstorm
activity.

&&

.MARINE...

Local effects and sea breeze variations will promote the
development of strong thunderstorms across the western side of the
islands that can move towards nearby waters. A weak tropical wave
moving across the Caribbean Sea, and a surface high pressure
system across the Atlantic Ocean will promote moderate easterly
winds with moderate chop across the regional waters through early
next week.

&&

.BEACH FORECAST...

The risk of rip currents will remain low through midweek. Even if
the risk of rip currents is low, life-threatening rip currents
often occur in the vicinity of groins, jetties, reefs, and piers.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for PRZ001>005-007-008-010>013.

VI...Extreme Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...MRR
LONG TERM....MMC
AVIATION...99