392
FXCA62 TJSJ 040830
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
430 AM AST Thu Sep 4 2025

.KEY MESSAGES...

 * Another warm day is expected today, with heat indices up to 111
   degrees across coastal and urban areas, and potentially even
   higher in localized areas during peak heating from 11 AM to 2
   PM AST.

 * Afternoon convection with thunderstorms will trigger an
   elevated threat of flooding and a limited lightning threat
   across northwestern Puerto Rico.

 * An increase in moisture and instability will induce a wet and unstable
   pattern from Friday into Monday, increasing the flood threat
   and the lightning threat along the islands.

 * For the U.S. Virgin Islands, variable weather conditions will
   persist, with warm temperatures expected through the upcoming
   weekend.

* There is a low risk of rip currents across the islands today
  and into the upcoming weekend.

&&

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

Mostly clear to partly cloudy skies were observed across the islands
overnight. Passing showers where noted mainly over the regional
waters under light steering easterly winds, with a few reaching
portions of eastern PR, where the Doppler radar estimated around a
quarter of an inch of rain in Humacao and vicinity. Minimum
temperatures were from the mid-70s to low 80s across the lower
elevations of the islands to the low-60s and mid-70s across the
higher elevations of Puerto Rico.

A broad surface high pressure will continue to build over the
central Atlantic during the next few days, promoting moderate trade
winds through the forecast period. Embedded in this flow today is a
drier air mass with precipitable water (PWAT) values below
climatology, which will move across the region. However, shallow
moisture content trapped below 850mb will be sufficient to aid in
the development of afternoon showers and thunderstorms over western
PR. Across the USVI, passing showers could move at times,
particularly tonight through Friday morning, as shower activity is
expected to increase ahead of a tropical wave currently located east
of the Leeward Islands. Temperatures will remain similar to
yesterday, with daytime highs ranging from the upper 80s to low 90s
across most coastal areas. Heat indices will peak around 108F,
mainly across western and northern PR. Regardless, due to plenty of
sunshine expected elsewhere, a Heat Advisory was issued for all
coastal municipalities in PR and the USVI.

The tropical wave is expected to cross the local area on Friday,
resulting in an increase in shower and thunderstorm activity through
the region. The PWAT is likely to recover at normal levels, around
2.00 inches, which is the high end of the 50th percentile. The main
hazards will be urban and small stream flooding, as well as gusty
winds and lightning strikes with the thunderstorms. Although hot
temperatures are still expected on Friday, the cloud cover and
showers could limit the heat threat in certain areas. A weak Saharan
Air Layer with minor concentrations of Saharan dust will arrive with
the wave passage, bringing hazy skies and affecting people sensitive
to these particles.

On Saturday, unstable conditions will continue across the region as
an upper-level low north of the area and associated trough combine
with lingering low-level moisture from the departing wave. The flood
threat will remain elevated, with possible isolated flash floods and
mudslides in areas of steep terrain, particularly across western PR.

.LONG TERM...Sunday through Thursday ...

The forecast for the first part of the long-term remains on track.
An upper-level low, just north of the region, will continue to
increase the instability aloft and induce colder temperatures at
500 Mb, ranging from -6 to -7F. Meanwhile, at the surface, plenty
of tropical moisture will move with Precipitable water (PWAT)
ranging between 1.8 to 2.0 inches will move into the area,
increasing the potential for shower activity. As a result, the
islands will experience a showery pattern from morning hours
across the windward section of the islands, and widespread
activity across western Puerto Rico in the afternoon hours.
Especially across northwestern Puerto Rico, where there is a
potential for scattered thunderstorms as the Galvez-Davison Index
(GDI suggests. Instability will remain in place from Monday into
Wednesday as the upper-level low/trough extends from another area
over the Western Caribbean. This upper-level feature will induce a
surface trough, resulting in plenty of cloudiness and moisture
along the islands. At the surface, winds will predominate from the
east-southeast as a surface high-pressure system over the central
Atlantic dominates the area. This wind flow will push the sower
activity along the San Juan metro area and northwestern sections
of Puerto Rico.

For Thursday, the forecast remain with a lot of uncertainty due
to two different solutions from the global model guidances (GFS &
ECMWF). According to the latest data, there are some discrepancies
in the weather patterns along the islands and the surface winds
due to the approach of the next tropical system monitored by the
National Hurricane Center, with a high chance of formation in the
next seven days. According to the GFS and with consistency, the
tropical system will move northeast of the region, resulting in
mostly east-southeast winds across the islands. On the other hand,
the ECMWF, with different solutions from yesterday, located the
system slightly closer to the islands for the weekend. Although
the long-term forecast only includes the Thursday forecast,
residents and visitors are urged to check the tropical emergency
plans and stay tuned for further official information regarding
this tropical wave.

&&

.AVIATION...
(06z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hour. However, locally induced TSRA is expected
to develop in and around TJBQ between 04/16-22Z, causing tempo MVFR
to brief IFR conditions. Elsewhere, mostly VCSH expected. East winds
will increase between 10 and 15 knots with sea breeze variations and
higher gusts after 04/13z.


&&

.MARINE...

A surface high pressure across the central Atlantic and an induced
surface trough just northeast of the islands over the Atlantic
will continue to promote moderate to locally fresh east to
southeasterly winds across the local waters today, with seas
generally up to 5 feet. Afternoon showers will result in choppy
seas, gusty winds, and localized hazardous seas, reducing
visibility in heavy rainfall.

Weather conditions are expected to deteriorate by late Friday
night into the weekend as a wet and unstable weather pattern
increases the frequency of showers and thunderstorms, leading to
periods of locally hazardous marine conditions

&&

.BEACH FORECAST...

There is a low risk of rip currents along all coastal areas today
through the upcoming weekend. Coastal conditions will be generally
favorable for beachgoers and visitors. However, despite the good
conditions, caution is advised as other coastal hazards like
lightning, especially during the afternoon hours.

&&

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

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

AM...None.
&&

$$

SHORT TERM & AVIATION...DS
LONG TERM & MARINE....LIS