104
FXCA62 TJSJ 041856
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
256 PM AST Thu Sep 4 2025

.KEY MESSAGES...

* Afternoon convective activity will produce showers and isolated
  thunderstorms over the western municipalities of Puerto Rico.

* A tropical wave, with it axis now crossing the Lesser Antilles,
  will increase moisture and instability over the region from
  Friday into early next week. Therefore, increasing the flood
  and lightning threat over the islands.

* For the U.S. Virgin Islands, variable weather conditions will
  persist during the next several days, with warm to hot
  temperatures expected through early next week.

* The National Hurricane Center is monitoring a broad area of low
  pressure (now Invest 91L) over the eastern tropical Atlantic
  with a high chance of development in the next 7 days. The
  closest approach could be sometime by the end of next week. It
  is important to continue monitoring the forecast as the week
  progresses.

&&

.SHORT TERM...Rest of Afternoon through Saturday...

During the morning hours, only isolated showers were observed across
portions of the U.S. Virgin Islands and eastern Puerto Rico.
Afternoon activity was less widespread and less intense than
previously expected, mainly due to reduced moisture content and
likely subsidence ahead of the approaching tropical wave, which
currently is located just east of the Leeward Islands. GOES-16 PWAT
imagery confirms the deeper moisture field remains just east of the
USVI.

Moisture levels are forecast to increase, with model guidance
suggesting precipitable water values exceeding 2.0 inches across the
region beginning late tonight. At the same time, the proximity of a
strong mid- to upper-level trough will provide additional
instability aloft, although the most favorable dynamics will stay
north of the forecast area.

As this pattern evolves, shower activity will increase late tonight
into early Friday morning, particularly across the USVI and eastern
Puerto Rico. A few isolated thunderstorms over the waters could
drift inland. By Friday afternoon, as the axis of the tropical wave
crosses the region, convection will focus across the interior and
western to northwestern Puerto Rico. Still, if extensive cloudiness
persists, this may limit the intensity and coverage of afternoon
storms. The wave axis should move into the Mona Passage by Friday
night, but trailing moisture and the influence of the upper-level
trough will sustain unsettled conditions into the weekend. Through
the short term period, the flooding threat will range from limited
to elevated. Urban and small stream flooding is possible, with
localized flash flooding in areas receiving the heaviest rainfall,
especially in flood-prone zones. Landslides over steep terrain
cannot be ruled out.

Meanwhile, hot conditions will continue. Heat indices are forecast
to reach around 108F or higher across urban and coastal areas of the
islands. However, if widespread cloud coverage develops,
particularly on Friday, it could mitigate heat stress in some
locations. The issuance of Heat Advisories are likely, particularly
during the weekend. Hazy skies are expected at times as a Saharan
Air Layer with minor dust concentrations accompanies the wave
passage.

.LONG TERM...Sunday through Thursday...
(from previous discussion)

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...

(18Z TAFS)

Prevailing VFR conds thru the aftn with only VCTS psbl mainly over
TJBQ. Sfc winds E 10-15 kt with sea breeze variations and ocnl
higher gusts thru 04/23z. Aft 05/03Z, SHRA/ISOLD TSRA expected to
increase across USVI and ern PR terminals (TIST/TISX/TJSJ) as deeper
moisture from a tropical wave arrives. Brief MVFR conds cannot be
ruled out. By Fri aftn, SHRA/TSRA will favor interior and W/NW PR,
affecting mainly TJBQ,  with mtn obsc likely. Light HZ due to
Saharan dust at times, but VSBY should remain mostly abv 6SM.

&&

.MARINE...

A surface high pressure over the central Atlantic and a surface
trough will continue to result in moderate to locally fresh east to
southeasterly winds across the local waters, producing moderate
chops to localized choppy seas. Seas will remain up to 5 feet across
the Atlantic offshore waters and even lower across the coastal
waters. From Friday onward, the arrival of a tropical wave is
expected to increase the frequency of showers and thunderstorms,
potentially resulting in localized hazardous marine conditions.

The National Hurricane Center is monitoring an area of low pressure
(Invest 91L) over the eastern tropical Atlantic. This system is
expected to approach the Caribbean region at some point by the end
of next week, deteriorating marine and coastal conditions across the
local and offshore Atlantic and Caribbean waters. Mariners are
advised to continue monitoring the progress of this system as it
moves over the Central Atlantic.


&&

.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 until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.

&&

$$

CVB/GRS