241
FXCA62 TJSJ 020739
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
339 AM AST Tue Sep 2 2025

.KEY MESSAGES...

 * An increase in low-level moisture and warmer temperatures will
   exacerbate heat indices in north-central areas and southern coastal
   plains. Given the expected conditions, an extreme heat warning
   is in effect from 10 AM to 5 PM AST today.

 * For today, there is a elevated flood and lightning threat
   along the northwestern Puerto Rico from 12 to 5 PM AST.

 * For the upcoming weekend, a tropical wave, combined with an upper-
   level trough, will increase the potential for showers and
   thunderstorms, increasing the flood threat.

 * For the U.S. Virgin Islands, variable weather conditions will
   persist, with warm temperatures expected through the upcoming weekend.
   The arrival of a tropical wave is anticipated to increase
   shower frequency across the islands.

&&

.SHORT TERM...Today through Thursday...

Clear to partly cloudy skies prevailed across the islands during the
overnight hours. Scattered showers with isolated thunderstorms were
noted mainly over the northwestern waters of Puerto Rico, and the
Anegada Passage just before midnight. Showers are expected to move
over portions of the USVI and the eastern sections of PR through the
rest of the early morning hours. Minimum temperatures were from the
upper-70s to low 80s across the lower elevations of the islands to
the mid-60s and 70s across the higher elevations of Puerto Rico. The
wind was light with an easterly component.

A broad surface high pressure over the central Atlantic and front
across the northwestern Atlantic will continue to promote light to
moderate (10-15 kt) southerly steering winds through Wednesday.
These winds will bring a surge in low-level moisture from the
southeastern Caribbean today. Meanwhile, an upper-level low north of
the area will move southwest, just north of Hispaniola by Wednesday,
promoting colder 500 mb temperatures and increasing instability in
general, which will aid in the development of strong afternoon
thunderstorms each day. A tropical wave will cross the region late
on Thursday, bringing scattered showers and thunderstorms across the
regional waters and over portions of the islands late in the
forecast period.

At least through Wednesday, a similar weather pattern is expected,
with showers increasing in and around the U.S. Virgin Islands and
across the east/southeast sections of Puerto Rico during the morning
hours. This will then be followed by afternoon thunderstorm
development over portions of the interior and the northwestern
quadrant of PR, as well as downwind from el Yunque area and over
portions of the Cordillera. The main weather hazards will be urban
and small stream flooding, strong gusty winds, and lightning
strikes. In addition, the light southeasterly winds will continue to
promote hot temperatures across the lower elevations of the islands,
and heat advisory conditions are expected before the onset of
afternoon showers.

On Thursday, winds are expected to shift more from the east,
remaining around 15 kt as the weak tropical wave moves across the
region. The best moisture associated with the wave will arrive late
in the day. Therefore, fair weather conditions are expected during
the morning hours across the islands, with afternoon showers and
thunderstorm development over portions of the Cordillera and western
PR, and downwind of the islands.

.LONG TERM...Friday through Tuesday...

The long-term forecast remains on track. According to the latest
global model guidance for Friday, the islands will be mostly
dominated by a broad surface high pressure over the central
Atlantic. Under this pattern, sufficient cloudiness and moisture
will support a typical weather regime, with variable conditions in
the morning followed by afternoon convection. An increase in
showers and thunderstorms is forecast from late Friday into the
weekend, as an upper-level trough near 250 mb enhances instability
across the region. This feature, combined with abundant tropical
moisture from a tropical wave, will favor stronger convection with
frequent lightning, gusty winds, and periods of heavy rainfall.
Based on the Glvez-Davison Index (GDI), there is a high chance of
widespread showers and thunderstorm activity each afternoon
through the weekend. This activity is expected to range from
moderate to intense, leading to urban and low-lying flooding.
Additionally, the threat of lightning will increase, particularly
across the western interior and the San Juan metro area. From
Monday to Tuesday, moisture will diminish and an upper-level ridge
will erode the instability over the region. As a result, a more
typical weather pattern is expected, with afternoon convection
across the islands.

The presence of moisture and 03 km winds will enhance the heat
threat across all coastal and urban areas during the long-term
period. These warmer conditions could exacerbate heat-related
illness symptoms associated with heat exhaustion. Residents and
visitors are urged to follow the recommendations issued by the
health department.


&&

.AVIATION...

(06z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hour. However, afternoon TSRA could develop near
TJBQ/TJSJ between 02/17-23Z, causing tempo MVFR to brief IFR
conditions. Elsewhere, mostly VCSH expected. Southeast winds
expected between 10-15 kt with sea breeze variations and higher
gusts aft 02/14z.

&&

.MARINE...

A surface high pressure over the central Atlantic will continue to
result in moderate to locally fresh east to southeasterly winds
across the local waters. Seas will remain up to 5 feet across the
Atlantic offshore waters and even lower across the coastal waters.
Localized hazardous marine conditions due to thunderstorms are
forecast from late Friday into Sunday as a tropical wave moves
into the local waters, resulting in an increase in cloudiness and
showers.

&&

.BEACH FORECAST...

There is a low risk of rip currents along all the coastal areas
of Puerto Rico, Culebra, Vieques and the U.S. Virgin Islands.
Therefore, coastal conditions are suitable for beachgoers and
visitors. Although conditions remain good stay alert to areas not
designated to beachgoers.

&&

.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