031
FXCA62 TJSJ 021813
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
213 PM AST Tue Sep 2 2025

.KEY MESSAGES...

* Strong afternoon convection will result in heavy rainfall,
  frequent lightning, and gusty winds across the islands through
  this evening.

* The heat spell will continue through the workweek, with heat
  index values above 102 degrees Fahrenheit, which could
  exacerbate heat-related illnesses and symptoms for Puerto Rico
  and the US Virgin Islands` residents and visitors.

* Beachgoers in the US Virgin Islands and Puerto Rico will
  observe a low risk of rip currents through at least Saturday.

* A tropical wave and an upper-level trough will promote an
  unstable and wetter pattern around Friday or Saturday.

* The National Hurricane Center is monitoring a tropical wave near
  Africa, south of the Cabo Verde Islands, which has a low chance
  (30 percent) of developing into a tropical cyclone within the
  next 48 hours and a high chance (70 percent) within the next 7
  days.

&&

.SHORT TERM...Tonight through Thursday

A trade wind perturbation brought showers across St. Thomas and
St. John this morning, followed by a few downwind from St. Croix
and Vieques, into eastern PR in the afternoon. However, the heat
spell continues with another warm to locally hot day, as the US
Virgin Islands and Puerto Rico weather network detected heat index
values mainly between 105 and 111 degrees Fahrenheit, but locally
higher, from mid-morning into the afternoon. Skies were clear
across the western two-thirds of PR, and mostly clear to partly
cloudy across the US Virgin Islands and the eastern third of
Puerto Rico early in the morning. Cloudiness gradually shifted
from partly cloudy to mostly cloudy over mainland Puerto Rico.
This change occurred as the moisture from the perturbation
interacted with the local topography, sea breezes, and significant
afternoon heating. As a result, additional convection developed
along the Cordillera and in areas to the west and north. Winds
were mainly from the east-southeast at 10 to 15 mph, with higher
gusts and fluctuations due to sea breezes.

Strong afternoon thunderstorms are likely in western Puerto Rico
due to a nearby mid-to-upper-level trough, which is expected to
lead to heavy downpours, localized flooding, frequent lightning,
and gusty winds this evening. In the US Virgin Islands, occasional
showers will continue to move inland, but we mainly expect a mix
of clouds and mostly clear skies, with above normal temperatures.

A similar weather pattern is expected to continue through
Wednesday, with morning showers in the US Virgin Islands and
eastern Puerto Rico. Afternoon thunderstorms are expected in the
interior, northwestern Puerto Rico, and areas near El Yunque and
the Cordillera. Still, the main weather hazards include urban and
minor stream flooding, strong winds, and lightning. Additionally,
light southeasterly winds will contribute to hot temperatures in
lower elevations, leading to significant heating conditions before
the afternoon showers. On Thursday, winds are expected to shift
more from the east, reaching around 15 kt. Expect fair weather in
the morning, with afternoon showers and thunderstorms in the
Cordillera, western PR, and downwind from the US Virgin Islands.

.LONG TERM...Friday through Tuesday...
PREV DISCUSSION... /issued 339 AM AST Tue Sep 2 2025/

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...
(18z TAFs)

Mainly VFR conds across all TAF sites. Aftn TSRA will continue
through at least 02/22z, promoting VCTS over JSJ, JPS, and JBQ.
MVFR/IFR conds are likely for JBQ btwn 01/18-01/21z due possible
TSRA that will reduce CIGs/VIS. SE winds will continue btwn 11 - 14
kt through 01/22z, with gusts up to 20 - 22 kt over JSJ and JPS,
becoming light and VRB btwn to 4 - 7 kt. Winds should increase to 12
- 14 kt around 02/12 - 02/13z.

&&

.MARINE...

A surface high-pressure across the central Atlantic will continue
to produce moderate to locally fresh east to southeasterly winds
across the local waters. Strong thunderstorms will develop along
the regional waters through this evening. This will result in
moderate chop and localized choppy seas. Mariners can expect seas
of up to 4 feet, while they will be even lower near the coast and
localized higher around the afternoon. A tropical wave will likely
increase the frequency of showers as early as Friday night and
into the weekend, potentially leading to localized hazardous
marine conditions.

&&

.BEACH FORECAST...

The risk of rip currents will remain low in Puerto Rico and the
US Virgin Islands through at least Friday. We forecast a moderate
risk of rip currents for the exposed beaches of St. Croix for
Friday and Saturday. Be aware that thunderstorms may impact the
western and northern regions of Puerto Rico through this evening.

Although the overall risk of rip currents is expected to remain
low during the forecast period, isolated but stronger rip currents
may occur in specific areas, especially near piers, jetties, and
channels.

&&

.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.
&&

$$

MORNING CREW...MNG/CAM
EVENING CREW...YZR/ICP