028
FXCA62 TJSJ 051352
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
952 AM AST Fri Sep 5 2025

.UPDATE...
Some minor changes were made to the weather forecast for the
weekend. Unsettled weather conditions are anticipated for the
region, especially in the afternoon and early evening hours of
each day.

For the Virgin Islands, occasional showers and thunderstorms will
stream at times, increasing the risk of water surges along guts,
ponding of water in roadways, with a risk of lightning strikes
too.

In Puerto Rico, rain in the east and Vieques and Culebra will be
scattered too, with showers and thunderstorms reaching these
areas at times. Rivers in the east are running drier than normal,
but it would not take a lot of water to cause water surges and
creeks overflowing in areas prone to flooding due to poor drainage
or low elevation.

Across the interior and western Puerto Rico, these showers and
thunderstorms are expected to be stronger and lasting longer.
Therefore, the risk of flooding here is elevated. Rivers are
running high in the west, so urban and small stream flooding, some
river reaching flood stage, mudslides, and isolated flash flooding
can be anticipated.

Please, stay tuned to any flooding developing this weekend, and
stay away from flood prone areas.


&&

.PREV DISCUSSION... /issued 750 AM AST Fri Sep 5 2025/

KEY MESSAGES...

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

 * Tropical moisture and an induced surface trough will increase
   the potential for shower activity along the islands from Friday
   into the weekend.

 * For the U.S. Virgin Islands, variable weather conditions will
   persist, with warm temperatures expected through the upcoming
   weekend. From Saturday into Sunday, a showery pattern is
   expected mostly for the morning and evening hours across the
   islands; however, no flooding threat is in the forecast.

 * The National Hurricane Center is monitoring a tropical wave
   (Invest 91L) with a high chance of formation in the next seven days.
   For now, there is a lot of uncertainty in the outcome of the
   forecast related to this tropical system; however, residents
   and visitors should stay tuned for further updates from the
   official sources.

SHORT TERM...Today through Sunday...

A tropical wave located across the Anegada Passage and Caribbean
waters will increase shower and thunderstorm activity today across
the islands. These showers could lead to minor flooding across
portions of the U.S. Virgin Islands during the later morning/early
afternoon hours. The flood threat across Puerto Rico will be
elevated, particularly across the eastern interior and western
sections of Puerto Rico during the afternoon hours. These
thunderstorms can produce brief gusty winds and cloud-to-ground
lightning strikes. A weak Saharan Air Layer with minor
concentrations of Saharan dust will arrive with the wave passage,
bringing hazy skies and reducing the air quality somewhat through at
least Saturday.

For the rest of the weekend, unstable conditions will persist across
the region as an upper-level low, located north of the area, and its
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 Puerto Rico. For the USVI, passing
showers will continue to move at times, with isolated thunderstorms
mainly developing over the regional waters.

Although cloudiness will increase during the day due to the expected
showers, the heat threat will remain elevated in general for all
lower elevations of the islands through the forecast period.
Particularly between the late morning and early afternoon hours, and
before the onset of afternoon showers. Heat advisories could be
issued each day.

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 instability aloft and induce colder temperatures at 500
Mb, ranging from -6 to -7C. Meanwhile, at the surface, plenty of
tropical moisture with Precipitable Water (PWAT) values 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. From
Wednesday into Thursday, instability will increase again as
another upper-level trough moves south of the region, favoring
colder temperatures near -7C at 500 Mb. This feature, combined
with PWAT values near the 75th percentile, will support enhanced
cloudiness and frequent showers and thunderstorms across the
islands. At the surface, east-southeast winds driven by high
pressure over the central Atlantic will prevail, steering
additional convection toward the San Juan metro area and the
northwestern sections of Puerto Rico.

From late Thursday into Friday, the forecast becomes more
uncertain due to the approach of a tropical system tracked by the
National Hurricane Center, and with a high chance of tropical
cyclone formation (90%). There are differences in the global
model guidance (GFS and ECMWF), particularly in the position and
the intensity of the system. The latest GFS run suggests a more
southerly track, aligning with the ECMWF, which continues to place
the system closer to the islands and therefore capable of
altering local weather conditions. Given how far this system is,
additional modifications in the forecast are likely. For now, the
most certain course of action is for residents and visitors is to
review their tropical emergency plans and to stay tuned to
official updates from the National Hurricane Center.

AVIATION...

(06z TAFs)

A tropical wave will increase SHRA/TSRA across the local area today.
Tempo MVFR to brief IFR conditions are possible at TISX/TIST btw
05/12z-18z, and across the PR terminals btw 05/16z-06/00z. HZ due to
minor concentrations of Saharan dust expected after the wave
passage, but VSBY should remain P6SM. East winds will increase
between 12 and 16 knots with sea breeze variations and higher gusts
after 05/13z.

MARINE...

A surface high pressure across the central Atlantic, a tropical
wave, and an induced surface trough just northeast of the islands
over the Atlantic will continue to promote moderate to locally
fresh east-southeasterly winds across the local waters today, with
seas generally up to 5 feet. 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, with small craft operators advised to exercise
caution.

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 4 PM AST this afternoon for PRZ001>003-005-
     007-008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

UPDATE...ERG
AVIATION AND RADAR...GRS
IDSS AND PUBLIC DESK...CVB