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667
FXCA62 TJSJ 090739
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
339 AM AST Wed Sep 9 2026

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST...

.KEY MESSAGES...
Issued at 330 AM AST Wed Sep 9 2026

- Two tropical waves will move over the local area today and tomorrow
  increasing the potential for shower and thunderstorm activity,
  including a limited flooding threat.

- Moderate concentrations of Saharan dust to affect the area today
  and tomorrow, with drier conditions and high concentrations
  expected late Thursday into Friday. Prepare for deteriorated air
  quality and reduced visibilities.

- Breezy to locally windy conditions expected over the next few days,
  especially along coastal and exposed areas.

- Moderate risk of rip currents expected today along the northern,
  eastern and southeastern beaches of Puerto Rico, Culebra,
  Vieques and the U.S. Virgin Islands. Increasing winds will
  expand the moderate risk for most beaches tonight.

- A limited to elevated heat threat will continue through the
  rest of the workweek; please continue to monitor the forecast
  for updates.

&&

.Short Term(Today through Friday)...
Issued at 330 AM AST Wed Sep 9 2026

The rest of the workweek will be influenced by a couple of tropical
waves and a Saharan Air Layer (SAL) moving over the islands. Both
tropical waves, along with a strengthening surface high pressure
across the central Atlantic, are tightening the pressure gradient
and will maintain breezy to locally windy ESE conditions through
the rest of the workweek, particularly across coastal and exposed
areas. The night started with mainly tranquil conditions, but an
increase in shower and isolated t-storm activity has been observed
over the U.S. Virgin Islands (USVI) and eastern Puerto Rico as
the first tropical wave begins to move across the region.
Overnight low temperatures have remained in the mid 70s to low 80s
in the urban areas and lower elevations of Puerto Rico, in the
low 80s across Vieques, Culebra and the USVI, and in the mid 60s
to lower 70s across the interior sections of Puerto Rico.
Satellite-derived precipitable water (PWAT) values are increasing
from around 1.41.5 inches (below normal for this time of the
year) to near 2.0 inches (high-end seasonal levels) as the
moisture field from the first wave moves in from the east.
Therefore, variable weather conditions with frequent fast-moving
passing showers are expected this morning across the USVI and
eastern Puerto Rico, with activity shifting toward the interior,
western and northwestern sections of Puerto Rico during the
afternoon, where scattered showers and isolated thunderstorms are
expected. Streamers downwind of El Yunque and the local islands
will also be possible. The bulk of the moisture field from this
wave (with PWAT values around 2.0 inches) is forecast to be over
western Puerto Rico and the Mona Passage by this afternoon, with
lingering moisture (PWAT values of around 1.7 inches) persisting
through the evening. This wave will favor advective showers and
t-storms across the windward sections, driven by the breezy to
locally windy ESE flow, which could limit rainfall totals despite
a continued limited flooding risk. 500 mb temperatures are
expected to drop to around -6 to -7C, supporting shower and
t-storm development. Available moisture will somewhat expand back
into the mid-levels, but will however be limited by low to
moderate concentrations of Saharan Dust also arriving from this
morning onwards. This drier air intrusion can also limit the
potential for widespread shower activity, and therefore, a
significant rainfall event is not anticipated. In terms of heat,
for now we are leaning toward a limited heat risk today. However,
depending on the evolution of the cloud cover, heat impacts could
be higher later today and a Heat Advisory could be issued. Please
continue to monitor the forecast for any updates.

The second tropical wave and its associated moisture field will
reach the region tonight into tomorrow, Thursday. Current
guidance indicates PWAT values near 2.0 inches moving in from the
southeast, reaching the USVI first before crossing Puerto Rico
overnight and Thursday morning. The bulk of the moisture is
expected to shift toward western Puerto Rico, the Caribbean
waters, and the Mona Passage by Thursday afternoon. More
pronounced rainfall activity is forecast with this wave due to its
broader moisture field and a more substantial mid-level moisture
intrusion. Passing showers and isolated t-storms are expected
across eastern areas during the overnight to morning hours,
followed by afternoon convective showers and t-storms mainly
across interior to western Puerto Rico, with streamers again
possible downwind of El Yunque and the local islands. With 500 mb
temperatures continuing at -6 to -7C and breezy to locally windy
ESE flow persisting, a similar pattern to Wednesday is forecast.
However, low to moderate Saharan Dust concentrations will continue
to affect the region. Current guidance suggests high Saharan Dust
concentrations could reach the USVI by around 5 to 6 PM tomorrow.
An earlier arrival of these higher concentrations could disrupt
tomorrow`s afternoon convection, leading to some uncertainty.

These high concentrations of Saharan Dust will be the dominant
feature Thursday night and Friday. PWAT values will rapidly
decrease (as drier and more stable air related to this SAL filters
in) and be at generally around 1.3 to 1.5 inches by Friday
evening (below normal for this time of the year). This will
promote hazy skies, deteriorated air quality and reduced
visibilities. The still breezy to locally windy ESE flow could
steer some passing showers along windward sectors as lingering
moisture exits the area and local effects can still result in more
limited afternoon convection. Overnight trapping by the SAL will
lead to warmer lows Thursday night and Friday. An elevated heat
risk is forecast for Thursday and Friday as above normal 925 mb
temperatures continue to be expected.

&&

.Long Term(Saturday through Wednesday)...
Issued at 330 AM AST Wed Sep 9 2026

A surface high-pressure is expected to promote easterly breezy winds
across the islands for most of the forecast period. The Saharan
dust layer will persist during the weekend, with concentration
values reaching high levels on Saturday. This will promote hazy
skies, reduced visibility, and poor air quality. Residents and
visitors are encouraged to follow medical recommendations and
closely monitor weather forecasts for updates. Precipitable water
values are expected to drop to around 1.25 to 1.5 inches over the
weekend, which is considered below-normal to seasonal values.
Nonetheless, passing showers during the morning cannot be ruled
out in portions of eastern Puerto Rico and the U.S. Virgin
Islands. Later on, afternoon showers and isolated thunderstorms
triggered by daytime heating and local effects are possible each
day, with a limited flood threat expected on Sunday afternoon
across western Puerto Rico.

A transition to a wetter pattern from Monday through the rest of
the long term, associated with a series of upper-level lows.
Current model guidance shows a drier air slot moving across the
area Tuesday morning. The persistent breezy easterly flow will
likely bring a few passing showers across windward areas of the
islands during the morning, followed by afternoon convection
across the western interior and western sectors of Puerto Rico.
Due to the aforementioned lingering upper-level lows, 500-mb
temperatures are expected to remain at or below -6 degrees
Celsius, which will likely trigger isolated thunderstorms during
the afternoon hours in combination with diurnal heating and local
effects.

A change in the pattern is anticipated by Tuesday night into midweek
as a tropical wave brings above-normal moisture, with PWAT values
exceeding 2 inches. This will bring an increasing chance of showers
and thunderstorms, along with a potentially elevated flood risk
across portions of the islands. Continue to monitor the forecast as
adjustments and updates become available.

Overall, hazy, drier, and warmer conditions are expected on
Saturday, followed by variable conditions Sunday into Tuesday, with
seasonal moisture and instability fueled by lingering upper-level
lows. By Tuesday night into Wednesday, moisture associated with a
tropical wave will increase the chance of showers and thunderstorms,
along with an increasing flood threat.

&&

.AVIATION...
(06Z TAFS)
Issued at 330 AM AST Wed Sep 9 2026

Mainly VFR conditions. Tropical waves will increase SHRA and
isold -TSRA over the region and can mainly affect the eastern and
southern terminals through the period. Locally induced VCTS/-TSRA
can develop and affect TJBQ btwn 09/17-21z, streamers can also
form towards TJSJ at this time. HZ (Saharan Dust) will also be
present and can reduce VIS. Winds up to around 10 kts through
09/13z, then ESE at up to around 15 to 18 kt, with sea breeze
variations and stronger gusts (locally higher near SHRA/TSRA).

&&

.MARINE...
Issued at 330 AM AST Wed Sep 9 2026

A surface high pressure strengthening across the central Atlantic
will promote moderate to fresh winds, with stronger gusts for the
rest of the week. This will result in choppy to locally rough
marine conditions. Seas will build up to 6 feet across the
offshore waters and local passages, with occasional seas up to 8
feet. A passing tropical wave will increase showers and
thunderstorms across the regional waters through this afternoon.
However, a second tropical wave will arrive later tonight through
Thursday afternoon promoting similar weather conditions. Gusty
winds, brief periods of torrential rainfall, and locally higher
seas are expected with these tropical waves.

&&

.BEACH FORECAST...
Issued at 330 AM AST Wed Sep 9 2026

A moderate risk of rip currents will persist across most east,
south, and north-facing beaches of the islands through the end of
the week due to increasing winds and building seas across the
local coastal waters. Life-threatening rip currents are possible
in the surf zone, and beachgoers are urged to exercise caution.

A passing tropical wave will enhance the development of showers
and thunderstorms across the region, promoting gusty winds and
lightning strikes during the afternoon hours. Beachgoers should
remain hydrated and limit prolonged sun exposure, and seek shelter
whenever they hear thunder.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION/KEY MESSAGES...MRR/KML
LONG TERM....YZR/JCP
MARINE/BEACH...GRS

NWS SJU Office Area Forecast Discussion