464
FXCA62 TJSJ 111827
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
227 PM AST Mon Aug 11 2025

.KEY MESSAGES...

 * A surge of moisture associated with a tropical wave will move in,
   increasing the frequency of showers from later tonight into
   Tuesday.

 * A significant area of suspended Saharan dust particles will
   arrive over the islands early Tuesday, lasting at least through
   Thursday. This will result in mostly hazy skies where showers
   do not develop and poor air quality conditions for individuals
   with asthma or allergies.

 * At 11 AM, the NHC issued the first advisory for Tropical Storm
   Erin, with maximum sustained winds of up to 45 mph. As of
   today, the forecast track keeps the system outside of our
   region; however, hazardous marine and coastal conditions are
   expected from Saturday into Monday.

 * For the U.S. Virgin Islands, residents and visitors should
   expect an increase in light to moderate showers from late this
   evening into early Tuesday, quickly transitioning to a more
   stable pattern by around 10 AM Tuesday.

&&

.SHORT TERM...Tonight through Wednesday...

Mostly stable weather conditions prevailed across Puerto Rico and
the U.S. Virgin Islands this morning. Satellite imagery showed
mostly clear skies, with only a few clouds and isolated showers
over some small areas along the east. Daytime temperatures ranged
from the upper 80s to lower 90s across coastal and urban areas,
and the lower 80s across the mountains. Heat indices climbed to
between 105 and 111 degrees along the northern coastal areas.
Winds remained from the east at around 15 knots, with sea breeze
variations.

For tonight, weather conditions will deteriorate as moisture from
a tropical wave moves westward into the region, increasing
cloudiness and showers. Forecast models indicate an increase in
shower activity from the early morning hours on Tuesday, spreading
from east to west across the islands and resulting in cloudy
skies. Although moisture is expected, the NASA Goddard Earth
Observing System Model V5 continues to indicate a pulse of Saharan
dust with moderate to high concentrations. Given these conditions,
residents can expect a variable pattern with warm temperatures,
hazy skies, and afternoon convection. On Wednesday, conditions
will once again deteriorate as upper-level divergence from an
upper trough, combined with low-level convergence from an induced
surface trough, increases the potential for showers and thunderstorms.
According to the Galvez-Davison Index (GDI), thunderstorm
activity is most likely along the Cordillera Central.

.LONG TERM...Thursday through Monday... from previous
discussion....

On Thursday, moderate to high concentrations of Saharan dust will
diminish, allowing for an improvement in air quality and
visibility. Model guidance indicates a patch of drier air mass,
with precipitable water (PWAT) values as low as one inch (below
the climatological normal) moving across the region, probably
reducing the potential for rainfall that day. By Friday, moisture
content will increase from seasonal to above seasonal levels.
Under these conditions, a typical summer weather pattern is
expected, with showers developing across eastern sectors in the
morning, followed by afternoon convection and isolated thunderstorms
fueled by available moisture, local effects, and daytime heating.
This activity could lead to ponding of water on roads and in poorly
drained areas, as well as flooding in urban zones and small streams.
Winds will begin to weaken due to a col region that will be
positioned over our area, resulting in slower- moving showers that
could lead to higher rainfall accumulations during the afternoon.

Heading into the weekend, weather models continue to show
discrepancies regarding various meteorological variables, making
the forecast more challenging at this time. The forecast will
largely depend on the development and path of Invest 97L, a well-
defined low-pressure area currently located over the eastern
portion of the Cabo Verde Islands. The system is being monitored
by the National Hurricane Center and currently has a high chance
of formation over the next 48 hours and the next seven days (70%
and 90%, respectively). At this time, model guidance suggests it
will track well northeast of the region, approximately 250 to 300
miles northeast of San Juan, though it could still affect us
indirectly by generating marine and coastal hazards over the
upcoming weekend, particularly across Atlantic waters and north-
facing beaches. Northerly winds are expected on Saturday. Then, as
the system moves away into the western Atlantic, winds will shift
to southerly on Sunday and Monday and remain light. PWAT values
will likely range from seasonal to above normal, as these
southerly winds transport trailing tropical moisture, potentially
increasing rain chances during the weekend into early next week.
At the moment, the flood risk ranges from limited to elevated.
Since it is still too early to determine any direct impacts on our
region, residents and visitors are encouraged to stay informed
and monitor future forecasts, as updates will be issued regularly.


&&

.AVIATION...
(18Z) VFR conditions will persist along all the TAF sites at
least until 19Z when SHRA and TSRA will develop along western PR
resulting in lower VIS and lower ceilings across the area. Winds
will decrease at 11/23Z peaking again at 12/15Z. An increase in
VCSH and SHRA is forecast at TIST & TISX from 12/09 spreading
along the islands during the day.

&&

.MARINE...

No changes were introduced to the actual forecast. Gentle to
moderate easterly winds are expected over the next few days,
resulting in seas of around 2 to 4 feet, locally higher up to 5
feet. Tropical moisture will bring periods of shower activity
across the regional waters and afternoon convection is expected to
develop each day across the coastal waters of western Puerto Rico
and into the Mona Passage. Hazardous marine conditions are
forecast as the now Tropical Storm Erin, intensifies and move near
the region as a hurricane by late Saturday.

&&

.BEACH FORECAST...

No changes to the actual forecast. There is a moderate risk of
life threatening rip currents along the north and east facing
beaches of Puerto Rico, Culebra, St. Thomas and St. John today. A
low risk of rip currents is present elsewhere, however, it`s
important to note that even when the risk is low, life-threatening
rip currents can still occur near groins, jetties, reefs, and
piers. Beachgoers are advised to exercise caution and follow the
recommendations of local officials. Additionally, with heat
indices expected to exceed 100F across coastal areas, it is
important to stay hydrated and take necessary precautions to avoid
heat-related illnesses. A moderate risk will likely persist
through the week. Looking ahead to the upcoming weekend,the now
Tropical Storm Erin had a trajectory track moving near the area
increasing the risk of rip currents.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010-011.

VI...None.
AM...None.
&&

$$

SHORT TERM & AVIATION...LIS
LONG TERM & MARINE....YZR