142
FXCA62 TJSJ 110917
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
517 AM AST Mon Aug 11 2025

.KEY MESSAGES...

* Afternoon showers and isolated thunderstorms are expected
  across the western interior to western Puerto Rico.

* For the U.S. Virgin Islands, warm day with passing showers
  expected tonight.

* Moderate to high concentrations of Saharan dust are expected to
  arrive tomorrow, Tuesday, with warm to hot conditions likely to
  persist across coastal and urban areas. A Heat Advisory is in
  effect once again today from 10 AM to 5 PM AST across urban and
  coastal areas of Puerto Rico.

* The NHC is monitoring the Invest 97L, which has a high chance of
  formation over the next 48 hours and the next 7 days (70% and
  90%, respectively. Residents and visitors are encouraged to
  stay informed and monitor future forecasts, as updates are
  issued regularly.

&&

.SHORT TERM...Today through Wednesday...

Current satellite derived precipitable water (PWAT) values show from
1.39 inches over western PR to 1.58 in over eastern PR, 1.65 in over
Vieques and Culebra, and 1.65 to 1.70 over the USVI. A patch of
relatively higher moisture is already over the USVI and making its
way towards PR during these early hours, under E to ESE steering
flow. PWAT values over 2 inches will be present during afternoon
convection and as a result of moisture trailing from Invest 96L over
the central Atlantic that is forecast to arrive by tonight, linger
on Tuesday, and gradually decrease on Wednesday. This moisture
arriving on Monday can also reach over 1.90 to 2 inches of PWAT.
Although Invest 96L itself will move well northeast of the
islands, this trailing moisture associated with the system will
reach the islands and therefore, enhancing the potential to
observe showers and isolated thunderstorms across the U.S. Virgin
Islands and Puerto Rico. Although this moisture will reach the
islands, Saharan Dust will also filter in during this period,
bringing some uncertainty to the forecast. By late Monday night
through Wednesday (and into the long term forecast period) a
Saharan Air Layer (SAL) with (according to the latest model
guidance) moderate to high concentrations of Saharan Dust will
move over the region, promoting hazy skies, reduced visibilities
and deteriorated air quality. Sensitive groups should take
precautions and monitor updates to the forecast.

With the upper level low that was present during the weekend, now
well west of the islands, ridging should persist in the upper
levels. An August diurnal pattern of morning and overnight passing
showers across windward areas of Puerto Rico and the US Virgin
Islands, and isolated to scattered afternoon convection, mainly over
interior to western Puerto Rico, will continue. This pattern can be
boosted by nearby surface disturbances and then by trailing moisture
from Invest 96L on Tuesday (when Galvez-Davidson Index values are
highest in the latest model run). Sea breeze convergence, local
effects, and diurnal heating will also promote shower and t-storm
activity over the Cordillera Central to western PR each afternoon
(under E to ESE steering flow through Tuesday, backing to become
more E to ENE on Wednesday) with the risk of excessive rainfall
being limited to elevated. Convective activity can also develop
downwind of El Yunque (towards the metro area), the USVI, Vieques
and Culebra. Outflow showers can also linger over land through the
evening hours, spreading towards other nearby areas.

Maximum temperatures will be in the upper 80s to low 90s over urban
and coastal areas of the islands, with heat indices surpassing
105F. A Heat Advisory will be in effect from 10 AM to 5 PM AST
today for urban and coastal areas of Puerto Rico. A limited heat
threat will be present during those hours for the U.S. Virgin
Islands, Vieques and Culebra. Similar heat conditions, with a
limited to elevated heat risk, are forecast for Wednesday and,
possibly higher tomorrow. Lows will be in the upper 70s and low 80s
at coastal and urban areas, and the upper 50s to low 70s at higher
elevations. Patchy fog will also be present during the overnight to
early morning hours, mainly over sectors of interior Puerto Rico.


.LONG TERM...Thursday through Monday...

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...
(06Z TAFs)

Mainly VFR conditions. Passing SHRA will continue to move in the
VCTY of TIST/TISX/TJSJ during the morning hours (Isol VCTS
possible). E to ESE steering flow up to around 12 to 18 kts with
higher gusts and sea breeze variations. Afternoon SHRA/ISOL TSRA in
the vicinity of TJBQ/TJPS and possibly near TJSJ, due to El Yunque
Streamer, at around 11/17z to 11/22z. Winds decreasing after 11/22Z.

&&

.MARINE...

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. A Saharan Air Layer is expected to arrive tomorrow,
leading to reduced visibilities through at least Thursday morning.
Additionally, the Invest 97L, monitored by the National Hurricane
Center, is forecast to move northeast of the region by the upcoming
weekend and it could trigger some marine hazards. Stay informed and
monitor future forecasts.

&&

.BEACH 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, Invest 97L should be monitored
closely, as it could potentially trigger coastal hazards in our
region.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010-011.

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

$$

SHORT TERM/AVIATION...MRR
LONG TERM/MARINE/BEACH FORECAST...YZR