767
FXCA62 TJSJ 101858
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
258 PM AST Sun Aug 10 2025

.KEY MESSAGES...

* A Flash Flood Warning is in effect for northern Bayamon, Catano,
  northern Guaynabo, San Juan, and Toa Baja until 4 PM AST.
  Various Flood Advisories over interior and northwestern Puerto
  Rico are in effect until 4:30 PM AST.

* Passing showers are expected to continue across the U.S. Virgin
  Islands during the afternoon, gradually reducing by this evening.

* Moderate to high concentrations of Saharan dust are expected to
  arrive by Tuesday, with warm and hot conditions likely to
  persist across the islands.

* The NHC is monitoring Invest 97L, which has a medium chance (50%)
  of formation in the next 48 hours and a high chance (90%) over the
  next 7 days. Residents and visitors are encouraged to stay
  informed and monitor future forecasts, as updates are issued
  regularly.

&&

.SHORT TERM...Tonight through Tuesday...

...HEAT ADVISORY IN EFFECT FOR URBAN AND COASTAL AREAS OF PUERTO
RICO THROUGH 5 P.M. AST...

During the morning hours, moderate to locally heavy showers moved
across the local waters and windward sections. Based on Doppler
radar rainfall estimates, localized areas over eastern Puerto Rico
received up to half an inch, while portions of St. Croix in the U.S.
Virgin Islands got up to 0.25 inches. Heat indexes gradually
increased by mid-morning, reaching Heat Advisory Criteria by midday
in Puerto Rico, while stations over the U.S. Virgin Islands
reported above 100 degrees Fahrenheit heat indexes. Convection
activity started near midday, with the San Juan streamer affecting
portions of San Juan, Guaynabo, Bayamon, and Catano, and showers
and thunderstorms over the mountain ranges of Puerto Rico.
Additionally, streamers over the U.S. Virgin Islands continued,
mainly affecting western/northwestern portions of the islands.
Flood Advisories are currently in effect for portions of the San
Juan Metropolitan Area and northwestern Puerto Rico.

The short-term forecast remains on track, with variable conditions
during the first part of the week. A surface high pressure over the
Central Atlantic shall continue promoting E-ESE winds for the rest
of the period. Current satellite-derived products show a drier air
mass east of the CWA due to ridging that should gradually filter
into the region this evening through Monday morning. Based on the
latest model guidance, Precipitable Water (PWAT) should slightly
decrease to more seasonal values (1.5 - 1.7 inches). Nevertheless,
isolated showers may move across the regional waters and local
passages, and even reach windward sections during the morning hours,
and afternoon convection over eastern interior, and western portions
of Puerto Rico, including the San Juan Metropolitan Area. Rainfall
accumulations could lead to ponding of water over roadways, urban,
and poorly drained areas. Due to past rainfall activity, some areas
may be vulnerable, leading to urban and small streams flooding with
a low chance of flash flooding.

Trailing moisture associated with tropical wave Invest 96L will
likely arrive late Monday night into Tuesday, increasing shower and
thunderstorm activity over the U.S. Virgin Islands and eastern
Puerto Rico. Nevertheless, the latest NASA GMAO Dust Extinction
product shows a dense Saharan Dust Layer (SAL) arriving by this
timeline as well. The interaction of these features introduces
uncertainty to the forecast, but for the moment, the afternoon
convection is the likeliest scenario, mainly over northwestern
Puerto Rico. Areas without shower activity could experience hazy
skies, reduced visibility, and deteriorated air quality. People
sensitive to these particles should monitor the conditions and
take necessary precautions.

The warm-to-hot pattern should continue due to above-normal
temperatures and available moisture content. Hence, the heat risk
should remain limited to elevated over urban and coastal areas of
the local islands for the rest of the period.

&&


.LONG TERM...

/PREV DISCUSSION... /issued 511 AM AST Sun Aug 10 2025/

Around mid-week, the Saharan Air Layer (SAL), with moderate to
high concentrations of dust particles, is expected to persist
across the region, promoting hazy skies and poor air quality.
However, these conditions should gradually improve from Thursday
afternoon onward. Moisture content is expected to oscillate
between normal and above-normal levels, as trailing tropical
moisture occasionally moves in on Wednesday within the trade
winds. Under these conditions, variable weather is anticipated
most days of the forecast period, followed by afternoon convection
and isolated thunderstorms, fueled by residual moisture, local
effects, and daytime heating. Additionally, lower heights and
cooler temperatures at the 500 mb level are expected on Wednesday,
which may enhance thunderstorm development during the afternoon
hours, particularly across the western municipalities of Puerto
Rico. The U.S. Virgin Islands can expect daily showers, mainly
during the overnight and morning hours, followed by warm to hot
daytime temperatures.

Another ongoing concern will be the continued warm to hot
temperatures. Temperatures at the 925 mb level will remain near
or above climatological normal throughout most of the period. Heat
indices are likely to exceed 100F each day. Residents and
visitors are advised to stay hydrated, limit outdoor activities
during peak heat hours, and take necessary precautions to avoid
heat-related illnesses. Dont forget to keep your pets hydrated as
well.

Toward the end of the forecast period uncertainty increases, and
the forecast will depend on the development of Invest 97L, an area
currently being monitored by the National Hurricane Center. It
currently has a 70% chance of formation over the next seven days.
At this time, model guidance suggests it will move well northeast
of the region, though it could still trigger marine and coastal
hazards over the upcoming weekend. This potential system is
currently a broad area of low pressure associated with a tropical
wave and is producing a large area of disorganized showers and
thunderstorms just to the east and southeast of the Cabo Verde
Islands. Environmental conditions appear conducive for gradual
development, and a tropical depression is likely to form by the
middle to latter part of the upcoming week. As 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 are issued regularly.

&&

.AVIATION...

(18z TAFs)

Mostly VFR conds are expect for TAF sites. Aftn convection is
producing TSRA across the CWA, with VCTS over JPS and JSJ. TEMPO was
issued btwn 10/18 - 10/20z for JBQ due to psbl TSRA, reducing
CIG/VIS and promoting brief periods of MVFR conds. Winds from the E-
SE should reduce btwn 4 - 8 kt by 10/22 - 10/23z, then increasing
btwn 12 - 14 kt and gsty winds up to 25 kt after 11/12 - 11/13z.

&&

.MARINE...

No major changes were introduced to the marine forecast. Easterly
moderate to locally fresh winds should continue for the next few
days. Afternoon convection is expected to develop each day across
the islands, with showers and thunderstorms moving across the
coastal and offshore waters of western Puerto Rico and into the Mona
Passage, small craft should exercise caution. A Saharan Air Layer is
likely to arrive by Tuesday,  leading to reduced visibility. The
National Hurricane Center is currently monitoring the Invest 97L,
which is located with a well-defined low center at 100 miles east of
the Cabo Verde Islands and is forecast to move well northeast of the
region. Nevertheless, this system could trigger some marine hazards
by the upcoming weekend.

&&

.BEACH FORECAST...

A gradual increase in the risk of rip currents is expected tonight
for the northern and eastern beaches of Puerto Rico, including
Culebra. The risk is expected to spread across the U.S. Virgin
Islands by tomorrow, Monday, but may vary for the next few days.
Afternoon convection is expected each day across the local islands,
with showers and thunderstorms moving over coastal areas. Beachgoers
must stay weather alert and exercise caution due to thunderstorm
activity producing lightning over the surf zone. As mentioned in
previous sections, the National Hurricane Center continues
monitoring the Invest 97L, which could trigger beach hazards by the
upcoming weekend. Beachgoers must continue following the latest
updates of this system.

&&

.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...MNG
LONG TERM....YZR
PUBLIC/UPPER AIR...LIS