Caribbean - Central America Weather

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
522 AM AST Sat Aug 9 2025

.KEY MESSAGES...

* An induced surface trough will likely bring showers across
eastern Puerto Rico and US Virgin Islands this morning, then
moderate to heavy showers with isolated thunderstorms across the
interior and western Puerto Rico by the afternoon.

* The US Virgin Islands will have daily showers and warm to hot
heat indices from mid-morning to afternoon each day.

* The warm-to-hot spell trend will persist throughout most of the
forecast period, especially during the peak of daily heating.

* Tropical moisture is expected to result in showers early next
week, followed by a Saharan Air Layer (SAL) expected to arrive
by Tuesday afternoon.

* The National Hurricane Center (NHC) is monitoring a tropical
wave with 30% chance of formation over the next seven days.
Residents and visitors are encouraged to stay well informed.

&&

.SHORT TERM...Today through Monday...

A surface induced trough increased the frequency of passing showers
the islands during the overnight hours. Since midnight, radar
estimated accumulations over land indicate that northern and eastern
PR, Culebra, Vieques and the U.S. Virgin Islands were affected by
these showers. By 425 AM, St. John saw the highest, radar estimated
since midnight, accumulations of the USVI with around 0.22 in.
Canovanas saw the highest, radar estimated since midnight,
accumulations of PR with around 0.58 in. Accumulations of up to
around 0.9 inches fell over the waters just north of Toa Baja and
Dorado. Current satellite derived precipitable water (PWAT) values
indicate slightly below normal to normal moisture (around 1.70 to
1.75 in) over northeastern Puerto Rico, Culebra, St. Thomas and St.
John. ENE steering flow is forecast to gradually veer to become more
E to ESE today, and continue being ESE during the rest of the period.

Although PWAT values over 2 inches will be present during afternoon
convection, notable advective patches of moistures (with PWAT values
around 1.85 in) are forecast to arrive by tonight into early
tomorrow and on Monday. This moisture arriving on Monday can also
reach over 2 inches of PWAT. This is trailing moisture from Invest
96L, located over the central Atlantic. Although the system itself
will likely move well northeast of the islands, trailing moisture
associated with this system will reach the islands on Monday, with
the highest amounts arriving late Monday into the long term period.
Therefore, enhancing the potential to observe showers and isolated
thunderstorms across the U.S. Virgin Islands and Puerto Rico.

An upper low will continue over the region to end the week today,
moving away on Sunday, leaving ridging in the upper levels. In
general the diurnal pattern of morning and night passing showers
across windward areas of Puerto Rico and the US Virgin Islands, and
isolated to scattered afternoon convection, mainly over western
Puerto Rico, will continue, boosted today by the induced surface
trough and on Monday by trailing moisture from Invest 96L. Sea
breeze convergence, local effects, and diurnal heating will promote
shower and t-storm activity over the Cordillera Central to western
PR each afternoon with the risk of excessive rainfall being limited
to elevated. Convective activity can also develop downwind of El
Yunque, the USVI, Vieques and Culebra. Saharan Dust that will
continue over the islands during the period with generally low
concentrations.

Highs are forecast to still reach the upper 80s to low 90s over
urban and coastal areas of the islands, with heat indices surpassing
105°F. A Heat Advisory is once again in effect today from 10 AM to 5
PM AST for urban and coastal areas of Puerto Rico. A limited heat
threat is once again present today for the U.S. Virgin Islands,
Vieques and Culebra. Similar heat conditions are forecast for
tomorrow and Monday, with a limited to elevated heat risk. 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...Tuesday through Saturday...

Based on the latest model guidance, lingering tropical moisture
is expected to remain in the area through early Tuesday, with
precipitable water (PWAT) values ranging from 1.75 to 2 inches.
Although the main axis of the tropical wave, Invest 96L, is
forecast to move away over the open Atlantic waters on Monday,
trailing moisture will continue to enhance the potential for
showers and isolated thunderstorms across Puerto Rico and the U.S.
Virgin Islands.

Following this moisture, a moderate to high Saharan Air Layer
(SAL) is expected to arrive from Tuesday afternoon through at
least Wednesday. Under these conditions, shower activity will be
limited. However, with residual moisture, local effects, and
daytime heating, afternoon convection and isolated thunderstorms
are likely each day, 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. The flood risk will
vary between limited and elevated.

Regarding temperatures, no significant changes are expected.
Temperatures at the 925 mb level will remain near or above
climatological normal throughout most of the period. Heat indices
are likely to exceed 100°F 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. Don’t forget to keep your pets hydrated as well.

Toward the end of the forecast period, uncertainty increases.
Model guidance suggests the possible approach of a system near the
region, but confidence remains low at this time. Forecasts will
continue to be updated daily. This potential system is currently a
tropical wave being monitored by the National Hurricane Center,
located near the west coast of Africa. Some slow development is
possible next week as it moves west-northwestward across the
eastern and central tropical Atlantic. It currently has a 30%
chance of formation over the next seven days. As it is still too
early to determine any impacts on our region, residents and
visitors are encouraged to stay well informed and monitor future
forecasts as we head into next week.

&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions during the period. Passing SHRA will continue
to move through the region, moving over the VCTY of TIST/TISX/TJSJ/
TJBQ during the morning hours (Isol VCTS possible due to an induced
trough moving over the area). E-ENE flow up to 13 to 18 kts with
higher gusts. SHRA/ISOL TSRA mainly near the vicinity of TJBQ/TJPS
and possibly TJSJ at around 09/17z to 09/22z. Winds gradually
becoming more E to ESE. Winds decreasing after 09/22Z.

&&

.MARINE...

Moderate to locally fresh easterly winds are mainly expected over
the next few days, resulting in seas of around 2 to 4 feet. A surface
trough will continue to enhance rain activity and maintain the
potential for thunderstorms today. Afternoon convection is expected
to develop daily across the coastal waters of western Puerto Rico and
into the Mona Passage. Invest 96L is forecast to move well northeast
of the northern U.S. Virgin Islands by early next week; however,
trailing tropical moisture will continue to bring periods of shower
activity across the regional waters. Additionally, a Saharan Air
Layer (SAL) is expected to arrive in the region by Tuesday afternoon,
leading to reduced visibilities.

&&

.BEACH FORECAST...

The low risk of rip currents continues today along all beaches in
Puerto Rico and the U.S. Virgin Islands. 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. A
moderate risk of life-threatening rip currents is expected to
return along the north- and east-facing beaches of Puerto Rico and
the U.S. Virgin Islands starting Sunday night and continuing
onward. Additionally, with heat indices expected to exceed 100
degrees across coastal areas, it's important to stay hydrated and
take necessary precautions to avoid heat-related illnesses.

&&