351
FXCA62 TJSJ 131721
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
121 PM AST Sun Jul 13 2025

.KEY MESSAGES...

* Hot week ahead with heat indices around 105F or more expected
  daily.
* Afternoon showers and thunderstorms are likely MondayTuesday,
  mainly in western PR and mountain areas.
* A tropical wave by midweek (WedThu) will bring a better chance
  for heavy rains and thunderstorms across PR and USVI.
* A Saharan Air Layer returns this weekend, causing hazy skies and
  poor air quality.

&&

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

The Doppler Radar detected a few showers across the islands in the
morning, with the majority of them over the waters. Later in the
morning, as showers slowly dissipated or moved away from the
islands, a layer of mid to upper-level clouds moved over,
increasing cloudiness and limiting the heat indices from reaching
even higher values, and leaving them around 105 degrees
Fahrenheit. The suspended Saharan dust particles slowly dissipated
throughout the morning hours. Diurnally induced showers and
isolated thunderstorms developed downwind from the US Virgin
Islands and la Sierra de Luquillo, form the interior to west and
southwest PR this afternoon. Maximum temperatures were from the
upper-80s to the low-90s across the lower elevations of the
islands, and from the mid-70s to low-80s across the higher
mountains of Puerto Rico. The wind was from the east to east-
northeast between 5 and 15 mph with sea breeze variations and
higher gusts.

We expect afternoon convection to continue and isolated
thunderstorms may form today. The afternoon activity will continue
into the evening, spreading into the waters or dissipating,
leaving mainly calm weather overnight. The advection of
additional rain will focus primarily on the regional waters and
windward locations of PR and the USVI through at least tomorrow
(Monday) morning. The Saharan particle concentrations will finally
exit the region, with only suspended traces, if any.

A mid to upper-level high pressure will slowly displace today`s
upper-level trough westward into the western Caribbean and Atlantic,
leaving the islands in between these two features on Monday. Thus, a
more stable atmosphere will return across the  Northeast Caribbean
for the beginning of the week. By Tuesday, the mid to upper-level
high will build over the islands, increasing even more stability.
Regardless of the stable conditions and hot weather pattern, a few
trade wind perturbations with increasing winds will arrive Monday
and Tuesday, bringing periods of showers across the windward
locations. This moisture increase, with the local effects and sea
breeze variations, will promote afternoon thunderstorms across the
mountains and western locations of PR, as well as downwind from the
US Virgin Islands and from El Yunque into the San Juan Metropolitan
area.

.LONG TERM...Wednesday through Sunday...

A wet weather pattern is expected for the first half of the long-
term forecast due to the approach of a broad tropical wave into the
region. This tropical wave is anticipated to reach the islands
between Wednesday and Thursday, promoting a high moisture
environment and unsettled weather conditions. The latest
precipitable water content guidance (PWAT) has values up to 2.10
inches, suggesting a very moist environment beginning by early
Wednesday and lasting through late Thursday. Therefore, showers and
thunderstorms are likely over portions of eastern, central and
western Puerto Rico, and the U.S. Virgin Islands during this period.
After the wave passage, a drier airmass will begin to filter into
the region. Nonetheless, pockets of moisture will continue to
filter from time to time, promoting isolated to scattered showers
over the islands. From Saturday onwards, a mid-level ridge is
expected to build near the region, promoting drier conditions and
stability aloft.

Recent model guidance continues to suggest the arrival of another
pulse of high concentrations of Saharan Air layer (SAL)into the
region. This event will also help to inhibit the development of
shower across the islands. Be prepared for hazy skies, reduced
visibilities and deteriorated air quality through at least the
upcoming weekend.

Hot temperatures will remain all week across the area with maximum
temperatures reaching upper 80s to low 90s along the coastal and
urban areas each day. Heat indices are expected to rise into values
above 105 degrees Fahrenheit each day as well, mainly due to the
combination of high temperatures and high moisture. Residents and
visitors, please be aware of these conditions and plan
accordingly if planning to do outdoor activities under sun
exposure.

&&

.AVIATION...
(18Z TAFS)

VFR conditions will prevail across all terminals thru the period.
Visibilities will improve as the SAL exits the region. Isolated
trade wind SHRA/-SHRA may briefly affect terminals. Afternoon
SHRA/TSRA will develop across the interior and west PR, affecting
the VCTY of JPS/JBQ thru 13/23z. A similar wx pattern will prevail
tomorrow, with a slight increase in coverage/intensity after 14/16z.
Winds E/ENE at around 15 kt with higher gusts and sea breeze (from
E/ESE tomorrow after 14/13z), becoming light/variable after 13/23Z.


&&

.MARINE...

A surface high pressure across the central Atlantic will continue
to promote moderate easterly winds across the region. Moderate to
locally fresh easterly winds are expected to return by early next
week. Therefore, small craft operators will likely be encouraged to
exercise caution. A tropical wave will approach the region by the
middle of next week, increasing moisture and the potential to observe
showers and thunderstorms.

&&

.BEACH FORECAST...

Rip current risk remains low across local beaches, but life-
threatening rip currents can still form near structures like groins,
jetties, reefs, and piers. Always swim near a lifeguard and follow
local beach safety guidance.

By late Tuesday, a moderate risk of rip currents will return along
north- and east-facing beaches of Puerto Rico, Culebra, Vieques, and
the US Virgin Islands, driven by increasing winds.

Hot and humid conditions will continue throughout the week, with
heat index values likely around 105F. Stay hydrated, take breaks in
the shade, and limit outdoor activity during peak heat hours.

&&

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

$$

SHORT TERM...CAM
LONG TERM....YZR
AVIATION...YZR