496
FXCA62 TJSJ 301759
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
159 PM AST Wed Jul 30 2025

.KEY MESSAGES...

* A weak tropical wave will increase shower activity on Thursday.

* A Saharan Air Layer with minor to moderate concentrations of
  Saharan dust will bring hazy skies and drier air on Friday.

* A wetter pattern is expected during the weekend with the passage
  of a tropical wave.


&&

.SHORT TERM...Today through Friday...

...A Heat Advisory is in effect until 5 PM this afternoon for the
coastal municipalities and lower-elevations of eastern PR...

A lingering drier air mass under the influence of a mid-level
ridge extending from the southwestern Atlantic and minor
concentrations of Saharan dust promoted fair weather conditions in
general throughout the day. The 30/12z TJSJ sounding indicated
a 500 mb temperatures of near -4C, and the precipitable water
content was 1.51 inches, below normal levels. However, this
available limited moisture content in combination with daytime
heating and the sea breeze convergence will cause showers and
possible isolated thunderstorms to develop this afternoon over
portions of the interior and west/southwest Puerto Rico.
Elsewhere, light showers from streamers can redevelop at times
until sunset.

Thereafter, the main weather features will be two tropical waves.
The first one on Thursday, with global models suggesting a weak
and disorganized wave that can still bring an increase in passing
showers across the USVI and windward areas of PR during the
morning hours. Then, as the wave moves across the region during
the peak of daytime heating, this will enhance the diurnal cycle
of afternoon convection with showers and isolated thunderstorms
developing over western PR and downwind from el Yunque into
portions of Bayamon, Toa Baja, Dorado and nearby municipalities.
On Friday, a quick drying pattern is expected as drier air with
minor to moderate concentrations of Saharan dust filters over the
area.

.LONG TERM...Saturday through Wednesday...

/From previous discussion issued at 432 AM AST Wed Jul 30 2025/

A vigorous tropical wave will bring an unstable weather pattern
across Puerto Rico and the U.S. Virgin Islands from Saturday
through Sunday. Abundant moisture associated with this wave will
support a wet pattern, with increased cloudiness and a higher
frequency of showers and thunderstorms. While there remain
discrepancies between the GFS and ECMWF models regarding the exact
timing of the waves axis passage, both agree on the potential
for widespread rainfall. The most significant impacts are expected
on Sunday, supported by favorable instability from an upper-level
low just northwest of the region, with 500 mb temperatures
ranging from -6 to -7C, as shown by the Glvez-Davison Index
values. The flood threat will be elevated, particularly across the
eastern slopes, western interior, and northwestern and
northeastern Puerto Rico due to persistent shower activity and
saturated soils. The U.S. Virgin Islands will mostly observe
showers early on Saturday and Sunday. Therefore, for the islands,
the flood risk will remain limited.

Trailing moisture will linger into Monday, keeping conditions
humid and favorable for showers across parts of the islands. These
conditions will lead to mostly warm and muggy conditions, leading
heat indices to surpass 100 degrees in most of the coastal and
urban areas. However, a drier and more stable air mass with
Saharan dust is forecast to arrive by Tuesday, leading to
improving conditions. This will result in hazy skies and reduced
shower activity through midweek. By Wednesday, as the Saharan dust
moves out of the area, a more seasonal weather pattern is
expected to resume, characterized by typical afternoon convection
mainly across the interior and western sections of Puerto Rico
under light to moderate easterly trade winds.


&&

.AVIATION...

(18Z TAFS)

Mainly VFR conditions are expected across all terminals through
the forecast period. However, SHRA/TSRA in and around the western
interior may cause brief MVFR cigs at TJPS and VCTS thru at least
30/21z. The 30/12z TJSJ sounding indicated east winds up to 13 kt
blo FL030.


&&

.MARINE...

A surface high pressure over the central Atlantic, in combination
with an induced surface trough, will continue to promote gentle to
moderate east-southeasterly winds. Today, portions of the
Caribbean waters may experience moderate to locally fresh winds,
leading to slightly choppy marine conditions. By Friday, an
approaching tropical wave and its leading edge will increase winds
across the local waters, enhancing the potential for thunderstorms
and resulting in localized hazardous marine conditions.


&&

.BEACH FORECAST...

There is a low risk of rip currents today for all the islands. However,
life-threatening rip currents often occur in the vicinity of groins,
jetties, reefs, and piers. The risk is expected to increase to
moderate late tomorrow (Thursday) for beaches in St. Croix,
Vieques, and Culebra.


&&

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

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

$$

DSR/GRS