180
FXCA62 TJSJ 040824
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
424 AM AST Fri Jul 4 2025

.KEY MESSAGES...


 * The increase in moisture from a tropical wave will enhance peak
   heat indices around the coastal areas, resulting in an
   elevated heat threat for vulnerable communities such as adults
   aged 65 and older and children. Stay hydrated throughout the
   day.

 * A tropical wave will increase the potential for showers
   across northwestern Puerto Rico.

 * A pulse of Saharan dust particles will move into the area,
   resulting in hazy skies at least until Monday.

 * There is a LOW risk of rip current for all beaches across the
   islands today 4th of July.

&&

.SHORT TERM...Today through Sunday...

The night was tranquil with very little rain observed over land or
near the local waters. There are some thunderstorms well to the
north associated with a trough, and another cluster of storms over
the Caribbean Sea.

A surface high pressure will maintain a pronounced east-southeast
wind flow today. This wind flow will cause temperatures to soar once
again, with highs in the upper 80s and low 90s, and elevated heat
indices for most coastal and urban areas. This level of heat will be
particularly dangerous for those most sensitive to heat or without
adequate hydration or cooling. Meanwhile, a tropical wave will move
south of the region, increasing moisture a bit. This wave will bring
some passing showers over eastern Puerto Rico and the Virgin
Islands, but the strongest activity is anticipated to develop in the
interior and northwest. The combination of strong diurnal heating
and the available moisture have the potential to trigger strong
thunderstorms across the interior and northwest, with a localized,
but elevated risk of urban and small stream flooding. Frequent
lightning can be anticipated with these storms as well.

As the wave departs, a dense Saharan Air Layer follows, with high
concentrations of particulate anticipated. The air mass will begin
to reach the Virgin Islands this morning, and then advancing into
Puerto Rico in the afternoon. Hazy skies will prevail through the
entire weekend, so anyone sensitive should take the necessary
precautions. It will be hot too, and it is very likely that the
criteria for heat advisories will be met in Puerto Rico each day. In
the Virgin Islands, Vieques and Culebra, the risk of heat will be
limited, so everyone is encourage to stay hydrated, seek shaded
areas, and take plenty of breaks if you need to spend long period of
times outside. Even with the dust, there is a chance of afternoon
thunderstorms each day along the interior and western Puerto Rico,
although rainfall accumulations are not expected to be too
significant or long-lasting.

.LONG TERM...Monday trough Friday...

From Monday into Tuesday, an upper-level cut-off low will position
itself over Hispaniola, providing enough instability aloft to
support some convective development across the region. However,
despite this instability, moisture levels across Puerto Rico and
the U.S. Virgin Islands will remain limited, particularly at mid
to upper levels, where relative humidity values are forecast to
stay low. This lack of deep moisture will prevent widespread or
long-lasting shower activity. At the surface, conditions will be
driven by a surface high-pressure system to the north of the
region, promoting easterly winds from Monday into early Tuesday.
By late Tuesday, as the broad surface high expands and
establishes itself over the Central Atlantic, surface winds will
gradually shift from the southeast. According to the Galvez-
Davison Index (GDI), the best chance for notable convective
development will occur on Tuesday, with the potential for a few
isolated thunderstorms during the afternoon hours. The focus of
the showers will be on the northwestern quadrant leaving the rest
of the islands with less potential to observed shower activity.

By Wednesday, a new plume of Saharan dust will begin to move into
the region, limiting afternoon shower development despite
lingering instability. These dust particles are forecast to
persist through Friday, contributing to hazy skies, reduced air
quality, and limited rainfall activity across the islands.
Although some low-level moisture may become trapped beneath the
dust layer, any resulting showers will be brief and isolated.
Surface winds will remain from the southeast through midweek as
the high maintains its position over the Central Atlantic, helping
to sustain variable but generally suppressed afternoon convection.
Overall, the weather pattern for the latter half of the week will
feature partly cloudy skies, hazy conditions, and limited rainfall
potential.

&&

.AVIATION...
(06Z TAFS)

Mostly VFR conditions expected to prevail early in the day. SHRA and
TSRA are expected after 17z for the western Cordillera Central and
TJBQ, with mountain obscuration and periods of reduced VIS and low
ceilings. A tropical wave will bring additional SHRA into the area,
but with little impacts to operations. HZ due to Saharan dust will
move into the USVI terminals after 13Z, and gradually progressing
into PR. VIS are expected to remain above 6SM. Winds will be from
the SE at 11-16 kts, with stronger gusts.

&&

.MARINE...

A broad surface high pressure over the central Atlantic and a
tropical wave will continue to result in moderate to fresh
easterly winds across the region. Therefore, wind-driven seas will
result in choppy marine conditions across the offshore Atlantic
waters and the offshore Caribbean Waters. For today into Monday,
Saharan dust particles will be in place, resulting in hazy skies
and limited shower activity.



&&

.BEACH FORECAST...

Good news for today 4th of July, all the coastal areas of Puerto
Rico and the US Virgin Islands the risk of rip current will remain
low and it will persist low for at least Sunday when turn moderate
to St. Croix. The risk of rip current will become moderate for the
northwestern sections of Puerto Rico by Monday.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon
     for PRZ001>005-007-008-010-011.

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

$$

SHORT TERM & AVIATION...ERG
LONG TERM & MARINE....LIS