637
FXCA62 TJSJ 040729
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
329 AM AST Mon Aug 4 2025

.KEY MESSAGES...

* Hazy skies associated with the arrival of Saharan dust will
  lead to reduced visibility and deteriorated air quality, today.

* Once again, high heat indices will prevail today.

* We encourage citizen and visitors to continue to monitor the
  long-term forecast as the National Hurricane Center is
  monitoring a tropical wave across the Central Tropical Atlantic
  with a 50 percent chance of development in the next 7 days.

&&

.SHORT TERM...Today through Wednesday...

Satellite and Doppler radar data indicated clear to partly cloudy
skies over the islands, with occasional passing showers over eastern
Puerto Rico during the nighttime hours. Overnight low temperatures
were one or two degrees warmer than in previous days, with
temperatures above 80 degrees Fahrenheit in coastal areas of the San
Juan metropolitan area, eastern and southern Puerto Rico, and the
local islands. Coastal observation sites and local buoys have also
recorded warm low temperatures, hovering around 83-84 degrees. Winds
were mainly light to calm and variable.

Satellite-derived precipitable water imagery shows a gradient of
moisture, with a moist air mass over Puerto Rico and a drier one
east of the Virgin Islands. This drier air mass will continue moving
westward over the local islands. The GEOS-5 dust extinction model
indicates a low to moderate Saharan Air Layer also reaching the
area, resulting in hazy skies. This dust is expected to linger into
tonight. With fewer clouds and less rain activity than yesterday,
temperatures are expected to soar once again, with hazardous heat
indices anticipated. A Heat Advisory has been issued for coastal and
urban areas of Puerto Rico. A limited heat risk will persist for
Vieques, Culebra, and the U.S. Virgin Islands. Residents are advised
to stay hydrated and take breaks from the sun, especially as air
quality deteriorates due to the Saharan dust.

A surface high-pressure centered north of the islands will also
enhance the steering flow, with winds from the east-southeast at 17
to 22 mph and occasional stronger gusts. Despite the presence of a
drier air mass, some showers will still move through at times, and
additional activity could develop in the afternoon. Strong daytime
heating may lead to isolated thunderstorms, mainly across
northwestern Puerto Rico this afternoon.

For the remainder of the short-term forecast period, as the Saharan
dust departs, some moisture is expected to filter in from the east.
Precipitable water values will increase from below-normal to near-
normal levels. Additionally, weak troughing aloft and a surface-
induced perturbation crossing the area by Wednesday could result in
a slight uptick in shower activity. Low-level winds will weaken
slightly but remain from the east at around 15 mph. As is typical
with the diurnal pattern of shower development, morning showers will
favor eastern Puerto Rico and the Virgin Islands, while stronger
convection is expected to develop over the interior and western
portions of Puerto Rico during the afternoon hours. This could lead
to an elevated risk of urban and small stream flooding.


.LONG TERM...Thursday through Monday...

The inherited forecast remains on track, with improving weather
conditions expected by Thursday. Mid-level dry air will dominate,
and low-level moisture will drop to below-normal levels,
increasing overall atmospheric stability. Despite this, isolated
to scattered showers are forecast from time to time, particularly
across the windward areas and western sections of Puerto Rico
during the afternoon hours, driven by diurnal heating and sea
breeze convergence. Through Friday, model guidance continues to
suggest 925 mb temperatures near or above climatological normals.
This indicates that hot daytime temperatures will persist.
Combined with sufficient low-level moisture, heat index values
exceeding 100F are likely. As a result, a limited to elevated
heat risk is expected for coastal and urban areas, especially
during peak afternoon hours. A localized significant heat threat
cannot be ruled out.

From Saturday into Monday, forecast confidence decreases slightly,
but there is general agreement on a gradual increase in moisture and
atmospheric instability. This trend is associated with the approach
of a Tropical Upper Tropospheric Trough (TUTT) north of the area. By
Monday, global models diverge in their solutions as the Global
Forecast System (GFS) depicts a vigorous tropical wave
approaching the northeastern Caribbean, combining with the TUTT to
enhance instability and increase rainfall potential, and the
European Centre for Medium Range Weather Forecast(ECMWF), on the
other hand, suggests possible tropical development, with the
system steering northward before reaching the region and weakening
rapidly.

At this time, the forecast leans slightly toward the GFS scenario,
which implies a transition into a wetter pattern late in the weekend
into early next week. This would bring a gradual deterioration in
weather conditions and a rising flood threat across Puerto Rico and
the U.S. Virgin Islands. The National Hurricane Center is now
monitoring this tropical wave over the Central Tropical Atlantic and
is giving it a 50 percent chance of development in the next 7
days. However, due to ongoing uncertainty in model guidance, it
remains too early to specify exact impacts. Residents are strongly
encouraged to stay informed and monitor future forecasts as we
move closer to early next week.

&&

.AVIATION...
(06Z TAFS)

Mainly VFR conditions across the terminals. Winds will be mainly
from the ESE at up to 12 to 18 kts aft 4/13Z with sea breeze
variations and higher gusts. Afternoon TSRA/SHRA are forecast for NW
PR, which can affect TJBQ, at around 4/17Z-22Z. This can promote
brief MVFR conditions. Brief VCSH across other terminals possible
during the period. HZ across the terminals with increasing Saharan
Dust today may reduce VIS.

&&

.MARINE...

A surface high pressure over the central Atlantic will promote
moderate to locally fresh winds through early this week. Hazy
skies are anticipated today due to the arrival of Saharan dust.

&&

.BEACH FORECAST...

The moderate risk of rip currents along the norther coast and St.
Croix will continue through at least tomorrow, with a low risk
elsewhere. The north- central to northwestern PR are forecast to
remain under a moderate chance of rip currents through early
Tuesday. Even if the risk for rip currents is low, life-
threatening rip currents often occur in the vicinity of groins,
jetties, reefs, and piers. Deteriorated air quality is forecast
today as a plume of saharan dust affects the islands.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010-011.

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

$$

SHORT TERM/AVIATION/MARINE...CVB
LONG TERM/MARINE/BEACH FORECAST...MMC