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366
FXCA62 TJSJ 041756
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
156 PM AST Sat Jul 4 2026

...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST, FIRE
WEATHER...

.KEY MESSAGES...
Issued at 155 PM AST Sat Jul 4 2026

* Moderate to high concentrations of suspended Saharan dust will
  affect the islands through early next week. Expect hazy skies,
  reduced visibility, and deteriorating air quality. Increased
  heat risk is anticipated each afternoon.

* Hot temperatures combined with abundant moisture will increase
  the risk of heat-related illnesses, especially for those
  spending extended periods outdoors or lacking proper cooling.

* A moderate risk of rip currents is expected at several local
  beaches today and will continue through the middle of next week.
  Life-threatening rip currents are possible. Swim near a
  lifeguard, heed beach warning flags, and follow the guidance of
  local beach patrols.

* A tropical wave may increase the risk of heavy rainfall,
  thunderstorms, and flooding by the middle of next week. Continue
  to monitor the forecast for updates.

&&

.Short Term(This evening through Monday)...
Issued at 155 PM AST Sat Jul 4 2026

A dense mid- to upper-level cloud deck produced mostly cloudy
skies through the morning before gradually eroding around midday.
Moderate concentrations of Saharan dust were observed today and
are expected to increase through tonight. Despite the cloud cover
and haze, afternoon temperatures climbed into the upper 80s and
lower 90s, producing peak heat indices around 100F. East to east-
southeast winds ranged from 5 to 15 mph, with higher gusts and
local sea-breeze variations.

Air quality is expected to deteriorate further as moderate to
high concentrations of Saharan dust continue filtering across the
islands while a weak tropical wave moves westward well south of
the region across the Caribbean Sea. The dry and dusty air mass
will promote hazy skies, reduced visibility, and generally
suppress widespread rainfall across Puerto Rico and the U.S.
Virgin Islands. Temperatures will remain above normal, especially
across urban and coastal areas. Nevertheless, daytime heating,
local effects, and pockets of low-level moisture will still
support afternoon showers across the interior and western sections
of Puerto Rico, as well as downwind of the U.S. Virgin Islands.

The moderate suspended dust concentrations will persist Monday
and Sunday, before gradually weakening as a surge of tropical
moisture arrives those days. This surge will increase the
frequency of passing showers across the U.S. Virgin Islands and
eastern Puerto Rico, followed by afternoon convection over the
interior and western portions of Puerto Rico. Isolated
thunderstorms cannot be ruled out if daytime heating and local
sea-breeze convergence become sufficient to weaken the trade-wind
inversion associated with a mid to upper level-ridge despite
lingering mid-level dry air.

The islands will remain under a warm-to-hot weather pattern
through the short-term period, especially from mid-morning through
the afternoon, when Heat Advisories may again be required.

&&

.Long Term(Tuesday through Saturday)...
Issued at 136 AM AST Sat Jul 4 2026

Mostly dry and stable conditions are expected in the long-term
forecast, though the flooding threat will increase on Wednesday. A
surface high pressure building north of the region will maintain
east-east-to-southeast winds throughout the forecast period, with
breezy to locally windy conditions across coastal areas of the
islands. A dense plume of SAL will linger on Tuesday, with moderate
concentrations  filtering into the region and limiting shower
activity. Concentrations will diminish early Wednesday, while low
and mid-level moisture should increase as a tropical wave
approaching the Caribbean Basin moves across the local area. From
the latest model guidance, PWAT values may range between 1.8 and 2.0
inches, with a low to medium chance of reaching 2.25 inches, well
above the climatological normal. Additionally, ensemble members of
the Grand Ensemble are tending toward wetter conditions, with low
variability among global models (PWAT difference of approximately
0.2 inches). In terms of instability, the vicinity of an upper-level
low will maintain cooler-than-normal mid-level temperatures (between
-8 and -9 degrees Celsius), while model guidance points out an
enhancement in vorticity across the CWA, indicating upward motion
favorable for cloud growth. Additionally, the Galvez-Davison Index
(GDI) suggests the potential for isolated to scattered thunderstorms
that may produce heavy rainfall. The most likely scenario could be
showers and isolated thunderstorms moving early Wednesday morning
across the U.S. Virgin Islands and eastern exposed portions of
Puerto Rico, followed by afternoon convection over northwestern
portions of Puerto Rico. Additionally, local island streamers could
bring shower and thunderstorm activity over eastern Puerto Rico,
including the San Juan Metropolitan Area. Given the expected
conditions, rainfall accumulations could lead to ponding of water
over roadways and poorly drained areas, along with urban and small
stream flooding over the aforementioned areas. The flooding threat
may vary, as winds will remain strong, making showers more
progressive rather than stationary. Additionally, the tropical wave
may be surrounded by SAL, which may weaken the system; changes to
the forecast will be introduced if necessary. As the tropical wave
continues its way across the Caribbean Basin, a drier air mass will
filter into the region by Thursday, along with another plume of
Saharan Dust. From the latest model guidance, PWAT values should
drop to below normal values (between 1.2 and 1.4 inches), with mid-
level moisture content plummeting to 20 - 30 %. Shower activity can
still be expected, but it should remain limited, with no flooding
threat expected across the CWA for the rest of the forecast period.

Model guidance continues to suggest seasonal 925 mb temperatures,
though combined with the available moisture, heat indices may reach
100 degrees Fahrenheit across urban and coastal areas of the
islands. Additionally, the presence of SAL may inhibit nighttime
cooling, with warmer minimum temperatures. Hence, the heat threat
will remain elevated throughout the forecast period.

&&

.AVIATION...
(18Z TAFS)
Issued at 155 PM AST Sat Jul 4 2026

VFR conditions will persist along the TAF sites during the period.
An increase in Saharan Dust will result in a reduction in VIS for
the rest of the period, diminishing for tomorrow in the evening.
Some SHRA are forecast from 04/18 to 04/22 along western interior.
Winds will remain up to 18 knot until 04/23Z decreasing and
increasing again at 05/12Z with gusty winds.

&&

.MARINE...
Issued at 155 PM AST Sat Jul 4 2026

A surface high pressure over the central Atlantic and a weak
tropical wave moving westward well south of the local islands will
maintain moderate to locally fresh east to east-southeast winds
through the weekend. As a result, seas will remain moderate to
locally choppy, especially across the Caribbean waters and local
passages. The high pressure will persist through much of the
forecast period, while another tropical wave approaching by the
middle of next week may increase the potential for showers,
thunderstorms, and locally hazardous marine conditions.
Additionally, a dense Saharan Air Layer will continue spreading
across the region through early next week, reducing visibility
and, at times, deteriorating air quality.

&&

.BEACH FORECAST...
Issued at 155 PM AST Sat Jul 4 2026

A moderate risk of rip currents will continue today for beaches
along eastern and southeastern Puerto Rico, Vieques, Culebra, St.
Croix, St. John, and St. Thomas before expanding to additional
beaches beginning Sunday as easterly winds strengthen and breaking
wave action increases. Even at beaches with a low risk, isolated
life-threatening rip currents remain possible, particularly near
jetties, groins, reefs, and piers.

Expect hot and humid conditions at the beaches, with dangerous
heat levels possible during the afternoon. Stay hydrated, seek
shade frequently, wear lightweight clothing, and limit prolonged
sun exposure during the hottest part of the day.

&&

.FIRE WEATHER...
Issued at 155 PM AST Sat Jul 4 2026

Elevated fire danger will continue across portions of Puerto Rico
as dry conditions persist. Limited rainfall over the past several
weeks has allowed fine fuels to continue drying, increasing their
flammability and fire potential. Recent observations indicate low
relative humidity across southern and southwestern Puerto Rico,
while moderate to locally fresh easterly winds will continue to
promote the rapid spread of any fires that develop.

Although Red Flag criteria are not currently expected to be met,
elevated fire weather conditions will persist, especially across
the southern coastal plains and lower elevations where fuels
remain driest. With Fourth of July celebrations underway,
residents and visitors should use extreme caution with fireworks,
grills, and any activity involving open flames, as even a small
spark could quickly ignite dry vegetation and lead to a fast-
spreading wildfire.

&&

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

$$

KEY MESSAGES/SHORT TERM/BEACH FORECAST/MARINE/FIRE WEATHER...CAM
LONG TERM....MNG
AVIATION...ICP

NWS SJU Office Area Forecast Discussion