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407
FXCA62 TJSJ 050752
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
352 AM AST Sun Jul 5 2026

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

.KEY MESSAGES...
Issued at 352 AM AST Sun Jul 5 2026

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

* Breezy to locally windy conditions will sustain up to a Moderate
  Risk of Rip Currents and "Small Craft Should Exercise Caution"
  conditions throughout most of the week.

* A tropical wave can increase the risk of heavy rainfall,
  thunderstorms, and flooding by Wednesday. Continue to monitor
  the forecast for updates.

* Elevated fire weather conditions are expected across the
  southern coastal plains of Puerto Rico today. Relatively dry and
  breezy conditions will favor the rapid spread of any fires that
  develop.

&&

.Short Term(Today through Tuesday)...
Issued at 352 AM AST Sun Jul 5 2026

During the overnight hours, partly to mostly cloudy skies
prevailed across the islands while moderate to high concentrations
of Saharan Dust filtered into the region. From midnight to 3 AM
AST, radar detected no rainfall accumulations. Due to overnight
heat trapping by the Saharan Air Layer (SAL), temperatures
remained in the upper 70s to low 80s across coastal and lower-
elevation sectors, while dropping into the 60s across the
interior. Recent satellite-derived precipitable water (PWAT)
imagery indicates generally normal values across the area. Today,
the continuous influx of moderate to high concentrations of
Saharan Dust will promote hazy skies, reduced visibilities, and
deteriorated air quality; sensitive groups should follow medical
advice. Although PWAT values are expected to stay generally within
a normal range despite some fluctuations, a drier mid-level air
mass will confine most available moisture below 800 mb,
suppressing widespread rainfall and passing showers over windward
sectors across Puerto Rico and the U.S. Virgin Islands.
Nevertheless, diurnal heating, local effects, and pockets of low-
level moisture will support limited afternoon showers across
interior and western Puerto Rico, as well as downwind of the USVI,
Vieques, Culebra, and El Yunque. Additionally, an upper-level low
lingering north of the islands (will linger through most of the
period), combined with cool (below normal) 500 mb temperatures and
Galvez- Davidson Indices (GDI) reaching 30, can support some
lightning strikes mainly over western Puerto Rico.

From tonight through Tuesday, breezy east toeast-southeast winds
will continue, steering both Saharan Dust and patches of tropical
moisture to maintain normal to locally above- normal PWAT values
over the islands. Moderate concentrations of Saharan Dust are
forecast to persist tonight through Tuesday, becoming locally high
at times, resulting in continued hazy skies and reduced air
quality; sensitive groups should follow medical advice. Although
500 mb temperatures and 700 to 500 mb lapse rates will be at more
seasonal values, the latest guidance indicates GDI values up to 40
on Monday and 30 on Tuesday. Combined with surges of tropical
moisture, this will increase the frequency of passing showers
across eastern Puerto Rico and the U.S. Virgin Islands, with
afternoon showers and t-storms subsequently developing over the
interior and western portions of Puerto Rico where a limited
flooding threat is possible, as well as downwind of the USVI,
Vieques, Culebra, and El Yunque. Despite lingering dry air in the
mid-levels, isolated thunderstorms remain a possibility if daytime
heating and local sea-breeze convergence become strong enough to
weaken the trade-wind inversion associated with a mid- to-upper-
level ridge.

A warm-to-hot weather pattern will encompass the islands during
the period due to diurnal heating, particularly from mid-morning
into the afternoon hours, and overnight trapping by the Saharan
dust layer. With 925 mb temperatures expected to persist at high-
end normal values, the issuance of a Heat Advisory may be required
later today, depending on the development of the cloud deck that
is currently moving over the region during the morning to early
afternoon hours.

&&

.Long Term(Wednesday through Sunday)...
Issued at 352 AM AST Sun Jul 5 2026

The long-term remains on track, though the flooding threat may
extend into Thursday. As mentioned in the previous discussion, the
wind pattern should remain mostly from the E-ESE, as a surface high
pressure builds north of the CWA and maintains breezy to windy
conditions across the islands. The latest model guidance continues
to suggest a well-defined tropical wave approaching the Caribbean
Basin and moving across the CWA on Wednesday, along with a wind
surge. This system should pool tropical moisture into the region,
with low- and mid-level RH peaking around 80%, well above the
climatological normal. Additionally, Grand Ensemble members
(including GEFS and ENS) continue to tend to wetter conditions, with
PWAT values reaching 2.0 - 2.2 inches. As mentioned in the previous
discussion, model guidance suggests cooler than normal mid-level
temperatures (500 mb between -8 and -9 degrees Celsius), enhanced
vertical motion, and vorticity, indicating favorable conditions for
deep convection. In addition, GDI values remain high on Wednesday,
particularly over the regional waters and portions of eastern and
northwestern Puerto Rico. Hence, the discussed scenario prevails,
with showers and isolated thunderstorms moving into windward
sections of Puerto Rico and the U.S. Virgin Islands during the
morning, followed by afternoon convection over interior and
northwestern Puerto Rico. Due to strengthening winds, daytime
heating, and local effects, local streamers could develop and move
over portions of eastern Puerto Rico, including the San Juan
Metropolitan Area. Rainfall accumulations could result in ponding of
water over roadways and poorly drained areas, along with urban and
small stream flooding over the aforementioned areas, with low flash
flood chance over northwestern Puerto Rico. Although the latest
NASA/GMAO Dust AOT suggests low concentrations of SAL that may
linger across the local area on Wednesday, this could enhance rather
than limit lightning activity, particularly during afternoon
convection.

In the previous discussion, model guidance suggested a drier air
mass filtering into the region by Thursday, though the latest global
solutions suggest an abundance of moisture content over the CWA,
associated with another tropical wave approaching the Caribbean
basin. Although the area of enhanced convection should remain well
south of the region, lingering moisture associated to the system
combined with daytime heating and local effects may result in
afternoon convection, once again, across western/northwestern Puerto
Rico, with the San Juan streamer as well. Although periods of heavy
rainfall can be expected, theres uncertainty regarding the arrival
of the tropical wave and the SAL. For the rest of the forecast
period, the dense plume of Saharan Dust should persist and limit
shower activity, with isolated showers moving from time to time.

The latest model guidance suggests seasonal 925 mb temperatures,
slightly near above climatological normal. Nevertheless, the
combination of available moisture and winds with a southerly
component will likely trigger heat indices to reach 100 degrees
Fahrenheit across urban and coastal areas of the islands. In
addition, the dense plume of Saharan Dust will likely inhibit
nighttime cooling, with warmer minimum temperatures. Hence, the heat
threat will remain elevated throughout the forecast period.

&&

.AVIATION...
(06Z TAFS)
Issued at 352 AM AST Sun Jul 5 2026

VFR conditions across the terminals. ESE winds around 15 to 20
kts with gusts near 24 to 28 kts. HZ will affect the TAF sites,
reducing VIS through the period, decreasing this evening. Passing
VCSH/-SHRA over windward sectors at times and developing from 05/18
to 05/22 mainly near TJBQ.

&&

.MARINE...
Issued at 352 AM AST Sun Jul 5 2026

A surface high-pressure system over the central Atlantic will
promote a moderate to locally fresh east to east-southeast wind flow
through Monday. As a result, seas will remain moderate to locally
choppy, especially across the Caribbean waters and local passages.
This high pressure will persist through much of the forecast period.
By the middle of the week, an approaching tropical wave will
increase the potential for showers, thunderstorms, and locally
hazardous marine conditions. Meanwhile, a dense Saharan Air Layer
(SAL) will continue to spread across the region through early this
week, reducing visibility and deteriorating air quality.

&&

.BEACH FORECAST...
Issued at 352 AM AST Sun Jul 5 2026

A moderate risk of rip currents is expected today along exposed
beaches of Puerto Rico, Vieques, Culebra, St. Croix, St. John, and
St. Thomas. A low risk is expected across beaches of western Puerto
Rico. Even at beaches designated as low-risk, isolated, life-
threatening rip currents remain possible, particularly near jetties,
groins, reefs, and piers.

Expect hot and humid conditions at the coast, with dangerous heat
levels possible during the afternoon. To stay safe, remain well-
hydrated, seek shade frequently, wear lightweight clothing, and
limit prolonged sun exposure during the hottest parts of the day.

&&

.FIRE WEATHER...
Issued at 352 AM AST Sun Jul 5 2026

A Fire Danger Statement has been issued for today (from 10 AM to
4 PM AST) across portions of the southern coastal plains of Puerto
Rico due to the combination of critically dry fuels, high KBDI
values, sustained winds of 15 to 20 mph with higher gusts, and
minimum relative humidity values in the low 50s as moderate to
high concentrations of Saharan Dust continue moving over the
region. Prolonged rainfall deficits, abnormally dry to severe
drought conditions, and critically dry fuels also continue. These
conditions will favor rapid wildfire ignition and extreme fire
behavior, meaning any fires that develop will likely spread
quickly and become difficult to contain. Outdoor burning is highly
discouraged.

&&

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

$$

SHORT TERM/AVIATION/FIRE WEATHER...MRR
LONG TERM....MNG
MARINE/BEACH...GRS

NWS SJU Office Area Forecast Discussion