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024
FXCA62 TJSJ 210751
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
351 AM AST Tue Jul 21 2026

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

.KEY MESSAGES...
Issued at 331 AM AST Tue Jul 21 2026

* Ponding of water on roads and minor flooding expected today
  across portions of the islands due to shower and thunderstorm
  activity.

* Temperatures reaching heat advisory/warning criteria expected on
  Wednesday.

* Hazy skies return by the end of the workweek due to moderate
  concentrations of Saharan dust.

* Across the USVI, scattered showers and brief periods of gusty
  winds can be expected today.

&&

.Short Term(Today through Thursday)...
Issued at 331 AM AST Tue Jul 21 2026

An upper-level low north of the region, along with an induced
surface trough, will bring an increase in shower and thunderstorm
activity through Wednesday. This setup will pull a surge of deep
tropical moisture across the northeastern Caribbean, pushing
precipitable water values up to 1.80 to 2.00 inches. Favorable
upper-level divergence combined with strong daytime heating and
local sea breeze convergence will support shower and thunderstorm
activity each afternoon, concentrated mainly over the interior and
western Puerto Rico. The wind will prevail from the NE today, and
this activity should favor mostly the west/southwest quadrant of
PR, and over NW PR on Wednesday as winds shift from the SE behind
the trough. The U.S. Virgin Islands and eastern Puerto Rico will
see periods of showers, mainly this morning, with isolated
thunderstorms developing across the coastal waters of the islands
as the low-level trough propagates westward. Quick moderate to
heavy downpours could lead to ponding of water on roads and poor
drainage areas, as well as localized urban flooding.

By late Wednesday and through Thursday, a marked drying and
stabilizing trend will take hold as a dense plume of the Saharan
Air Layer (SAL) moves into the northeastern Caribbean.
Precipitable water values are forecast to drop sharply back down
toward 1.20 to 1.40 inches, accompanied by a robust low-to-mid
level capping inversion and dry mid-level air. This dust/drier
airmass will bring hazy skies, degraded air quality, and sharply
suppressed convective development for Thursday. Shower activity on
Thursday will be capped and restricted mainly to brief, isolated
afternoon showers over western PR. Warm 925 mb temperatures
combined with lower wind speeds and dewpoints in the upper 70s to
low 80s will also favor elevated (106F-112F) heat indices across
coastal and low-elevation areas, particularly on Wednesday, and
heat advisories/extreme heat warnings could be issued for
portions of the islands.

&&

.Long Term(Friday through Tuesday)...
Issued at 331 AM AST Tue Jul 21 2026

By the end of the week and into the weekend, moderate concentrations
of Saharan dust are expected to persist across the region. Surface
high pressure over the Atlantic will reestablish itself as the upper-
level low weakens and moves away, resulting in breezy easterly
winds, particularly on Friday and Saturday. During this period,
available moisture will remain primarily confined to the surface
through the 850 mb layer, with PWAT values fluctuating between 1.6
and 1.9 inches. These values are considered seasonal for this time
of year based on climatology, although locally above-normal values
may develop during afternoon convection. The easterly steering flow
will continue to favor passing showers across windward areas during
the nighttime and morning hours, followed by afternoon showers over
western Puerto Rico driven by local effects and available moisture.
The flood risk is expected to remain low, although a few isolated
thunderstorms will remain possible each afternoon across western
Puerto Rico. Temperatures at 500 mb will remain around -6 to -7 C,
which is near climatological normal, but still sufficient to support
isolated thunderstorms associated with afternoon convection.

From Sunday through Tuesday, current guidance suggests that moisture
associated with a tropical wave and an accompanying wind surge will
reach the area, increasing PWAT values to above-normal levels while
producing breezy to locally windy east-southeasterly flow. PWAT
values are expected to remain above 1.8 inches from Sunday onward,
peaking around 2.1 inches between Monday and Tuesday (near the 75th
percentile). Additionally, current model guidance indicates that an
upper-level low will develop and move across the northeastern
Caribbean. This is reflected by lower 250 mb heights and cooler 500
mb temperatures, both decreasing toward the 25th percentile of the
climatological distribution. As a result, more favorable upper-level
dynamics are expected to enhance the potential for showers and
isolated thunderstorms across the forecast area. Continue to monitor
the forecast for updates.

Warm to hot conditions will persist through most of the forecast
period, with an elevated heat risk expected across coastal and lower
elevations of the islands. Residents and visitors are encouraged to
continue practicing heat safety measures to reduce the risk of heat-
related illnesses.

&&

.AVIATION...
(06Z TAFS)
Issued at 331 AM AST Tue Jul 21 2026

Tempo MVFR to brief IFR conds possible thru 21/12z across the USVI
terminals and TJSJ due to -TSRA/SHRA from a broad sfc trough
moving across the northeastern Caribbean. Similar conditions are
possible at TJPS btw 21/17-21z, while VCTS could prevail across
the rest of the PR terminals. SCT/BKN cigs btw FL025-FL150 and mtn
tops obscd expected. Sfc winds NE at 12-16 kt with stronger gusts
around 25-30 kt. A southerly sea breeze variation is expected at
TJPS aft 21/14z.

&&

.MARINE...
Issued at 331 AM AST Tue Jul 21 2026

An approaching induced surface trough will weaken the influence of
a surface high pressure over the central Atlantic and promote
more moderate east-northeast winds through late tonight. The
surface high will then reestablish itself and promote moderate to
fresh east to east-southeast winds early tomorrow, Wednesday,
onwards. This induced surface trough will also enhance the
likelihood of showers and thunderstorms, as well as increase sea
conditions and wind strength near any thunderstorms today and
tomorrow. Additionally, another tropical wave is forecast to move
near the Lesser Antilles by Thursday and approach the region by
Friday. Moderate concentrations of Saharan Dust will also reach
the region on Thursday and into the weekend.

&&

.BEACH FORECAST...
Issued at 331 AM AST Tue Jul 21 2026

A low risk of rip currents will persist today and tomorrow
(Wednesday) however, life-threatening rip currents often occur in
the vicinity of groins, jetties, reefs, and piers. A moderate risk
of rip currents (life-threatening rip currents are possible in the
surf zone) will return on Thursday and persist through the weekend.
Beachgoers should swim near lifeguards, obey beach warning flags,
and never swim alone. Showers and thunderstorm chances increase
today and tomorrow, with showers moving towards windward sectors
under east-northeast winds today and east-southeast winds tomorrow,
while afternoon showers and t-storms are forecast mainly over
western Puerto Rico. Beachgoers should remain weather-aware and be
cautious of thunderstorms, as lightning can strike far from rain. If
thunder is heard, leave the beach and seek shelter in a substantial
building or hard-topped vehicle. There is also a risk of extreme
heat, especially from late morning to afternoon, due to high
temperatures, humidity, and limited shade. To minimize heat-related
illnesses, beachgoers should stay hydrated, seek shade, and apply
sunscreen regularly.

&&

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

$$

SHORT TERM...DSR
LONG TERM....YZR
AVIATION...DSR
BEACH/MARINE...MRR

NWS SJU Office Area Forecast Discussion