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291
FXCA62 TJSJ 211731
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
131 PM AST Mon Sep 21 2026

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

.KEY MESSAGES...
Issued at 125 PM AST Mon Sep 21 2026

- A typical weather pattern will persist across Puerto Rico and
  the U.S. Virgin Islands during the next few days, with
  afternoon showers and thunderstorms across central, northern,
  and western Puerto Rico.

- A Heat Advisory is remains in effect through 3 PM AST today for
  all Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands.
  Heat concerns will continue each day, particularly before the
  onset of afternoon showers.

- A wetter and more unstable pattern will return by mid-week,
  increasing the potential for widespread heavy rainfall and
  stronger thunderstorms.

- Low rip current risk will continue through the day on Wednesday
  across all local beaches. Moderate rip current risk is expected
  to return starting late Wednesday for the northern and eastern
  beaches of Puerto Rico, Vieques, Culebra and the northern U.S.
  Virgin Islands.

&&

.Short Term(This evening through Wednesday)...
Issued at 125 PM AST Mon Sep 21 2026

Clear to partly cloudy skies prevailed across the islands during the
morning hours, with passing showers moving across eastern to
southern portions of Puerto Rico and the U.S. Virgin Islands. Based
on radar estimates, accumulations were not significant overall, but
locally, 0.75 to nearly 1 inch of rain was reported in the
municipality of Naguabo. From mid-morning into the afternoon hours,
cloudiness began to increase, accompanied by additional persistent
passing showers moving into the interior municipalities. In terms of
winds, the overall low-level wind flow has been from the southeast
at around 5 to 12 mph. At 11 AM AST, daytime temperatures ranged
from the upper 80s to the low 90s, while heat indices reached the
low 100s F, with localized areas across north-central Puerto Rico
reaching up to 110 F at that time. A Heat Advisory remains in
effect through 3 PM AST this afternoon for both Puerto Rico and the
U.S. Virgin Islands.

Latest satellite-derived Precipitable Water (PWAT) imagery indicates
values remain near seasonal levels to slightly above normal (1.9 to
2.1 inches). The morning sounding (12Z TJSJ) showed 1.91 inches of
PWAT and a persistent light southeasterly flow from the surface to
around 3 km. This, in combination with daytime heating, local
effects, and 500-mb temperatures around -6 C, should support the
potential for strong thunderstorms capable of frequent lightning and
gusty winds. As a result, for the rest of the afternoon, an elevated
flood threat remains in place, particularly across portions of the
interior, north-central, and northwestern Puerto Rico, while a
limited flood threat remains for portions of eastern Puerto Rico
into the metropolitan area with the development of the El Yunque and
U.S. Virgin Islands streamers. An elevated flood threat means
flooding is possible in urban areas, roads, small streams, and
washes, while a limited flood threat means ponding of water is
possible on roads and in poorly drained areas.

For the rest of the short-term period, seasonal moisture and
easterly to southeasterly winds will continue, driven by a surface
high-pressure system establishing over the eastern Atlantic. Aloft,
a series of upper-level lows will linger, gradually cooling mid-
level temperatures to -7 to -8 C on Tuesday and Wednesday. As a
result, a similar weather pattern is expected to continue over the
coming days, with heat and afternoon convection as the main hazards.
However, one difference expected on Wednesday is that drier air will
filter into the mid-levels, slightly suppressing convective
activity. Nevertheless, local effects and low-level moisture should
be sufficient to support afternoon activity. Continue to monitor the
forecast as forecast adjustments and updates become available.

Overall, above-normal heat and strong afternoon thunderstorms posing
a limited to elevated flood risk are expected to continue through
the forecast period.

&&

.Long Term(Thursday through Monday)...
Issued at 215 AM AST Mon Sep 21 2026

Surface high pressure across the central to eastern Atlantic will
promote mainly southeasterly low-level wind flow during most of the
long-term period, becoming more easterly early next week, at around
10 to 18 knots. The latest models show that precipitable water
content will remain mainly near seasonal values, between 1.75 and
2.0 inches, except from Friday night into early Saturday, when
values may decrease to around 1.5 inches (25th percentile), as low
concentrations of Saharan dust particles are expected to start
arriving.

By Thursday and Friday, models continue to indicate mid to upper-
level lows, bringing some instability and 500-mb temperatures
cooling to below-seasonal values, around -8 to -9 degrees Celsius.
Models have been adjusting, bringing drier air into the 850 to 700-
mb levels and also into the mid to upper levels during the weekend.
With marginal instability aloft, daytime heating, and local effects,
at this time Thursday and Friday look like two days with frequent
passing showers across windward areas and the U.S. Virgin Islands,
along with strong afternoon thunderstorms across parts of the
interior into northwestern Puerto Rico. If this scenario
materializes, it could produce heavy rainfall, frequent lightning,
and locally gusty winds. A limited to elevated flooding risk is
possible during this period, meaning that flooding in urban areas,
on roads, and in small streams and washes is possible.

As previously mentioned, Saturday looks like the day Saharan dust
will start to infiltrate the islands. Additional mid-level drying is
expected. As a result, rain chances are expected to decrease, and
afternoon showers and isolated thunderstorms, driven by local
effects, should remain more confined to northwestern Puerto Rico and
decrease in intensity. From Sunday into early next week, low-level
winds are also expected to strengthen somewhat and available
moisture gradually increase. That should allow for a more typical
pattern, with morning showers across eastern Puerto Rico and the
U.S. Virgin Islands, followed by afternoon convection across western
sectors of Puerto Rico.

A persistent hazard during the long term will be the heat. It will
largely depend on cloud cover and the coverage and timing of
showers, but overall, the combination of seasonal moisture, mainly
southeasterly winds, and 925-mb temperatures around two standard
deviations above normal will likely result in warm to hot
conditions, particularly from 11 AM to 4 PM, which are typically the
peak heat hours.

&&

.AVIATION...
(18Z TAFS)
Issued at 125 PM AST Mon Sep 21 2026

Mainly VFR conditions are expected across all TAF sites through most
of the forecast period. However, VCTS & TSRA will likely result in
intermittent MVFR to brief IFR conds and mountain obscurations at
TJSJ/TJBQ thru 21/22z. VCSH are expected across the rest of the
terminals. Mainly ESE winds up to 10 kt and sea breeze variations.
Light and variable winds expected overnight. A similar weather
pattern is expected to continue on Tuesday.

&&

.MARINE...
Issued at 125 PM AST Mon Sep 21 2026

A weak surface high pressure northeast of the region will maintain
gentle to moderate east-southeasterly trades of 5 to 15 knots over
the next few days, with winds gradually becoming easterly late
Tuesday into midweek as the high pressure becomes better organized
over the northeastern Atlantic. Seas will remain around 2 to 4 feet
across the Atlantic and Caribbean waters, including the local
passages. A small long-period northeasterly swell will reach the
area late Wednesday into late in the workweek, becoming the dominant
period across the local Atlantic Waters, including the Anegada and
northern sections of the Mona Passages. However, so far wave heights
are expected to remain between 2 to 4 feet across the region.
Passing showers may briefly increase winds and seas, while afternoon
thunderstorms moving into the western and northwestern coastal
waters of Puerto Rico may produce locally strong gusts, torrential
rainfall, and frequent lightning through the early evening hours.

&&

.BEACH FORECAST...
Issued at 125 PM AST Mon Sep 21 2026

Low rip current risk is expected across all local beaches for the
rest of today, tonight and tomorrow, Tuesday. From late Wednesday
onward, a moderate risk is expected to return across the north
and east-facing beaches of the islands, due to a small long-period
northeasterly swell and a slight increase in wind speeds across
the coastal waters. Additionally, daily diurnal convection will
continue to generate afternoon showers and isolated thunderstorms
across the northwest quadrant and northeastern Puerto Rico,
bringing localized hazards of lightning and gusty winds.
Beachgoers should monitor changing conditions and seek shelter
immediately. An elevated heat risk is forecast through at least
Tuesday, enhancing potential heat impacts. Residents and visitors
should remain hydrated and limit prolonged sun exposure during
peak afternoon heating. Follow guidance from local health
officials.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 3 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory until 3 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/LONG TERM/AVIATION...YZR
MARINE/BEACH...KML

NWS SJU Office Area Forecast Discussion