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687
FXCA62 TJSJ 200545
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
145 AM AST Sun Sep 20 2026

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

.KEY MESSAGES...
Issued at 145 AM AST Sun Sep 20 2026

- Hot and humid conditions will continue through much of the
  week, with extreme heat concerns each day.

- Afternoon showers and thunderstorms may produce locally heavy
  rainfall that could lead to flooding, mainly across interior and
  western Puerto Rico.

- A wetter and more unstable pattern is expected mid to late next
  week, increasing the potential for more widespread heavy rainfall
  and stronger thunderstorms.

- Increasing winds mid to late next week may bring a moderate rip
  current risk to north- and east-facing beaches.

&&

.Short Term(Today through Tuesday)...
Issued at 145 AM AST Sun Sep 20 2026

A generally seasonal but locally active weather pattern is expected
today, with relatively light winds allowing local effects to play a
larger role in shower and thunderstorm development. Precipitable
water values will remain near to locally above normal, supporting
passing morning showers across the U.S. Virgin Islands and eastern
Puerto Rico, followed by afternoon showers and thunderstorms across
the interior and western to northwestern Puerto Rico. Streamers are
also expected to develop downstream of the islands and higher
terrain, and may produce locally significant rainfall where they
persist. With relatively weak steering winds, showers and
thunderstorms may linger somewhat later than usual this evening
before transitioning to more typical passing overnight showers
across the local waters, U.S. Virgin Islands, and eastern Puerto
Rico. Low concentrations of Saharan dust will remain across the region.

On Monday and Tuesday, a weak mid-level ridge will help introduce
some drier air, while surface high pressure over the Atlantic
becomes better organized and winds gradually strengthen. Despite
the drying, enough low-level moisture will remain and, combined
with strong daytime heating, will support active afternoons Monday
and Tuesday. Winds will shift from east-southeast Monday to more
easterly Tuesday, favoring afternoon convection from the interior
into western Puerto Rico with a more westward movement of showers
and thunderstorms. The driest period is expected Tuesday evening,
while colder temperatures aloft associated with an approaching
mid- to upper-level trough begin the transition toward a more
unstable pattern Tuesday night into Wednesday.

The main hazards through the period will be heat, locally heavy
rainfall leading to flooding, lightning, and gusty winds. A Heat
Advisory is in effect today, with extreme heat concerns likely to
continue Monday and Tuesday. The greatest rainfall threat will occur
during the afternoon, mainly from the interior into western Puerto
Rico, where slow-moving or repeated convection may produce urban and
small-stream flooding, isolated flash flooding, and landslides in
steep terrain, especially where soils remain locally saturated from
recent heavy rainfall.

&&

.Long Term(Wednesday through Sunday)...
Issued at 145 AM AST Sun Sep 20 2026

At this time, Wednesday is forecast to continue the transition toward
a more unstable pattern as the mid- to upper-level trough exerts a
greater influence across the region. By Thursday, a tropical wave is
expected to reach the islands, providing additional moisture and
low-level support while conditions aloft become increasingly
favorable for convection. The latest model guidance shows mid-level
temperatures falling to around -9 degrees Celsius. With favorable
mid-level moisture also in place, Thursday into Friday appears to be
the most favorable period for stronger convection.

More frequent passing showers are expected across windward areas and
the U.S. Virgin Islands, while afternoon thunderstorms across Puerto
Rico could produce heavy rainfall, frequent lightning, and locally
gusty winds. An elevated flooding risk is possible during this period,
particularly across urban areas, small streams, and locations
receiving repeated heavy rainfall.

Although instability aloft will persist into Saturday, guidance
indicates that mid-level moisture will begin to decrease,
particularly within the 700-500 mb layer. Afternoon thunderstorms
will remain possible, especially across the interior and western
portions of Puerto Rico, but coverage should become more localized as
moisture gradually decreases.

By Sunday, additional mid-level drying is anticipated while
temperatures aloft remain relatively cold and lapse rates remain
favorable. Therefore, sufficient instability should remain for
afternoon convection, although reduced moisture should limit both
thunderstorm coverage and lightning activity compared with Thursday
and Friday. Low-level winds are also expected to strengthen somewhat
toward the end of the period.

Heat risk will remain a concern throughout the period, but it will
largely depend on cloud cover and the coverage and timing of showers
associated with the upper-level trough and tropical wave. Greater
cloudiness and earlier rainfall could limit daytime heating in some
areas, while locations experiencing longer periods of sunshine could
reach higher heat indices. Therefore, residents and visitors should
continue monitoring the forecast as confidence increases regarding
the evolution of the trough, tropical wave, rainfall, and heat
risk.

&&

.AVIATION...
(06Z TAFS)
Issued at 145 AM AST Sun Sep 20 2026

Mostly VFR conds expected thru the period. Passing SHRA may briefly
lower VIS/CIGs at TJSJ/TIST/TISX overnight. SHRA/TSRA increase
across PR after 20/16-17Z, with greatest impacts at TJBQ/TJSJ around
20/17-20Z; PROB30 groups for 4SM TSRA BKN/OVC020-030. VCTS psbl
at TJPS. Winds light/VRB overnight, becoming E-ESE 10-15 kt after
20/13Z with sea-breeze variations and higher gusts near TSRA, then
returning to light/VRB after 20/22Z.

&&

.MARINE...
Issued at 145 AM AST Sun Sep 20 2026

Weak surface high pressure north of the region will maintain gentle
to moderate easterly trades of 5 to 15 knots today, with winds
gradually becoming east to southeast through midweek as high pressure
over the northeastern Atlantic becomes better organized. Seas will
remain around 2 to 4 feet across the Atlantic and Caribbean waters,
including the local passages. Passing showers may briefly increase
winds and seas, while afternoon thunderstorms moving into the western
coastal waters of Puerto Rico may produce locally strong gusts,
torrential rainfall, and frequent lightning through the early
evening hours.

&&

.BEACH FORECAST...
Issued at 145 AM AST Sun Sep 20 2026

A low risk of rip currents is expected across the beaches of Puerto
Rico and the U.S. Virgin Islands through around midweek. However,
life-threatening rip currents can still occur near groins, jetties,
reefs, and piers. Mid to late week, a moderate risk is expected to
return across some north- and east-facing beaches as winds increase
and a small long-period northeasterly swell reaches the region.

Other beach hazards will include heat and afternoon thunderstorms,
particularly across western Puerto Rico. Thunderstorms may produce
gusty winds and frequent lightning. Beachgoers should remain
hydrated, limit prolonged sun exposure, and seek shelter indoors
whenever thunder is heard.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 3 PM AST this afternoon
     for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 3 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

EVE...DSR/MMC
MID...ICP

NWS SJU Office Area Forecast Discussion