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180
FXCA62 TJSJ 190655
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
255 AM AST Sat Sep 19 2026

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

.KEY MESSAGES...
Issued at 255 AM AST Sat Sep 19 2026

* Heat will remain a concern through midweek next week, mainly
  across lower elevations, interior valleys, and urban areas.

* Localized flooding will remain possible each afternoon into the
  evening through early next week with slow-moving showers and
  thunderstorms, especially across central and western Puerto
  Rico.

* A more active weather pattern is expected late in the week as
  an upper-level trough and tropical wave increase the potential
  for heavy rainfall, frequent lightning, and gusty winds.

* Increasing winds by mid to late week may bring locally choppy
  seas and a moderate rip current risk to some beaches.

&&

.Short Term(Today through Monday)...
Issued at 255 AM AST Sat Sep 19 2026

A generally seasonal and locally driven pattern will prevail
through the short term, with only subtle day-to-day changes.
Large-scale forcing will remain weak, leaving available moisture,
daytime heating, sea-breeze convergence, and terrain effects as
the main drivers of shower and thunderstorm development. Moisture
will remain near to above seasonal levels, supporting passing
overnight and early morning showers across the US Virgin Islands
and eastern Puerto Rico, followed by afternoon showers and
isolated thunderstorms from the interior toward western Puerto
Rico. Conditions near 700 mb will help modulate convective depth
and coverage, as periods of relative drying may limit development
despite sufficient moisture elsewhere in the column. Minimal
concentrations of Saharan dust will also remain across the region.

The main changes will involve the steering flow and mid-level
environment. Easterly flow today will favor mainly westward-moving
convection, while a northeasterly component near 700 mb may carry
some activity farther southwest. Winds will gradually become more
east-southeasterly Sunday into Monday, shifting the favored
downstream corridor toward western and northwestern Puerto Rico.
Sunday may support somewhat deeper convection as mid-level
moisture improves and temperatures aloft cool slightly. By Monday,
stronger east-southeast flow and renewed drying near 700 mb may
again limit the persistence and coverage of afternoon convection.
An upper-level trough will begin moving southeast toward the
northeastern Caribbean late in the period.

The main hazards will be excessive heat and heavy rainfall. Above-
normal low-level temperatures, high humidity, and generally light
to moderate winds will support elevated to locally significant
heat risk each day, with a Heat Advisory in effect today for
Puerto Rico and the US Virgin Islands. The greatest rainfall
concern will be across portions of the interior and western Puerto
Rico, with a lower-end flooding risk extending into the San Juan
metropolitan area and portions of northern and central Puerto
Rico. Urban and small-stream flooding is possible, along with
isolated flash flooding and isolated landslides in steep terrain.
Thunderstorm coverage should remain isolated overall, with Sunday
appearing somewhat more favorable for deeper convection.

&&

.Long Term(Tuesday through Saturday)...
Issued at 255 AM AST Sat Sep 19 2026

Weather conditions will become increasingly unstable during the
long-term period as a mid- to upper-level ridge is replaced by a
mid- to upper-level trough later in the week. The ridge will
still be located west of the area on Tuesday but will be pushed
farther away as the upper-level trough begins to dig into the
local area from the north-northeast and remains in place for the
remainder of the period. This pattern will continue to support
occasional induced surface perturbations, with moisture reaching
the area throughout the period and keeping precipitable water
values between 1.75 and 2.00 inches, which are seasonal values for
this time of year. Additionally, a tropical wave is expected to
cross the area between Thursday and Friday, providing additional
low-level support for more frequent showers and thunderstorms.
Moderate steering winds will prevail through the period, with east
to east-southeasterly flow between 10 and 15 knots.

On Tuesday and Wednesday, a more typical pattern is expected, with
the usual passing showers across windward areas and afternoon
convection across the interior, western, and northwestern
portions of Puerto Rico. A more unstable period is expected late
in the week (Thursday through Saturday), when the tropical wave
crosses the area and the upper-level trough becomes more favorably
positioned for convective activity. However, uncertainty remains,
as conditions will depend on the exact position of the trough,
whose axis is currently forecast to be over the local area. If the
current forecast materializes, 500-millibar temperatures during
the favorable period are forecast to be around -9 degrees Celsius,
with mid-level lapse rates (between 700 and 500 millibars) around
7 degrees Celsius per kilometer. Both are around 2 standard
deviations favorable for convective support, suggesting active
afternoons with scattered showers and the potential for strong
thunderstorms with frequent lightning and locally strong gusty
winds, especially across the interior, western, and northwestern
sections of Puerto Rico. Additionally, urban and small-stream
flooding is expected each afternoon during the second half of the
week across these areas, as well as downwind from El Yunque and
the U.S. Virgin Islands. On Thursday and Friday, more frequent
passing showers are possible across windward areas, including the
U.S. Virgin Islands, due to the tropical wave.

An elevated heat risk will continue through the long-term period
due to elevated moisture content, which will push heat indices
above 100 degrees Fahrenheit each day, especially across lower
elevations and urban areas that do not experience rainfall during
the warmer part of the day.

&&

.AVIATION...
(06Z TAFS)
Issued at 255 AM AST Sat Sep 19 2026

Mostly VFR conds expected thru the period. VCSH expected overnight
at TIST/TISX and TJSJ, with passing SHRA psbl at times. VCTS
expected near TJSJ, TJBQ, and TJPS mainly 19/17-21Z, with brief
MVFR/IFR conds psbl in heavier SHRA/TSRA. Some passing SHRA may
also affect TJPS/TJBQ overnight. Winds light/VRB to calm thru
early morning, increasing to 10-15 kt after 19/13Z with strong
sea-breeze variations, then becoming light/VRB to calm again after
19/21-23Z. Weak steering flow may result in slow-moving SHRA/TSRA
during the aftn.

&&

.MARINE...
Issued at 255 AM AST Sat Sep 19 2026

A surface high pressure extending across the Atlantic north of the
region will maintain gentle to moderate easterly winds across the
local waters through the weekend. The surface high will strengthen
northeast of the area early next week, leading to a slight
increase in winds and locally choppy seas across the offshore
waters. Passing showers will remain possible across the regional
waters each day, while afternoon thunderstorms may linger into the
evening hours across the western waters of Puerto Rico.

&&

.BEACH FORECAST...
Issued at 255 AM AST Sat Sep 19 2026

A low risk of rip currents will likely prevail across most beaches
of Puerto Rico and the U.S. Virgin Islands through early next
week. However, even under a low risk, life-threatening rip
currents can still occur near groins, jetties, reefs, and piers.
By midweek, a moderate risk is expected to return across some
north- and east-facing beaches of the islands as winds increase
across the coastal waters.

Another beach hazard through the weekend will be the heat.
Beachgoers should remain hydrated and limit prolonged sun
exposure. Additionally, afternoon thunderstorms over western
Puerto Rico may produce gusty winds and lightning. Beachgoers
should seek shelter whenever thunder is heard.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

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

NWS SJU Office Area Forecast Discussion