984
FXCA62 TJSJ 191811
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
211 PM AST Sat Sep 19 2026

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

.KEY MESSAGES...
Issued at 208 PM AST Sat Sep 19 2026

- Limited to elevated flooding risk will remain possible each
  afternoon through early next week with slow-moving showers and
  thunderstorms, especially across interior and western PR

- Warm to hot conditions are expected through at least midweek,
  with the possibility of issuing heat-related products.

- A more active weather pattern is expected late in the week as a
  series of troughs and tropical wave increase the potential for
  heavy rainfall, frequent lightning, and gusty winds.

- The risk of rip currents is low over the coming days, but toward
  the end of next week, increasing winds may bring moderate rip
  current risk along north and east exposed areas.

&&

.Short Term(This evening through Monday)...
Issued at 208 PM AST Sat Sep 19 2026

A seasonal pattern prevailed during the morning hours, with a few
passing showers over the waters and windward coastal areas, but with
minimal rainfall accumulations. Heat index values have already
reached Heat Advisory criteria in localized areas, and additional
locations are expected to experience increasing heat indices through
the rest of the afternoon. However, increasing cloud cover and
shower activity may locally limit the heat threat. Nevertheless, the
Heat Advisory remains in effect through 5 PM AST this afternoon. The
19/12Z TJSJ sounding measured a precipitable water value of 2.15
inches, near the 90th percentile of climatological normal. This
above-normal moisture, combined with sufficient instability and
daytime heating, will support showers and isolated thunderstorms
through the afternoon, particularly across the interior and
western/northwestern Puerto Rico, as well as in streamers developing
downwind of the smaller islands. As of 1:30 PM AST, isolated showers
had developed, including a thunderstorm embedded within a streamer
affecting portions of the San Juan metro area.

Tonight into Sunday morning, passing showers will favor the US
Virgin Islands and eastern Puerto Rico under the prevailing easterly
flow. Low- to mid-level moisture is expected to improve somewhat by
Sunday, while 500 mb temperatures and mid-level lapse rates remain
near seasonal value. This should support somewhat deeper convection
compared with today. At the same time, the low-level steering flow
will gradually become more east-southeasterly, shifting the favored
area for afternoon convection toward northwestern Puerto Rico.
Although widespread organized convection is not anticipated, locally
heavy rainfall and isolated thunderstorms will remain possible, with
urban and small-stream flooding and isolated flash flooding possible
where repeated or persistent rainfall occurs.

By Monday, a somewhat drier layer is expected to return near 700 mb,
while precipitable water values decrease slightly. This should once
again limit the overall coverage of afternoon convection. However,
the 700-500 mb layer is forecast to remain relatively moist, while
lapse rates aloft become somewhat more favorable. Therefore,
although convection may be more isolated in coverage, any
thunderstorms that develop could still produce brief heavy rainfall
and isolated thunderstorms. An upper-level trough will also begin
moving southeastward toward the northeastern Caribbean late in the
period, with its more significant influence expected beyond the
short-term forecast period.

The main hazards through the short-term period will continue to be
excessive heat and locally heavy rainfall. Very warm low-level
temperatures will support another round of elevated heat indices on
Sunday and Monday, particularly across coastal and urban areas.
Afternoon convection may locally reduce temperatures, but where
sufficient heating persists, heat indices could again reach Heat
Advisory criteria. The greatest rainfall threat will remain across
the interior and western portions of Puerto Rico, with localized
flooding also possible across the San Juan metro area and other
locations affected by streamers or persistent showers.

&&

.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...
(18Z TAFS)
Issued at 208 PM AST Sat Sep 19 2026

Mostly VFR conds expected thru the period. SHRA/TSRA will affect
mainly TJSJ, TJPS, and TJBQ this aftn, with brief MVFR/IFR conds
from reduced VIS/CIGs and possible mtn obscurations. Activity should
diminish after 19/22Z, though VCSH will persist overnight at
TJSJ/TIST/TISX. Another round of SHRA/TSRA is expected after 20/17Z,
again mainly affecting TJSJ/TJPS/TJBQ. Winds E-ESE 10-15 kt with sea
breeze variations and occasional gusts near 20 kt, becoming
light/VRB overnight.

&&

.MARINE...
Issued at 208 PM AST Sat Sep 19 2026

A weak surface pressure gradient across the region is supporting
gentle to moderate easterly winds across the offshore waters and
local Caribbean passages this afternoon through the remainder of
the weekend, with light to gentle winds elsewhere. A surface high
will briefly build across the northeastern Atlantic early next
week, resulting in gentle to moderate east to southeast winds
across most waters and moderate chop through at least midweek.
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 208 PM AST Sat Sep 19 2026

The risk of rip currents is expected to remain low across most
beaches of Puerto Rico and the U.S. Virgin Islands through at
least midweek next week. However, even with a low risk, life-
threatening rip currents can occur near groins, jetties, reefs,
and piers. Beachgoers should always remain cautious when entering
the water. By the end of next week, a moderate risk of rip
currents will likely return along north and east exposed beaches.

From the remainder of the weekend into next week, the main beach
hazards will be heat and afternoon thunderstorms. Beachgoers
should stay hydrated, limit prolonged exposure to the sun, and
seek shelter indoors whenever thunder is heard.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

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