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676
FXCA62 TJSJ 291702
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
102 PM AST Tue Sep 29 2026

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

.KEY MESSAGES...
Issued at 1236 PM AST Tue Sep 29 2026

- Limited heat risk expected for the next few days, as easterly
  trade winds strengthen, allowing only localized areas across
  Puerto Rico and the US Virgin Islands to experience brief
  periods near heat advisory criteria.

- Trade wind showers will continue to move across the USVI and
  eastern sections of PR during the night/early morning hours,
  followed by afternoon thunderstorms over western PR each day.

- Higher moisture content is expected from Thursday through
  Saturday.

- A moderate risk of rip currents is expected to continue along
  the west, north, and east facing beaches of the islands through
  the end of the week.

&&

.Short Term(This evening through Thursday)...
Issued at 1236 PM AST Tue Sep 29 2026

A couple of synoptic features are currently setting the scene for
the observed weather pattern over the CWA. A lingering upper-level
trough located to the northeast of the US Virgin Islands is
currently promoting a region of enhanced upper-level convergence
which is acting to limit convective activity. Furthermore, the
steering flow for the duration of the short term forecast will be
driven by a building surface high pressure system currently located
over the east Atlantic Ocean. This system will lead to the
strengthening of the steering flow with light to locally breezy
winds expected today and Thursday, and breezy to locally windy
conditions expected by Wednesday. This increase in wind flow will
strengthen the potential for the development of advective showers
and streamers from the US Virgin Islands, Vieques, Culebra, and EL
Yunque rainforest during the morning into the afternoons,
potentially impacting the windward waters and eastern sectors of
Puerto Rico. This means that the activity that develops over the
remainder of the short term forecast will be driven by the building
surface high pressure and local effects.

For today, while mid-to-upper levels will act to inhibit the
development of showers and thunderstorms, there are other features
that may aid in the eventual development of rainfall activity,
particularly across the southwest of Puerto Rico during the
afternoon hours. There are lower than average mid-tropospheric
temperatures, this favors the potential for deep convective cells if
activity does develop over the region. In spite of the drier mid-
levels, precipitable water values are expected to increase by the
afternoon across southwestern sector of Puerto Rico. Model guidance
suggest that these conditions coupled with local effects will lead
to the development of showers and thunderstorms over the region.
Given the light winds, activity may linger, this means there is a
limited risk of urban and small stream flooding across the region.
Where rainfall occurs you may expect the ponding of water along
roads and poorly drained areas.

Similar conditions are expected by Wednesday, with the difference
that mid-tropospheric temperatures will increase and become less
amenable for the development of deep convective cells. Nonetheless,
the steering flow will also increase and shift to be predominantly
from the E. This will favor the potential for the development of
advective showers and streamers from el Yunque and the smaller
islands. Hence, a more active morning can be expected with impacts
predominantly focused on the US Virgin Islands, windward waters and
eastern Puerto Rico in the morning. The shift in the wind flow will
also move the focus of the afternoon convection to the western and
western interior regions of Puerto Rico. Rainfall associated impacts
are expected to be similar to those for today.

A similar pattern is forecasted for Thursday. With activity expected
to be focused on the western interior and the northwestern regions
of Puerto Rico during the afternoon. This pattern will by similarly
set by local effects and breezy-to-locally windy ESE winds. It will
be further enhanced by increasing precipitable water values over the
west of Puerto Rico during the afternoon.

Given the breezy-to-locally windy conditions along with average
precipitable water values, the low-level temperatures are expected
to range around average values for the season. Thus, lower
temperatures to those experienced during the previous days may be
expected. Individuals that are sensitive to heat should remain alert
as the heat risk remains limited throughout the short term forecast.
As always, citizens and visitors are invited to keep updated with
the evolving weather conditions.

&&

.Long Term(Friday through next Monday)...
Issued at 133 AM AST Tue Sep 29 2026

Unstable conditions are still expected on Friday and Saturday,
transitioning to a typical weather pattern Sunday onward. Model
guidance continues to suggest a surface high pressure the western
Atlantic, promoting an E-SE wind flow throughout the period. As
mentioned in the previous discussion, a surface trough may weaken
the surface high, with isolated breezy conditions across coastal
areas of the islands. Friday and Saturday remain the active days of
the long-term, as the latest model guidance suggests that the
presence of upper-level troughs should introduce instability across
the eastern Caribbean. In terms of moisture, the deterministic
guidance of the GFS and ECMWF continues to suggest abundant tropical
moisture across the region, with PWAT values between 1.8 and 2.0
inches, mainly in the low and mid levels. Additionally, model
sounding plots continue to show skinny profiles, with weak inversion
caps that could easily be broken and high CAPEs.  Due to the
proximity of the troughs, mid-levels might get cooler than expected
(500 mb temperatures reaching -8 C) with steeper lapse rates in the
mid-levels, allowing cloud growth and instability aloft. Therefore,
the most likely scenario should be for isolated to scattered showers
across the windward sections during the night and morning hours,
with deep convection in the afternoon, particularly across the
interior and western/northwestern. Additionally, local island
streamers are also expected, affecting the San Juan Metro Area. On
Sunday, drier air is expected to filter in the low-to-mid-levels
(850 - 500mb), inhibiting deeper convection over the islands, as
precipitable water (PWAT) values are expected to drop to seasonal or
below-seasonal values. However,  local effects and daytime heating
driven by an easterly flow should promote afternoon convection
mainly over the interior to western half of Puerto Rico. A similar
pattern is expected on Monday, as drier air persists in the low-to-
mid levels. In general, shower and thunderstorm activity is expected
each day, with the highest flooding threat on Friday and Saturday.
Rainfall accumulations could enhance the chance of urban and small
stream flooding over the aforementioned areas. Meanwhile, Sunday and
Monday rainfall activity may lead to ponding of water over roadways
and poorly drained areas, including minor flooding. In addition to
flooding, thunderstorm activity could produce gusty winds and
frequent lightning, without ruling out small hail.

Warmer conditions are expected throughout the long term, with
seasonal to above-seasonal 925-mb temperatures. Although heat
indices should exceed 100 F, a limited risk of excessive heat will
persist throughout the forecast period. Individuals sensitive to
these conditions should stay hydrated and avoid prolonged outdoor
activities.

&&

.AVIATION...
(18Z TAFS)
Issued at 1236 PM AST Tue Sep 29 2026

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, streamers developing downwind
of the islands may cause brief MVFR conds at TISX/TIST, while
locally induced TSRA/SHRA over S-SW PR could lead to tempo MVFR
cigs at TJPS. The 29/12z TJSJ sounding indicated ENE winds up to
20 kt blo FL120.

&&

.MARINE...
Issued at 1236 PM AST Tue Sep 29 2026

Building high pressure north of the region will maintain choppy
conditions across the Atlantic waters and Mona Passage, with winds
strengthening tonight into Wednesday and becoming locally fresh.
Seas will generally range from 3 to 5 feet, occasionally up to 6
feet across the Atlantic and Caribbean waters, and the local passages.
Periods of passing showers will persist, and afternoon showers
and isolated thunderstorms are expected to develop mainly near
western Puerto Rico over the next few days. Thunderstorms may
produce locally higher winds and seas, lightning, and reduced
visibility.

&&

.BEACH FORECAST...
Issued at 1236 PM AST Tue Sep 29 2026

A moderate risk of rip currents will persist into tonight along the
exposed beaches of western, northern, and eastern Puerto Rico, as
well as Culebra and the U.S. Virgin Islands. Breaking waves of
around 5 feet are expected along portions of the northern coastline,
with lower heights elsewhere. Tomorrow, the moderate rip current
risk will continue for those areas and expand to exposed beaches of
Vieques and southeastern Puerto Rico due to increasing easterly
trade winds. This means that rip currents are possible in the surf
zone.

Afternoon thunderstorms may also pose a lightning threat to beachgoers,
especially across southwestern Puerto Rico for the rest of today,
and western Puerto Rico over the next few days. A limited heat
threat and locally breezy conditions will continue over the next
few days.

&&

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

$$

SHORT TERM...RVM
AVIATION/KEY MESSAGES...DSR
BEACH/MARINE...KML
LONG TERM....MNG/JCP

NWS SJU Office Area Forecast Discussion