921
FXCA62 TJSJ 300854
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
454 AM AST Wed Sep 30 2026

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

.KEY MESSAGES...
Issued at 445 AM AST Wed Sep 30 2026

- Passing showers will continue to affect the USVI and
eastern/northern Puerto Rico during the night and morning hours,
followed by afternoon showers and thunderstorms over the interior
and western Puerto Rico each day.

- The potential for locally heavy rainfall and flooding will
increase Thursday through Saturday, especially across the interior
and western portions of Puerto Rico. Urban and small stream flooding
will be possible.

- A few stronger thunderstorms are possible Friday and Saturday as
conditions become more favorable for thunderstorm development. The
strongest thunderstorms could produce frequent lightning and gusty
winds. Gusty winds may knock down tree limbs and blow around
unsecured objects.

- The heat threat will remain limited today and Thursday. By Friday,
a warmer and more humid air mass may produce localized hazardous
heat, particularly across urbanized portions of north-central and
western coastal Puerto Rico. Stay hydrated, take frequent breaks,
and limit strenuous outdoor activity during the hottest part of the
day.

&&

.Short Term(Today through Friday)...
Issued at 445 AM AST Wed Sep 30 2026

During the overnight hours, an advective weather pattern prevailed
across the local area, with scattered showers moving across the
northern and northeastern portions of Puerto Rico and the smaller
islands. However, rainfall accumulations remained generally below
two tenths of an inch.

For today, a similar weather pattern to the past few days is
expected. East to east-northeast winds will maintain a relatively
fast steering flow, with breezy conditions persisting,
particularly across coastal areas. Precipitable water values will
remain near seasonal levels, generally between 1.7 and 1.9 inches,
providing sufficient moisture to support passing showers across
windward areas during the morning hours and locally induced
convection during the afternoon. However, warming at mid-levels
will gradually weaken the 700-500 mb lapse rates, limiting the
potential for deeper convection. Afternoon shower and isolated
thunderstorm activity will favor the interior and western to
southwestern portions of Puerto Rico. Given the relatively fast
steering flow, showers and thunderstorms should move quickly, and
with warming aloft, limit rainfall accumulations and the overall
flooding threat. Nevertheless, ponding of water on roads and in
poorly drained areas is possible, along with a low chance of urban
and small stream flooding.

Late tonight into Thursday, GOES-19 precipitable water imagery shows
a surface-induced perturbation approaching the local area, which is
expected to move across the region during the day. Winds will
gradually shift from the east-northeast to the east-southeast.
Although this perturbation will provide some dynamical support and
sufficient low- to mid-level moisture will be available, warm-
air advection near 500 mb will result in temperatures further warming
to around - 4 to -5 degrees Celsius, above normal for this time
of year. Therefore, 700-500 mb lapse rates will decrease,
providing less favorable conditions for deep convection.
Nevertheless, daytime heating, local effects, and the passage of
the perturbation should still promote showers and isolated
thunderstorms, particularly across the interior and western
portions of Puerto Rico during the afternoon. The flooding threat
will be somewhat higher than today, with urban and small stream
flooding possible where persistent or locally heavier rainfall
occurs.

By Friday, weather conditions aloft will become more favorable for
deeper convection as a series of mid- to upper-level troughs
deepen and sink southward northwest of the region. Cold-air
advection aloft will lower 500 mb temperatures to around -7 to -8
degrees Celsius, resulting in a renewed steepening of the 700-500
mb and the 850-700 mb lapse rates. At the same time, increasing
moisture and east- southeast low-level flow will support
additional shower and thunderstorm development. Afternoon
convection will again favor the interior and western/northwestern
portions of Puerto Rico, where an elevated flooding threat is
expected. With more favorable dynamics aloft, some thunderstorms
could become stronger, producing frequent lightning and gusty
winds in addition to locally heavy rainfall. Breezy conditions
will gradually diminish through the end of the short-term period
as low-level winds weaken somewhat.

The heat threat will remain limited today and Thursday, primarily
affecting individuals who are extremely sensitive to heat,
especially those outdoors without effective cooling or adequate
hydration. By Friday, east-southeast winds will advect a somewhat
warmer and more humid air mass into the region, with 925 mb
temperatures increasing to slightly above-normal levels. Although
widespread conditions warranting Heat Advisories may not be
necessary, hazardous heat may develop locally, particularly across
urbanized portions of north-central and western coastal Puerto Rico.
Individuals planning prolonged outdoor activities should take
preventive actions, including staying well hydrated, taking frequent
breaks in shaded or air-conditioned areas, and limiting strenuous
activity during the hottest part of the day.

&&

.Long Term(Saturday through Wednesday)...
Issued at 445 AM AST Wed Sep 30 2026

The long-term forecast remains on track, with slightly unstable
conditions on Saturday, transitioning to a typical weather pattern
by the beginning of next workweek. A surface high pressure over the
western Atlantic is expected to promote a southeasterly wind flow on
Saturday, becoming E-ESE later in the night. Abundant moisture
content should linger on Saturday, as the latest model guidance
suggests PWAT values between seasonal and near above climatological
normal (1.8 and 2.0 inches), with low variability between ensemble
members (around 0.3 inches). Additionally, the cross section over
the San Juan area shows abundant low- and mid-level moisture content
(RHs between 60 - 70%), with skinny sounding profiles and high
CAPEs (around 2000 and 3000 J/kg). As mentioned in the previous
discussion, the proximity of a series of upper-level troughs should
introduce  instability across the area, with the latest model
guidance suggesting cooler-than-normal mid-level temperatures
(between -7 and -8 C) and steeper lapse rates (between 6.0 and 6.5
C/km). These conditions, combined with the position of the troughs,
should allow cloud growth and ventilation, increasing the likelihood
of deeper convection. The most likely scenarios remain, with showers
and isolated thunderstorms moving over windward sections of the
islands during the morning, followed by deep afternoon convection
over interior and western/northwestern Puerto Rico. Additionally,
expected local island streamers are developing, moving over eastern
Puerto Rico, including the San Juan Metro Area. Lingering moisture
on Sunday combined with daytime heating, local effects, and sea
breeze convergence should trigger afternoon convection as well,
though a drier airmass filtering into the region and an approaching
ridge may limit the flood threat.

Monday onward, a lingering upper-level trough in our vicinity will
enhance instability through the end of the long-term period.
Combined with daytime heating and local effects, available low-level
moisture will drive localized shower and thunderstorm activity,
mainly over the interior and western half of Puerto Rico. By
Tuesday, precipitable water (PWAT) values are expected to return to
seasonal levels. This moisture increase, along with a shift to a
more east-southeasterly wind flow, will slightly enhance deeper
convection over the interior and northwestern areas.

Overall, the primary hazards for the long-term period remain a
limited-to-elevated risk of excessive rainfall, with the highest
threat on Saturday, with a limited-to-elevated heat risk persisting
throughout the forecast period. As a result, urban and small-stream
flooding is possible over roadways and poorly drained areas, and
flash flooding cannot be ruled out. For those sensitive to the heat,
staying hydrated and avoiding prolonged outdoor activities during
peak heating hours is highly recommended.

&&

.AVIATION...
(06Z TAFS)
Issued at 445 AM AST Wed Sep 30 2026

Mainly VFR conds are expected across all terminals thru the period.
Passing SHRA may result in brief MVFR cigs/vsbys at TIST/TISX and
occasionally TJSJ thru the morning. Winds will increase aft 13-14Z,
with E-ENE winds 15-18 kt and hir gusts. Afternoon TSRA may result
in VCTS at TJPS/TJBQ and mtn obsc after 17-18Z, with brief MVFR
conds possible in TSRA and mtn obscurations developing along the
Cordillera Central. The 30/00Z TJSJ sounding indicated ENE winds up
to 22 kt blo FL050.

&&

.MARINE...
Issued at 445 AM AST Wed Sep 30 2026

Building high pressure north of the region will maintain moderate to
locally fresh easterly trade winds through Thursday, resulting in
choppy conditions across the Atlantic and Caribbean waters and the
local passages. Winds should become mainly moderate from Friday into
the weekend and early next week. Seas will generally range from 3 to
5 feet, occasionally up to 6 feet across the exposed Atlantic and
Caribbean waters and passages, with somewhat lower seas in protected
coastal waters. Periods of passing showers will continue, while
afternoon showers and isolated thunderstorms are expected mainly
across the western and northwestern coastal waters of Puerto Rico.
Thunderstorms may produce locally higher winds and seas, lightning,
and reduced visibility.

&&

.BEACH FORECAST...
Issued at 445 AM AST Wed Sep 30 2026

A moderate risk of rip currents is expected across most north- and
east-facing beaches of Puerto Rico, Culebra, Vieques, and the U.S.
Virgin Islands through much of the forecast period. Life-threatening
rip currents will remain possible, especially near reefs, jetties,
and piers.

Additionally, showers and isolated thunderstorms may develop each
afternoon, particularly along the western coast of Puerto Rico and
nearby waters. Beachgoers should remain alert for rapidly changing
weather conditions and seek shelter if thunder is heard.

&&

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

$$

SHORT TERM/AVIATION...CVB
LONG TERM....MNG/JCP
BEACH/MARINE...CAM