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713
FXCA62 TJSJ 301731
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
131 PM AST Wed Sep 30 2026

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

.KEY MESSAGES...
Issued at 128 PM 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.

- 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.

- There is a moderate rip current risk for the north, east and
  west facing beaches of the islands.

&&

.Short Term(This evening through Friday)...
Issued at 128 PM AST Wed Sep 30 2026

As in previous discussions, similar synoptic features will continue
to serve as drivers for the weather conditions observed across the
islands. An upper-level low pressure system is currently positioned
to the northeast of the US Virgin Islands and is leading to a region
of upper-level convergence across the forecast area, inhibiting the
development of deep convection. This system will linger in the
vicinity of the region until Thursday. Nonetheless, a set of high
and low pressure systems in the low-levels located over the western
and central Atlantic Ocean will continue to promote breezy-to-
locally windy conditions across coastal and exposed areas through
Friday. These enhanced wind conditions may aid in the development of
advective showers during the morning hours and streamers from el
Yunque and the smaller islands during the afternoons, impacting the
US Virgin Islands and eastern Puerto Rico. Additionally, a moisture
surge associated with a surface-induced perturbation will reach the
local waters by tonight into Thursday. The associated moisture will
linger across the region until the end of the short term period and
will aid in the development of convection and thunderstorms. These
features combined with local effects are forecasted to bring periods
of enhanced activity over the remainder of the short term period.

Model guidance suggests that today will be dominated by drier mid
levels, with precipitable water values ranging from below average-to-
average values for the season (1.5-2 inches). In addition there will
be a gradual warming of mid-tropospheric temperatures which will
weaken the 700-500 mb lapse rates, this will act to decrease the
potential for the development of deep convective cells. The enhanced
steering flow from the ENE will favor the development of afternoon
convection leading to streamers across the US Virgin Islands,
Culebra, Vieques, and El Yunque rainforest. In combination with
local effects, the flow will also lead to afternoon convection
across the western interior and southwestern Puerto Rico. Given the
enhanced wind flow, these showers are expected to move quickly,
limiting the flooding risk. Ponding of water along roads and poorly
drained areas is possible.

Conditions are expected to become more favorable for the development
of convection by Thursday as the moisture associated to the surface-
induced perturbation makes its way to the forecast region. Thursday
will be characterized by a more humid vertical profile and gradually
decreasing mid-tropospheric temperatures. These features will favor
the development of deep convection. The steering flow will shift to
a ESE direction favoring the development of showers across the US
Virgin Islands and eastern Puerto Rico in the morning and
northwestern Puerto Rico during the afternoon. Given that the
conditions will be more conducive to the development of deep
convection the flooding risk for Thursday is elevated. This means
that flooding in urban areas, roads, small streams and washes where
frequent downpours occur is possible, along with a low chance for
isolated flash floods.

Friday is forecasted to be the most active weather day in the long
term period. With the upper-level low pressure system leaving the
region and similar favorable conditions to those seen on Thursday
persisting across the CWA, the potential for the development of
showers and thunderstorms will be higher. The rainfall and flooding
potential is also expected to be more widespread including the
northwestern, western interior, interior, and eastern regions of
Puerto Rico.

The breezy-to-locally windy conditions will gradually decrease by
Friday. Whereas precipitable water and low-level temperature values
are expected to increase by the end of the short term period. Thus,
the heat risk remains limited for today and Thursday, while it is
expected to be elevated by Friday. These levels of heat may affect
individuals sensitive to heat, without effective cooling and
adequate hydration. Citizens are invited to remain informed of the
evolving weather conditions.

&&

.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...
(18Z TAFS)
Issued at 128 PM AST Wed Sep 30 2026

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. However, tempo MVFR conds possible at
TJPS btw 30/18-22z due to locally induced TSRA over the Cordillera
Central, while mostly VCTS expected at TJBQ/TJSJ. Also, from 01/06-
12z -SHRA en route from the Leeward Islands may result in brief MVFR
conditions at TIST/TISX/TJSJ. The 30/12z TJSJ sounding indicated
east winds up to 26 kt blo FL050. Sea breeze variations and gusts up
to 25-30 kt expected to continue thru 30/21z, with lighter land
breeze variations overnight.

&&

.MARINE...
Issued at 128 PM 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. Tonight, seas will range from 5 to 6 feet,
occasionally up to 8 feet across the offshore Atlantic and
Caribbean waters and the Anegada Passage. 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 128 PM 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. A limited
heat threat will continue through tomorrow, increasing to elevated
on Friday and Saturday.

&&

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

$$

KEY MESSAGES...CVB
SHORT TERM...RVM
LONG TERM...MNG/JCP
AVIATION...DS
MARINE/BEACH FORECAST...KML

NWS SJU Office Area Forecast Discussion