429
FXCA62 TJSJ 010812
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
412 AM AST Thu Oct 1 2026

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

.KEY MESSAGES...
Issued at 353 AM AST Thu Oct 1 2026

- Passing showers will continue to affect the USVI and eastern Puerto
  Rico during the morning hours, followed by afternoon showers
  and thunderstorms across the interior and western portions of
  Puerto Rico.

- An elevated flooding threat will persist across portions of
  western Puerto Rico through Saturday. Urban and small stream
  flooding will be possible.

- Conditions will become increasingly favorable for deeper and stronger
  thunderstorms Friday and Saturday as moisture increases and
  temperatures cool aloft. Stronger thunderstorms may produce
  frequent lightning and gusty winds.

- The heat threat will persist through the short-term period and
  is expected to become elevated on Friday, 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.

- A moderate rip current risk will continue along the north-, and
  east-facing beaches of Puerto Rico, as well as across portions
  of the U.S. Virgin Islands. Life-threatening rip currents are
  possible in the surf zone.

&&

.Short Term(Today through Saturday)...
Issued at 353 AM AST Thu Oct 1 2026

During the overnight hours, GOES-19 precipitable water imagery
showed the leading edge of a surface-induced perturbation beginning
to move across the U.S. Virgin Islands and eastern Puerto Rico,
increasing the frequency of passing showers across these areas.
However, 700-500 mb Advected Layered Precipitable Water (ALPW)
imagery indicated that the bulk of the deeper moisture associated
with the perturbation remained southeast of the region. Despite
periods of moderate rain affecting windward coastal areas, as
observed by Doppler radar, estimated rainfall accumulations
generally remained below three-tenths of an inch. Through the
remainder of the morning, passing showers will continue to affect
windward sections of the islands, with activity gradually spreading
westward.

For today, precipitable water values will generally remain near
seasonal levels, around 1.8 to 2.0 inches. Although moisture will
gradually increase, relatively modest mid-level lapse rates and some
lingering drier air aloft should initially limit the depth and
persistence of convection. Nevertheless, the combination of the
surface-induced perturbation, daytime heating, sea-breeze
convergence, and local effects will support afternoon shower and
thunderstorm development, particularly across the interior and
western portions of Puerto Rico. Given the prevailing easterly flow,
showers and thunderstorms should move at a moderate pace; however,
repeated activity may still result in urban and small-stream
flooding, and a low chance of isolated flash flooding. Therefore, an
elevated flooding threat will continue across portions of western
Puerto Rico, with a limited threat elsewhere where locally heavy
rainfall develops.

Conditions will become more favorable for deeper convection by
Friday as the bulk of the perturbation moisture moves across the
region. Successive deterministic model runs and ensemble guidance
continue to support a gradual moistening of the mid-levels from
Thursday into Friday and Saturday, increasing confidence in a
progressively more favorable environment for sustained deep
convection. At the same time, a series of mid- to upper-level
troughs sinking southward will promote cooling aloft, with 500 mb
temperatures forecast to fall to around -7 to -8 degrees Celsius
while 700-500 mb lapse rates steepen to well above normal levels.
This will increase instability aloft and, when combined with
sufficient columnar moisture, afternoon sea-breeze convergence, and
local orographic effects, should support deeper and occasionally
stronger thunderstorms. As a result, locally heavy rainfall will
remain a concern, particularly across western and northwestern
Puerto Rico, while stronger thunderstorms may also produce frequent
lightning and gusty winds.

By Saturday, the favorable mid-level dynamic pattern will persist,
with relatively cold temperatures aloft and steep mid-level lapse
rates continuing to support deep convection. Moisture availability
may become somewhat more variable, particularly in the lower to mid
levels, which may modulate the overall coverage of showers and
thunderstorms. Even so, where sufficient moisture overlaps with
local convergence, thunderstorms could become locally vigorous and
produce heavy rainfall. The elevated flooding threat will therefore
persist across western Puerto Rico, with a limited threat across
adjacent areas and portions of eastern Puerto Rico.

The heat threat will also persist through the short-term period.
East to southeast winds, increasing moisture, and periods of
sunshine will continue to favor warm and humid conditions,
particularly across coastal and urban areas. The heat threat is
expected to become elevated on Friday, especially for individuals
sensitive to heat or those without effective cooling or adequate
hydration. Cloud cover and convective development may provide some
localized relief during the afternoon, but heat impacts will remain
possible through Saturday.

&&

.Long Term(Sunday through Thursday)...
Issued at 353 AM AST Thu Oct 1 2026

A variable and seasonal pattern will persist for the first part of
the long term. On Sunday, broad surface high pressure will remain in
place across the central Atlantic through Sunday into Monday, while
its combines with an induce disturbance over the central Atlantic,
maintaining an easterly wind flow within the 03 km layer across
Puerto Rico and the U.S. Virgin Islands. Aloft, a cut-off low
anchored over the western Atlantic and Caribbean, just north of
eastern Cuba, will enhance southerly winds at 250 mb, favoring the
development of an extensive upper-level cloud deck over the islands.
The influence of this upper-level low will also be evident at 500
mb, where southerly flow and slightly colder temperatures, near -7C
to -8C, are expected. Despite the colder temperatures aloft,
available moisture will remain limited, with PWAT values generally
near climatological normals and most of the moisture confined to the
lower levels. Therefore, although periods of upper-level cloudiness
will persist, especially into Monday, the overall weather pattern
should remain relatively stable with limited deep moisture available
for significant convective development. On Monday, similar weather
conditions will persist with possibly less upper cloudiness as the
advecting upper-level moisture pattern with the high clouds moves
out of the region. As a result, warmer temperatures and heat indices
are possible, with a limited heat threat along the islands, with
some isolated areas that have an elevated heat threat, especially in
the coastal areas and urbanized areas.

On Tuesday, the mid-level ridge will still be present, marginally
limiting deep convection. However, lapse rates returning to seasonal
values signal a better chance of typical afternoon showers and
thunderstorms driven by an easterly flow. This pattern should allow
afternoon convection to move over the interior and then over western
portions of Puerto Rico. A tropical wave will reach the islands by
Tuesday evening, increasing the PWAT values to above-seasonal levels
through Wednesday. Therefore, Wednesday is forecast to be the more
active day in terms of shower and thunderstorm activity, supported
by mid-level lapse rates at above-seasonal levels (exceeding 6.0
C/km). Additionally, a more east-southeasterly flow could drive
further activity over northwestern PR. As a result, for Tuesday into
Thursday there is a high chance of observer a limited to elevated
flooding threat especially over western sections of Puerto Rico.

Overall, the primary hazards for the long-term period will remain a
limited-to-elevated heat risk and a limited risk of excessive
rainfall from Sunday through Tuesday. 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 outdoor activities during peak
heating hours is highly recommended.

&&

.AVIATION...
(06Z TAFS)
Issued at 353 AM AST Thu Oct 1 2026

Mainly VFR conds are expected across all terminals thru the forecast
period, with generally FEW-SCT clds btw FL025-FL050, becoming BKN at
times in SHRA. Passing -SHRA may affect TIST/TISX and TJSJ during
the early morning hrs, with brief MVFR conds possible, particularly
at TISX thru 13Z. SHRA/TSRA development is expected across interior
and western PR aft 16Z, with VCTS at TJPS/TJBQ. Brief MVFR or lower
conds are possible in TSRA, mainly at TJBQ btwn 17-21Z. E-ESE winds
around 7-12 kt overnight will increase aft 13-14Z to 12-18 kt with
hir gusts, along with sea breeze variations, before weakening again
aft 22-23Z.

&&

.MARINE...
Issued at 353 AM AST Thu Oct 1 2026

A surface high pressure north of the region will maintain moderate
to locally fresh easterly trade winds through at least Friday,
producing choppy conditions across the Atlantic and Caribbean waters
and local passages. Winds will become mainly moderate over the
weekend into early next week, maintaining a moderate chop. A
disturbance moving through the region today will increase showers
and thunderstorms, potentially creating locally hazardous conditions
for small craft due to gusty winds, higher seas, lightning, and
reduced visibility. Lingering moisture will sustain unsettled
conditions into the early weekend.

&&

.BEACH FORECAST...
Issued at 353 AM AST Thu Oct 1 2026

A moderate risk of rip currents will persist across most exposed
north- and east-facing beaches of Puerto Rico and the U.S. Virgin
Islands through at least Sunday. Beginning Monday, the moderate risk
is expected to become more localized. Avoid unnecessary risks and
remain cautious, as dangerous rip currents can still occur,
particularly near piers, jetties, groins, and rocky areas. Swim near
a lifeguard and follow beach patrol guidance and posted warning
flags.

Thunderstorms may develop each afternoon. If you hear thunder, leave
the water and seek shelter inside a substantial building or enclosed
vehicle.

&&

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

$$

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