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744
FXCA62 TJSJ 131702
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
102 PM AST Tue Oct 13 2026

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

.KEY MESSAGES...
Issued at 1154 AM AST Tue Oct 13 2026

- Heavy afternoon and evening showers may cause localized urban
  and small stream flooding across interior and western PR, with
  isolated landslides possible. Brief showers will affect the US
  Virgin Islands and eastern PR, mainly overnight and during the
  morning hours.

- Weather conditions will improve late Wednesday into Thursday,
  reducing the potential for widespread flooding, lightning, and
  landslides.

- Heat indices around 102 and 107 degrees Fahrenheit will pose a
  risk to outdoor workers and vulnerable populations after 10 AM,
  when mostly clear skies prevail.

- A moderate rip current risk will prevail along the northern and
  eastern-facing beaches of Puerto Rico through the workweek and
  will include the USVI by Saturday, elsewhere a low risk is
  expected.


&&

.Short Term(This afternoon through Thursday)...
Issued at 1154 AM AST Tue Oct 13 2026

A lingering upper-level trough to the west will maintain favorable
convective instability across the local islands today. However, a
building upper-level ridge to the east will gradually increase 250
mb heights and mid-tropospheric thicknesses through midweek. Deep-
layer atmospheric moisture remains abundant through early Wednesday,
with precipitable water (PWAT) values fluctuating near or slightly
above 2.00 inches, which is between the 50th and 75th percentiles of
climatology. Low-level steering winds will prevail from the east-
northeast at 10 to 15 knots, steering passing showers across
windward coastal areas of PR and the U.S. Virgin Islands during the
late night and early morning hours. By early this afternoon, the
available moisture combined with daytime heating, the local sea
breeze convergence, and topography will trigger scattered to
numerous showers and isolated thunderstorms across the interior and
western sections of PR. This activity could produce localized urban
and small-stream flooding, as well as isolated mudslides in steep
terrain.

The atmospheric profile dries and stabilizes late Wednesday into
Thursday as the ridge strengthens and holds over the northeastern
Caribbean. PWAT drops steadily toward 1.65 to 1.75 inches, falling
near or below the seasonal normals. At the same time, mid-level
moisture collapses as 700500 mb relative humidity values plummet
from near 60 percent down to 30 to 40 percent. This mid-level drying
is accompanied by pronounced subsidence warming, with 500 mb
temperatures rising from around -7.5C up to -4.0C and lapse rates
dropping into a more stable 5.3 to 5.8C/km range. This thermal cap
aloft will suppress vertical development, limiting afternoon
activity to shallow, localized showers over the interior and
southwestern PR. Elsewhere across PR and the USVI, sunny to partly
cloudy skies and predominantly dry weather will prevail.

In terms of the heat risk through the short-term period, the 925 mb
temperatures are expected to range between 21-23C, yielding daytime
highs in the upper 80s to low 90s across the coastal plains and
urban areas. With lingering low-level humidity, afternoon heat
indices are expected to climb to 102-107F in general, posing a
limited heat risk for outdoor workers and vulnerable populations.

&&

.Long Term(Friday through next Tuesday)...
Issued at 1154 AM AST Tue Oct 13 2026

Some changes were introduced to the long term forecast, as the
stable weather conditions may linger in the weekend. As mentioned in
the previous discussion, winds on Friday should continue under a E-
ENE pattern due a frontal boundary north of the CWA. As a surface
high builds over the Western Atlantic, winds may veer and become
from the E-SE by Saturday, becoming easterly on Sunday and Monday.
In the previous discussion, Friday was forecast as the less "active"
day due to a mid-level ridge dominating the weather pattern through
Saturday, becoming unstable by Sunday due to an upper-level trough
approaching the Caribbean. Nevertheless, the latest model solutions
are tending to more stable conditions as the upper-level trough may
linger over the Central Atlantic while the ridge gets closer to the
local area. With the presence of the ridge, warmer than usual mid
level temperatures (above -6 C) are very likely, along with
subsidence, indicating stability aloft. Although moisture content
remains near seasonal values (PWATs up to 1.75 inches), a dry slot
into the mid levels (below 50%) filtering on Friday will also
limit deep afternoon convection. Additionally, the Galvez-Davison
Index (GDI) suggest that the best chance of observing convection
activity should be in the afternoons over interior and western
Puerto Rico, but mainly shallow. In general, daytime heating,
local effects, and seabreeze convergence are likely the main
drivers of the afternoon convection, though the weather conditions
won`t necessarily support long-lasting thunderstorms, lowering
the flood threat. Over the weekend, these rainfall accumulations
may lead to ponding of water over roadways and poorly drained
areas over the aforementioned areas. The latest guidance suggest a
warmer conditions this weekend, with 925 mb temperatures warmer
for this time of the year. During peak hours of the day,
particularly in urban and low-lying areas, heat indices could
exceed 100 F, affecting mainly sensitive groups. Therefore, the
heat threat will remain limited to elevated throughout the
forecast period.

&&

.AVIATION...
(18Z TAFS)
Issued at 1154 AM AST Tue Oct 13 2026

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, tempo MVFR to brief IFR conds are
possible at TJPS thru 13/22z due to locally induced +SHRA/TSRA over
the interior. Mtn tops obscd expected along the Cordillera. Across
the rest of the PR terminals, mostly VCTS expected thru the same
period, while VCSH prevails across the USVI terminals. The 13/12z
TJSJ sounding indicated ENE winds up to 17 kt blo FL100. Light winds
with a northeasterly component expected overnight, increasing 8-12
kt aft 14/13z.

&&

.MARINE...
Issued at 1154 AM AST Tue Oct 13 2026

The lingering upper-level low will maintain unsettled conditions
across the regional waters through midweek, supporting showers and
isolated thunderstorms, particularly during the afternoon and
extending into the evening hours as daytime heating increases
instability. Lightning remains the primary hazard, with locally
gusty winds and reduced visibility also possible. Otherwise,
moderate east to east-southeast winds and generally tranquil seas
will prevail. A persistent northeasterly swell will affect the
Atlantic waters, with seas gradually building across the northern
and offshore waters by the weekend. Mariners should remain alert
for rapidly changing conditions near thunderstorms.

&&

.BEACH FORECAST...
Issued at 1154 AM AST Tue Oct 13 2026

A moderate risk of rip currents along the north and east facing
beaches of Puerto Rico, including Culebra should remain through
the workweek and expand to additional beaches, including portions
of the U.S. Virgin Islands, by Saturday. Elsewhere, low rip
current risk will prevail. Afternoon thunderstorms will also pose
a lightning threat to beachgoers, particularly along the coast of
Puerto Rico. Additionally, heat concerns are expected to return
toward the end of the workweek and continue into the weekend,
especially during the late morning and afternoon hours.

&&

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

$$

SHORT TERM/AVIATION...DSR
LONG TERM...MNG
MARINE/BEACH FORECAST...MMC

NWS SJU Office Area Forecast Discussion