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189
FXCA62 TJSJ 171803
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
203 PM AST Sat Oct 17 2026

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

.KEY MESSAGES...
Issued at 203 PM AST Sat Oct 17 2026

* Heat today will primarily affect people especially sensitive to
  heat, particularly those outdoors without effective cooling or
  adequate hydration.

* Localized afternoon thunderstorms may produce heavy rainfall and
  isolated flooding the rest of today. The potential for stronger
  thunderstorms and flooding increases Sunday and Monday, when
  weak steering winds may favor slow-moving storms.

* A moderate risk of rip currents persists this afternoon along
  Atlantic-facing beaches of Puerto Rico and the U.S. Virgin
  Islands, particularly north-facing beaches.

* A more energetic northwesterly swell arrives Monday evening,
  builds into Tuesday, and becomes northerly by Wednesday,
  bringing building seas, higher surf, and increasingly hazardous
  rip currents through midweek.

&&

.Short Term(This evening through Monday)...
Issued at 203 PM AST Sat Oct 17 2026

A patch of moisture, with GOES-19-derived total precipitable
water estimates ranging from 1.5 to 1.9 inches, supported brief,
passing showers across the local waters and windward portions of
Puerto Rico and the U.S. Virgin Islands this morning. By around 11
AM AST, showers had spread farther west into Puerto Rico. Doppler
radar detected the San Juan Streamer developing downwind from the
Yunque into the San Juan Metropolitan area. Another area of
convection developed from Coamo`s mountains into the west coast of
PR. East-southeast winds ranged from 5 to 15 mph, with higher
gusts and local sea breeze variations. Temperatures reached the
upper 80s to lower 90s and, combined with high surface humidity,
resulted in heat indices of 100 to 107 degrees Fahrenheit.

A building mid- to upper-level ridge will promote subsidence,
while drier air at those levels will limit widespread convection
this afternoon. Nevertheless, available moisture, daytime heating,
sea breeze convergence, and terrain effects will support
localized showers and thunderstorms, mainly across interior,
north-central, and northwestern Puerto Rico. Additional showers
may develop downwind of St. Thomas, St. Croix, Vieques, and
Culebra and move over adjacent waters. The flood threat will
remain limited, with heavier showers potentially producing ponding
of water on roads and in poorly drained areas. Isolated urban and
small stream flooding cannot be ruled out across northeastern,
northwestern, and interior Puerto Rico.

Overnight into early Sunday morning, patches of moisture embedded
in the prevailing east-southeast flow will bring occasional
passing showers, mainly to windward areas. Leeward locations will
generally experience mostly clear skies between showers.

By Sunday afternoon and continuing into Monday, a mid- to upper-
level low northeast of the region is expected to weaken the
ridge`s influence, allowing a more favorable environment for
afternoon and evening convection. Additional moisture and local
convergence will increase the potential for heavier showers and
stronger thunderstorms, particularly across interior and northern
Puerto Rico. Weak steering flow may favor slow-moving cells and
repeated development over the same areas, increasing the potential
for localized rainfall accumulations and flooding. The Hazardous
Weather Outlook has been updated to reflect the increased
thunderstorm and flooding potential for both days.

&&

.Long Term(Tuesday through Saturday)...
Issued at 306 AM AST Sat Oct 17 2026

A similar weather pattern will be observed throughout the long-term
forecast with convective activity being predominantly driven by
diurnal heating and local effects. As the mid- to upper-level trough
moves further into the east Atlantic, a mid-level ridge will begin
its gradual development over the region. This will maintain warmer
than average temperatures at these levels and weaken the lapse rates
into mid-week, thus inhibiting the development of deep convective
cells. A surface ridge will also build from Tuesday into Wednesday
weakening the pressure gradient and winds at these levels until
Friday when locally breezy conditions return to the area. This
feature will also steer the flow to be from E on Tuesday to SE for
the remainder of the long-term forecast. The models show a
predominantly moist column, particularly by late week. Given that
the synoptic conditions will be acting to limit the development of
convection, any activity that does develop will be driven by diurnal
heating, available moisture, and local effects.

Thus, the potential for excessive rainfall- and lighting- associated
risks will increase as the week progresses. The focus of the
activity will be predominantly the western interior and northwest of
Puerto Rico during the afternoons with the exception of Wednesday
which should favor the northern half of Puerto Rico during the
afternoon. This level of risk may lead to flooding in urban areas,
roads, small streams, and washes as well as to a low chance of
isolated flash floods. The available moisture, above average low
tropospheric temperatures, and low surface winds will lead to a
level of heat that may affect those individuals extremely sensitive
to heat especially when outdoors without effective cooling and/or
adequate hydration. Individuals are invited to continue monitoring
the weather conditions as they evolve.

&&

.AVIATION...
(18Z TAFS)
Issued at 203 PM AST Sat Oct 17 2026

The likelihood of -TSRA in increasing in the TAF prd, though VFR
conds should persist across all terminals. Aftn cvnt will cont to
bring -SHRA/+SHRA, though JBQ remains with VCTS and TEMPO btwn 1719-
21z due psbl -TSRA. E-ESE btwn 10 - 15 kt will cont this aftn,
becoming light and VRB aft 17/23z. -SHRA may move across USVI TAFs
around 18/06z, reducing CIGs/VIS and leading to tempo MVFR conds.
Winds should pick up by 18/12-14z btwn 8 - 12 kt, though steering
flow will remain weak.

&&

.MARINE...
Issued at 203 PM AST Sat Oct 17 2026

As a surface low and its associated frontal boundary move across
the western Atlantic, the local pressure gradient will weaken,
supporting gentle to moderate east-southeast winds through the
weekend into early next week. A small northerly swell will
gradually subside tonight across the Atlantic waters and exposed
passages. A more energetic northerly swell will arrive Monday
night, peak Tuesday, and persist through midweek, resulting in
building seas. Isolated thunderstorms may develop over the coastal
waters each afternoon this weekend, with activity potentially
extending into the evening on Sunday and Monday.

&&

.BEACH FORECAST...
Issued at 203 PM AST Sat Oct 17 2026

A moderate risk of rip currents will persist this afternoon along
Puerto Rico`s Atlantic shoreline and the U.S. Virgin Islands,
particularly at north-facing beaches. A more energetic
northwesterly swell will arrive Monday evening, build into
Tuesday, and become northerly by Wednesday. This swell will
increase surf heights and the risk of life-threatening rip
currents along exposed beaches, with hazardous conditions
potentially persisting through midweek. Afternoon thunderstorms
may bring lightning and locally gusty winds to beaches each day.
Beachgoers should leave the water and seek shelter when they hear
thunder.

&&

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

$$

KEY MESSAGES/SHORT TERM/MARINE/BEACH FORECAST...CAM
LONG TERM....RVM
AVIATION...MNG

NWS SJU Office Area Forecast Discussion