918
FXCA62 TJSJ 060916
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
516 AM AST Mon Oct 6 2025

.KEY MESSAGES...

* A strong, long-period northeasterly swell will continue to
  produce hazardous beach and marine conditions through at least
  Tuesday afternoon.

* Increasing moisture from a tropical wave approaching the
  northeastern Caribbean region will enhance shower and
  thunderstorm activity across the local islands today and Tuesday.
  There is a limited to elevated flood risk, particularly for
  central and southern Puerto Rico.

* Across the U.S. Virgin Islands, passing showers and isolated
  thunderstorms are expected with a limited flood risk today.

&&

.SHORT TERM...Today through Wednesday...

Current satellite derived precipitable water (PWAT) values indicate
generally normal values over the islands ranging from 1.75 to 2.10
inches. Since midnight, showers and isolated t-storms have reached
windward sectors of the islands under northeasterly steering flow
with radar estimated accumulations (as of 4 AM) observed over north-
central, eastern, and southeastern Puerto Rico, as well as over
Culebra, Vieques, St. Thomas and St. Croix. As of 4 AM, the highest
radar estimated accumulations over PR where around three quarters of
an inch over isolated sectors of Canovanas, Carolina, Patillas,
Maunabo and Yabucoa. As of 4 AM, the highest radar estimated
accumulations over the USVI where around 0.19 in over northeastern
St. Thomas. Fog was also detected over areas of the interior. Lows
where in the mid 70s to low 80s over coastal and urban areas of the
islands and in the low 60s over interior Puerto Rico.

The main features during this period will be an upper level low,
centered southwest of Puerto Rico, and a tropical wave, soon moving
over the Lesser Antilles. 500 mb temperatures are forecast to remain
at normal values at around -6 to -5 degrees Celsius. 700 mb to
500 mb lapse rates are forecast at normal to slightly above normal
values 5.7 to 6.3 degrees C/km. PWAT values will also remain at
normal to above normal (peaking around 2.1 in when the bulk of the
moisture plume from the wave moves over the islands) values, with
the lowest values forecast for Wednesday. The upper level low
will gradually move northward as the period progresses, late in
the period an upper level ridge is forecast to develop.
Northeasterly steering flow is forecast to veer and become more
easterly during the morning but the approach of the tropical wave
will again promote northeasterly steering flow late afternoon. As
the wave moves over the region overnight into tomorrow,
southeasterly steering flow will dominate for the rest of the
period. With the above mentioned features and values, as well as
with diurnal heating, sea breeze convergence and orographic
effects, active afternoons are expected today and Tuesday. A
limited to elevated flooding risk will persist each day with
convective showers and thunderstorms forecast for interior to
western, south and southwestern Puerto Rico this afternoon and for
Tuesday, for interior to western, northern and northwestern
Puerto Rico. Lines of showers are also possible downwind of the
local islands and El Yunque. Advective shower and isolated t-storm
activity is also forecast to affect windward sectors of the
islands during the morning and overnight hours as the nearby above
mentioned features provide enough instability and moisture. Low
concentrations of Saharan Dust will also move over the islands
today. Drier and somewhat more stable conditions are forecast for
Wednesday but diurnal heating, sea breeze convergence and local
effects will promote convection over mainly the northwestern
quadrant of PR during the afternoon. Hazards include frequent
lightning, ponding of water on roadways, urban and small- stream
flooding and possible isolated flash floods. Additionally, gusty
winds, small hail, and possible isolated funnel clouds are also
hazards during the short term period, especially on Tuesday. 925
mb temperatures will be normal to slightly above normal today but
increase to above normal values Tuesday and Wednesday under
southeasterly flow.

.LONG TERM...Thursday through Monday...

The long-term forecast depends on the development of a tropical wave
located southwest of the Cabo Verde Islands. The National Hurricane
Center (NHC) continues to monitor this wave, assigning it a high
(70%) chance of tropical cyclone formation within the next seven
days as it moves quickly across the central Atlantic.

Regardless of its development, the latest model guidance suggests a
track that will pass to the northeast of the Lesser Antilles by the
end of the workweek. Moisture associated with this system is
expected to reach the northeast Caribbean by early Friday,
increasing the potential for showers and thunderstorms across the
regional waters and portions of Puerto Rico and the U.S. Virgin
Islands.

By Saturday, as the system moves just north of the area, winds will
shift from the south. This southerly flow will steer convection
inland, focusing showers and thunderstorms over the northern
portions of Puerto Rico and the U.S. Virgin Islands. By Monday,
winds will become from the southeast as the trailing edge of the
system moves over the islands, further enhancing the potential for
showers and thunderstorms across Puerto Rico and the U.S. Virgin
Islands.

Uncertainty remains high regarding specific rainfall amounts and any
potential local threats. Residents and visitors are therefore urged
to monitor the progress of this system closely over the coming days.


&&

.AVIATION...
(06Z TAFs)

Mainly VFR conditions expected to prevail. During the period, SHRA
and TSRA are forecast to continue reaching or moving close to
TJSJ/TIST/TISX, and possibly TJBQ, under ENE steering flow. These
can produce brief periods of reduced visibilities and lower
ceilings. After around 17Z, SHRA/TSRA over the Cordillera will
spread mainly to western/south/southwestern PR. Although VCTS
continues for TJSJ/TJBQ, impacts are expected to be more significant
at TJPS. ENE to E winds at around 6-13 kts, decreasing after 6/23Z.

&&

.MARINE...

A long-period northeasterly swell will continue to spread across the
local Atlantic waters and passages through at least Tuesday.
Therefore, marine and coastal conditions will continue deteriorated
through mid-week. A tropical wave will approach the region today,
increasing the frequency of showers and thunderstorms across the
regional waters, particularly during the afternoon hours.

&&

.BEACH FORECAST...

The strong long-period northeasterly swell will continue to produce
hazardous beach conditions through at least Tuesday evening along
the Atlantic exposed beaches. As a result, the high risk of life-
threatening rip currents continues through at least Tuesday
afternoon and early evening across western, northern, eastern
coasts of Puerto Rico, Culebra, Vieques and U.S. Virgin Islands.

A High Surf Advisory remains in effect for mentioned areas of
Puerto Rico, Culebra and U.S. Virgin Islands through at least 6 AM
AST Tuesday. Potential impacts include: high waves that can wash
over jetties and sweep people and pets onto jagged rocks. Large
breaking waves will result in localized beach/dune structure
erosion and dangerous swimming conditions, as well as minor
coastal flooding in the most vulnerable areas.

Residents and visitors are strongly encouraged to keep an eye on
the beach forecast and follow the beach flag warning system.
Beachgoers, dont take unnecessary risks, it`s safest to stay out
of the water at least through next Tuesday! For more information
about each hazard, please consult our Coastal Hazard Message
(CFWSJU).

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Tuesday night for PRZ001-002-
     005-008-010-012.

     High Surf Advisory until 6 AM AST Tuesday for PRZ001-002-005-008-
     010-012.

     High Rip Current Risk through Tuesday afternoon for PRZ013.

VI...High Rip Current Risk through late Tuesday night for VIZ002.

     High Surf Advisory until 6 AM AST Tuesday for VIZ001-002.

     High Rip Current Risk through Tuesday afternoon for VIZ001.

AM...Small Craft Advisory until noon AST Tuesday for AMZ711-712-741-
     742.

     Small Craft Advisory until 6 AM AST Tuesday for AMZ716-723.

&&

$$

MRR/GRS