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758
FXCA62 TJSJ 131704
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
104 PM AST Mon Apr 13 2026

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

.KEY MESSAGES...
Updated at 1241 PM AST Mon Apr 13 2026

 * Flash flooding likely across Puerto Rico and the U.S. Virgin
   Islands. Rapid river rises, urban flooding, and inundated
   roads are expected, especially over western, interior, and
   eastern PR and all USVI.

 * Landslides and significant runoff hazards in Puerto Rico. Saturated
   soils in mountainous areas increase the risk of landslides,
   while downstream flooding may impact coastal areas.

 * Repeated heavy rain and thunderstorms will prolong wet and unstable
   weather conditions.

 * Moderate rip current risk and hazardous marine conditions. Beachgoers
   should exercise caution, and small craft operators are urged
   to navigate carefully due to rough seas and unsettled weather.

&&

.Short Term(This evening through Wednesday)...
Issued at 1241 PM AST Mon Apr 13 2026

As expected, the upper-level trough began to deepen early this
morning while sinking southward toward the CWA. During the early
morning hours, very active weather was detected over the Atlantic
waters by satellite data and Doppler radar, accompanied by heavy
showers and thunderstorm activity. The Geostationary Lightning
Mapper indicated frequent lightning associated with this activity
over the Atlantic waters. Some of this activity moved over
northeastern Puerto Rico, prompting a Flood Advisory for Ceiba,
Humacao, and Fajardo. For the remainder of the morning, the active
area of showers and thunderstorms over the Atlantic waters moved
mainly over Vieques, Culebra, and the U.S. Virgin Islands,
prompting the issuance of Flood Advisories and Flash Flood
Warnings for St. Thomas and St. John. Radar estimates and rain
gauges indicate rainfall accumulations of 2 to 3 inches, with
locally higher amounts.

For this afternoon and evening, active weather is also expected to
develop over interior Puerto Rico and gradually move southward.
Hi-Res models indicate that strong, organized convection will
develop into a wide area in a southwest-to-northeast oriented
line, moving over the Caribbean waters and impacting mostly
Vieques, Culebra, and the USVI, particularly St. Croix.

For the remainder of the short-term period, model guidance suggests
the upper-level trough will continue to deepen and strengthen
into a cut-off low, reflected at the surface as a low-pressure
system just north of Puerto Rico. Although the most active part of
the surface low will be located north of Puerto Rico, all
variables indicative of a strong trough will be present, such as
below-normal 250 mb height fields, cooler-than-normal 500 mb
temperatures, steeper low-to-mid-level lapse rates, and well above
normal moisture. Therefore, expect periods of heavy showers and
thunderstorm activity across the CWA. With this setup, expect
excessive runoff to result in an elevated to significant flooding
risk, including urban and small stream flooding, flash flooding,
rapid river rises, and landslides in areas of steep terrain. Heavy
rain from previous days has resulted in saturated soils and
elevated stream flows, particularly over the interior and eastern
third of Puerto Rico. This will further exacerbate the flooding
risk across these areas. Additionally, near-severe thunderstorms
could produce frequent lightning and gusty winds.

Model guidance suggests that the highest rainfall accumulations
are expected over the interior, northern, and eastern portions of
Puerto Rico, as well as Vieques, Culebra, and the U.S. Virgin
Islands. However, even in areas that might not receive large
quantities of rainfall, such as southwestern Puerto Rico, river
flooding could still occur as excessive runoff from higher
elevations drains downstream into coastal areas near rivers.
Please stay tuned for further updates as this event unfolds.

&&

.Long Term(Thursday through next Sunday)...
Issued at 1241 PM AST Mon Apr 13 2026

No major changes were made to the long-term forecast, as the wet and
unsettled weather pattern is likely to persist into the weekend,
gradually improving by the first part of the next workweek. A
surface high pressure is expected to build over the Western
extending into the Central Atlantic, promoting SE-E winds through
Sunday. As a surface trough over the Hispaniola weakens, the
pressure gradient will likely increase, with winds gradually
strengthening throughout the period, resulting in breezy to windy
conditions across the coastal areas of the islands. In the mid to
upper levels, the deep-layered trough may linger and deepen into the
tropics, with our CWA under the favorable side for convection. From
the latest model guidance, moisture content will remain higher for
this time of the year, with PWAT values likely between 1.8 and 2.0
inches low chance of higher values. Additionally, the low to mid-
level moisture content should remain higher than normal, with model
soundings suggesting skinny profiles, particularly on Friday and
Saturday.  In terms of convection, the influence of the trough will
maintain slightly cooler than normal mid-level temperatures (500 mb
temperatures around -8 degrees Celsius), a jet streak with 250 mb
winds between 60 and 80 knots, and divergence aloft, triggering
upper-level dynamics. Under these conditions, forced vertical motion
will result in cloud growth and ventilation, favorable for deep
convection.  Additionally, the Galvez-Davison Index (GDI) continues
to suggest the potential for isolated to scattered thunderstorms
across the CWA, particularly in the afternoons over interior into
north/northwestern Puerto Rico and in the night across regional
waters, eastern Puerto Rico, and the U.S. Virgin Islands. As
mentioned in the previous discussion, the weather event in the short-
term forecast may result in saturated soils and higher-than-normal
river levels. A factor that may slightly inhibit deeper convection
is the winds, which may lead to more progressive showers instead of
stationary ones. Nevertheless, with the expected weather forecast
through Sunday, showers and thunderstorms over the CWA will increase
the flooding and lightning potential. Rainfall accumulations will
likely result in urban and small stream flooding, including
localized flash floods, including landslides, and rapid river rises.
Additionally, thunderstorms may result in gusty winds, reduced
visibility, and lightning. Residents and visitors of the islands are
encouraged to keep monitoring conditions and the next issuance of
the long-term forecast. By Monday, as another surface high pressure
builds over the western Atlantic and wind shifts from the NE, the
weather pattern should gradually improve with a drier air mass
filtering into the region.

&&

.AVIATION...
(18Z TAFS)
Issued at 1241 PM AST Mon Apr 13 2026

Unstable weather conds will continue to produce SHRA/TSRA across all
terminals with brief MVFR/IFR possible. Recent PIREPs rpt LGT/MOD
TURB, especially nr TJSJ/TIST/TISX. TURB may persist through the
night at terminal sites and En Route as unsettled weather
continues, particularly near areas of strong TS. BKN/OVC cigs
FL020FL070 expected at times. Winds will turn light and vrb aft
13/22Z, shifting more E/ENE at around 10-15 kt with higher gusts
and sea breeze aft 14/13z.

&&

.MARINE...
Issued at 1241 PM AST Mon Apr 13 2026

A broad surface high pressure across the central Atlantic and a
front over the western Atlantic will promote light to moderate
trades through the rest of the weekend. Pulses of northerly swell
will continue to move through the Atlantic waters and passages, where
small craft should continue to exercise caution for the next several
days. For today through Wednesday, the combination of the front and
associated pre-frontal trough will increase showers and thunderstorms
across the local waters, creating locally hazardous marine
conditions.

&&

.BEACH FORECAST...
Issued at 1241 PM AST Mon Apr 13 2026

A moderate risk of rip currents is in place for most north-
and east-facing beaches of Puerto Rico, including Vieques and
Culebra, as well as the U.S. Virgin Islands, meaning rip currents
are still possible and can be life-threatening. A low risk will
likely persist across western Puerto Rico, from Anasco and
Mayaguez southward and east along the Caribbean coast through
southeastern Puerto Rico, including Patillas, Maunabo, and
Yabucoa. Increasing winds early to midweek could lead to a return
of more hazardous marine and coastal conditions. Exercise caution
at the beach, especially along exposed coasts. Sharing is saving.

&&

.HYDROLOGY...
Issued at 1241 PM AST Mon Apr 13 2026

As a mid-to upper-level trough approaches from the northwest, the
likelihood of flooding is expected until at least midweek. An
elevated to significant flooding risk is forecast through this
period. The heaviest activity is anticipated for this evening and
Tuesday afternoon. Although a level of uncertainty is still
present, the days with the highest potential for flooding are
today and Tuesday. However, above normal moisture will persist
through late in the week. The likelihood of urban and small stream
flooding, isolated flash flooding, landslides and rapid river
rises continues to increase. Residents and visitors are urged to
remain alert to changing weather conditions and to avoid flooded
roadways. A Flood Watch (FFASJU) and an Hydrologic Outlook
(ESFSJU) are currently in effect to highlight this flooding
potential.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Flood Watch through Wednesday morning for PRZ001>013.

VI...Flood Watch through Wednesday morning for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION...CVB
LONG TERM...MNG
MARINE/BEACH FORECAST...MMC
HYDROLOGY...ALL

NWS SJU Office Area Forecast Discussion