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509
FXCA62 TJSJ 130613
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
213 AM AST Mon Apr 13 2026

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

.KEY MESSAGES...
Updated at 210 AM AST Mon Apr 13 2026

 * A wet and unstable weather pattern will bring widespread
   showers and thunderstorms across the area, with a Flash Flood
   Watch in effect from 10 AM today through Wednesday.

 * An elevated to significant flooding risk is expected across
   the islands; residents are urged to exercise caution, avoid
   flood- prone areas, and never cross flooded roads.

 * The wettest period is expected from tonight into early Tuesday
   morning, when the heaviest rainfall and most persistent shower
   activity are likely.

 * A moderate risk of rip currents will persist across all local
   beaches, creating hazardous conditions for swimmers.

 * Across the U.S. Virgin Islands, an elevated to significant flooding
   risk is expected from early Tuesday morning through the day as
   periods of heavy rainfall impact the area.

&&

.Short Term(Today through Wednesday)...
Issued at 210 AM AST Mon Apr 13 2026

Moderate to strong showers affected the Atlantic offshore and nearshore
waters for most of the night, resulting in hazardous marine
conditions due to locally higher seas and gusty winds. Given the
prevailing conditions, several Special Marine Warnings and Marine
Weather Statements were issued throughout the night. As the night
progressed, by 2 AM, showers shifted southward into coastal areas,
affecting portions of eastern and northeastern Puerto Rico,
including municipalities near Naguabo, Fajardo, and Rio Grande.
Overnight temperatures remained in the lower 70s across coastal
areas, with even cooler conditions across the mountains.

For today, the forecast of a wetter and more unstable weather
pattern remains on track. As a result, a Flash Flood Watch will be
in effect from 10 AM today through Wednesday across the entire
area. At upper levels, a deep trough with its axis just over
Hispaniola places the islands under a divergent region, favoring
vertical development and widespread thunderstorm activity. This
upper-level feature will allow colder temperatures at 500 mb,
ranging between -9C and -10C, which are around two standard
deviations below climatological normals. At the surface, a frontal
boundary and the associated pre-frontal trough will reach the
local area. Winds will shift to a northerly component from this
afternoon into early Tuesday as the frontal boundary moves
southward across the region. Additionally, abundant deep moisture
will be in place, with relative humidity values from 850 mb up to
500 mb near the 75th percentile. All of these factors provide a
well-defined setup for a very convective day. Model guidance
suggests an increase in showers during the morning hours, with
activity expanding and intensifying through the afternoon.
According to the highest-resolution models, a very active weather
day is expected, with the strongest convection developing across
the interior and then moving into northwestern and interior Puerto
Rico. By late afternoon into the evening, showers will affect
northeastern areas, extending into Culebra, Vieques, and the U.S.
Virgin Islands. An important factor for today will be the surface
wind pattern, as slow-moving showers will favor higher rainfall
accumulations in addition to the widespread shower activity. As a
result, an elevated to significant flooding threat is expected
across the entire forecast area today. Residents are advised to
avoid rivers and not to cross flooded roads or low-lying areas.

On Tuesday in the early morning hours into Wednesday, conditions
will remain on the unstable side, as the upper-level trough
continues to deepen, leaving a cut-off low just over the area.
This feature will be reflected through the column, resulting in a
low at 500 mb and an induced surface trough extending from around
1000 mb up to 700 mb. The presence of colder temperatures aloft
and the slow movement of this feature will continue to support
enhanced instability and prolonged convective activity across the
region. Given the expected conditions deteriorate weather
conditions are forecast across the islands. Given the expected
rainfall accumulations and the already saturated soils, this
pattern will maintain an elevated to significant risk of urban and
river flooding, as well as potential mudslides in areas of steep
terrain. Again, a Flood Watch is in effect from 10 AM this morning
through 8 AM Wednesday for all of Puerto Rico and the U.S. Virgin
Islands. Residents and visitors should continue to monitor the
latest weather information.

&&

.Long Term(Thursday through Monday)...
Issued at 210 AM 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...
(06Z TAFS)
Issued at 210 AM AST Mon Apr 13 2026

MVFR to IFR conditions are expected across all TAF sites today due
to an increase in SHRA and TSRA activity. Periods of reduced VIS
and lowering CIGs are anticipated, with tempo IFR conditions
possible in heavier showers and thunderstorms. Mountain
obscurations will persist, especially across interior and western
sections. Winds will remain VRB through 13/15Z, then increase to
around 10 KT from the E-SE as the surface trough lifts north of
the area. Thereafter, winds will become light and variable again,
before gradually shifting to the NE around 14/03Z.

&&

.MARINE...
Issued at 210 AM AST Mon Apr 13 2026

Moderate easterly trades will persist through the period
as high pressure remains over the central Atlantic, with periods of
locally fresh winds. Combined with ongoing northerly swell, this
will maintain choppy conditions across the Atlantic waters and
passages. Increasing showers and thunderstorms early next week will
lead to locally hazardous marine conditions, including gusty winds
and reduced visibility, though impacts will be brief and variable.
By midweek, a combination of stronger winds and building swell may
result in hazardous conditions for small craft, particularly across
the Atlantic waters.

&&

.BEACH FORECAST...
Issued at 210 AM AST Mon Apr 13 2026

Rip current conditions have improved as wave
energy has decreased, no longer supporting a High Rip Current Risk.
However, 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 210 AM 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 flooding risk is forecast through this period. The
heaviest activity is anticipated for this evening into early
morning hours on Tuesday. 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 from 10 AM AST this morning through Wednesday
     morning for PRZ001>013.

VI...Flood Watch from 10 AM AST this morning through Wednesday
     morning for VIZ001-002.

AM...None.


SHORT TERM, AVIATION,KEY MESSAGES...LIS
LONG TERM....MNG
MARINE & BEACH FORECAST...ICP

NWS SJU Office Area Forecast Discussion