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671
FXCA62 TJSJ 110859
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
459 AM AST Sat Apr 11 2026

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

.KEY MESSAGES...
Updated at 459 AM AST Sat Apr 11 2026

* A mid to upper level trough approaching from the northwest will
  result in wetter and more unstable conditions with an increased
  flooding risk as it reaches the islands late Sunday into the next
  workweek. The heaviest activity is forecast for Monday night into
  Tuesday. A Flood Watch (FFASJU) will likely be issued later today.

* A High Risk of Rip Currents is in effect for the northern coast of
  Puerto Rico and Culebra through early next workweek due to a long-
  period NNW swell.

* A limited to elevated flooding risk will be present today, mainly
  due to afternoon showers and t-storms over interior to W and NW
  PR, with showers also affecting the eastern region today and
  tonight.

* Limited heat risk today for coastal and urban areas.

&&

.Short Term(Today through Monday)...
Issued at 459 AM AST Sat Apr 11 2026

Since midnight, radar estimated rainfall accumulations showed
minimal amounts over eastern to southeastern PR, including Vieques.
Official and unofficial stations reported lows in the 70s across
coastal areas, with some stations reaching up to 80 degrees, and in
the 60s across interior PR. Current satellite derived precipitable
water (PWAT) values indicate a band of moisture approaching from the
east with values up to 1.65 in (at normal values for this time of
the year). A broad surface high over the central Atlantic
interacting with a frontal low over the western to central Atlantic
will continue to result in E to ESE steering flow through the
weekend; 925 wind speeds are forecast to continue to around 15 kts
through the weekend. Another building high and a trough near the
area will then result in winds backing to ENE and increasing to
breezy to locally windy on Monday. PWAT values will increase from
high end normal to above normal (PWAT values above 1.75 in) as the
period progresses with model guidance suggesting a peak in PWAT
(above 2 inches) Monday night into the long term period. Drier air
in the mid levels will continue to intrude through Sunday afternoon,
with Sunday night into Monday having deep columnar moisture. ESE
flow will steer moisture and showers towards the region today and
tomorrow Sunday.

For today, a limited (ponding of water in roads and poorly drained
areas with a low chance of urban and small stream flooding) to
elevated (flooding in urban areas, roads, small streams and washes
with a low chance of isolated flash floods) flooding risk will is
forecast. This is mainly due to afternoon convective showers and t-
storms over interior to W and NW PR due to diurnal heating, local
orographic effects and sea breeze convergence, with advective
showers also affecting the eastern region. Afternoon showers can
also develop downwind of the local islands and El Yunque. The ESE
steering flow will also result in a limited heat risk, with 925 mb
temperatures forecast to be warmer than normal, with highs reaching
the upper 80s to around 90 at urban and coastal areas.

For tomorrow, Sunday, an enhanced pattern will be present with
broader limited to elevated flooding risk forecast during the
afternoon over W-NW PR due to convective showers and t-storms.
Showers can also develop downwind of the local islands. Advective
showers will also continue to reach windward sectors of the islands
during the morning and overnight hours. At the same time a mid to
upper level trough (with a surface reflection) is forecast to
approach the region from the NE on Sunday, reaching the local
islands late Sunday into Monday. Conditions will gradually become
more wet and unstable as the short term period progresses. Galvez-
Davison Index (GDI) values are forecast at up to 30 today, up to 40
on Sunday and up to 40 at most of the region on Monday. 500 mb
temperatures will be at normal values today but cool to below normal
values as the period progresses. 250 heights will also continue to
decrease during the period and 850 to 500 relative humidities will
increase to above normal values (possibly even 2 standard deviations
above normal) as the period continues. Similarly other variables and
general model guidance indicate these increasingly unstable
conditions with shower and t-storm frequency increasing. This will
ultimately promote an elevated flooding risk for most of the area
during this period.

The first round of showers related to this trough are expected to
reach the region Sunday night into early Monday. Showers and
thunderstorms will then increase in frequency on Monday as the above
mentioned deep moisture and the approaching trough continue to
result in wet and unsettled conditions. Afternoon convection is
forecast to be further enhanced on Monday resulting in showers and t-
storms developing under the above mentioned NE steering flow over
the interior, and moving into south-central, west and southwest
Puerto Rico. The heaviest activity and the highest potential for
flooding is still forecast for Monday night into Tuesday (long term
period) as the trough axis reaches the islands. Model guidance
continues to suggest that this activity will be particularly strong
around St. Thomas, St John, Vieques, Culebra, eastern and northern
Puerto Rico. The likelihood of urban and small stream flooding,
isolated flash flooding, landslides and rapid river rises is
increasing to start the workweek. For additional details, including
rainfall expectations and hydrologic conditions Sunday onwards,
refer to the latest Hydrologic Outlook (ESFSJU). A Flood Watch
(FFASJU) will also likely be issued later today. Patchy fog over
will be present during the overnight hours over areas of the
interior. Satellite and model guidance also suggest low
concentrations of Saharan Dust over the area and gradually
decreasing during the period. Although Sunday could still see above
normal 925 mb temperatures, these will decrease to normal and
possibly below normal values as the period continues.

&&

.Long Term(Tuesday through Saturday)...
Issued at 459 AM AST Sat Apr 11 2026

A wetter and unstable pattern will continue from Monday night into
Tuesday as a deep-layered trough and upper-level low remain
northwest of the region, supporting favorable conditions aloft. Deep
tropical moisture will remain in place through midweek, with
precipitable water values at times exceeding climatological maxima.
Although some drying is possible late Wednesday into early Thursday,
moisture is expected to increase again late week, maintaining a very
moist pattern through at least Saturday.

The primary concern is an elevated to locally significant flooding
risk, with the highest impacts expected Monday evening through
Tuesday. Periods of heavy and repeated rainfall may result in urban
and small stream flooding, isolated flash flooding, and localized
landslides. A Hydrologic Outlook (ESFSJU) is currently in effect to
highlight this potential. Rainfall amounts will be refined in later
forecasts as confidence increases. While it is still too early for
more specific flood headlines, a Flood Watch may be needed if
conditions warrant.

Although activity may become somewhat less organized at times late
week, sufficient moisture and instability will persist to support
continued showers and thunderstorms with localized flooding impacts,
especially over areas that become saturated earlier in the period.
Confidence is moderate overall.

&&

.AVIATION...
(06Z TAFS)
Issued at 459 AM AST Sat Apr 11 2026

Mainly VFR conditions. ESE winds today, bcmg E by around 11/22Z.
Light and variable winds for TJSJ/TJPS/TJBQ; winds up to around 10
kts for TIST/TISX through 11/13Z. Winds increasing to around 15 kts
with higher gusts and sea breeze variations, before decreasing again
after 11/23Z. VCSH/-SHRA to continue mainly over eastern/southern
terminals from time to time throughout the period. TSRA in the VCTY
or at JBQ at 11/17-22Z, this could promote brief MVFR conditions
with lower cigs and reduced visibilities.

&&

.MARINE...
Issued at 459 AM AST Sat Apr 11 2026

Moderate trades will prevail through the weekend as a broad surface
high pressure builds from the Central Atlantic. A 4 to 5 feet
northwest to north swell will continue to spread across the Atlantic
waters and passages, building seas up to 6 feet for the next few
days. Therefore, small craft should continue to exercise caution,
mainly across the Atlantic waters. Another pulse of the northerly
swell is expected on Sunday. By early next week, a frontal boundary
and a pre-frontal trough will increase showers and thunderstorms
across the regional waters. Increasing winds and seas building up to
7 feet are also expected, and Small Craft Advisories could be issued
for the Atlantic waters during this period.

&&

.BEACH FORECAST...
Issued at 459 AM AST Sat Apr 11 2026

Buoy data from Rincon and Arecibo indicated a NNW swell at 4 to 5
feet with wave periods of 11 to 12 seconds. This swell is expected
to linger today across the Atlantic waters and passages, while
another pulse arrives on Sunday. Therefore, a High Rip Current Risk
is in effect for the northwest to northeast beaches of Puerto Rico
and Culebra through at least Monday. Across the USVI and Vieques, a
moderate risk of rip currents is expected to persist over the next
several days, while a low risk will continue along the southern and
more protected beaches of Puerto Rico. More information is available
at weather.gov/beach.sju.

&&

.HYDROLOGY...
Issued at 459 AM AST Sat Apr 11 2026

As a mid to upper level trough approaches from the northwest, the
likelihood of flooding is increasing for late Sunday into the next
workweek. An elevated flooding risk is forecast through at least
midweek. The first round of showers are expected to reach the region
Sunday night into early Monday. Showers and thunderstorms will then
increase in frequency on Monday, but the heaviest activity is
anticipated for Monday night into Tuesday. Although a level of
uncertainty is still present, the days with the highest potential
for flooding are Monday 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 is increasing. Residents and
visitors are urged to remain alert to changing weather conditions
and to avoid flooded roadways. A Hydrologic Outlook (ESFSJU) is
currently in effect to highlight this potential. A Flood Watch
(FFASJU) will also likely be issued later today.

&&

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

VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION/HYDROLOGY...MRR
LONG TERM....ICP
MARINE/BEACH...DSR

NWS SJU Office Area Forecast Discussion