277
FXCA62 TJSJ 131646
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1246 PM AST Sun Sep 13 2026

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

.KEY MESSAGES...
Issued at 1242 PM AST Sun Sep 13 2026

* Showers and thunderstorms have already begun to develop and will
  continue through the rest of the afternoon, mainly across interior
  and western Puerto Rico. Gusty winds, frequent lightning, and
  localized flooding are possible.

* Periods of showers and thunderstorms will continue through the
  next several days, with localized flooding possible. The flooding
  threat may increase at times as additional moisture moves across
  the region.

* The heat threat will persist throughout the week. Heat Advisories
  may be needed on some days, although the extent of the heat will
  depend on cloud cover associated with unsettled weather conditions.

* A moderate risk of rip currents will continue across exposed
  northern, eastern, and southern beaches of the local islands.

&&

.Short Term(This evening through Tuesday)...
Issued at 1242 PM AST Sun Sep 13 2026

Scattered showers have been moving across the islands since the
early morning, particularly affecting portions of the U.S. Virgin
Islands as well as northern, eastern, and southeastern Puerto Rico.
Doppler radar and satellite imagery also detected isolated
thunderstorms developing over the coastal waters from Manati to
Hatillo; however, these remained offshore. Shower frequency
increased by mid-morning across the previously mentioned areas and
will continue to move westward as the day progresses.

Atmospheric instability will increase as an upper-level low remains
positioned to the west-northwest of the local area. The combination
of upper-level divergence and cooling temperatures at the 500 mb
level (-7 to -8C) will create ideal conditions for strong
thunderstorm formation. The latest precipitable water analysis
suggests seasonal values of approximately 2.00 inches over the next
few days. Consequently, diurnal heating, local orographic effects,
and sea breeze convergence will drive scattered afternoon showers
and thunderstorms across the interior, northwestern, and western
sections of Puerto Rico. Similar weather patterns are anticipated to
persist daily through the short-term period. Localized flooding in
urban areas, roads, and small streams will be possible with the
strongest activity each afternoon through at least Tuesday.

By Tuesday, low-level moisture associated with a passing tropical
wave south of the area will reach the islands, once again increasing
the potential for widespread showers across the region.

Saharan dust concentrations will continue to gradually dissipate,
though pulses of low-concentration of dust will return to the
northeastern Caribbean by midweek. Warm, hot, and humid conditions
will persist daily, with heat indices reaching up to 100F,
particularly across urban and coastal areas of Puerto Rico and the
U.S. Virgin Islands.

&&

.Long Term(Wednesday through next Saturday)...
Issued at 346 AM AST Sun Sep 13 2026

Confidence is increasing in the long-term forecast, with Thursday
being the wettest day of the period. A surface high pressure
building in the Central Atlantic should maintain east-southeast
winds, leading to breezy conditions across coastal areas. Slightly
drier conditions are expected on Wednesday, as the latest NASA GMAO
AOT model suggests low to moderate concentrations of Saharan Dust
moving across the CWA. From the latest model guidance, PWAT values
may drop below climatological normal (below 1.70 inches),
highlighted in the 850 - 700 mb. Additionally, a mid-to-upper-level
ridge lingering across the eastern Caribbean will likely lead to
warmer-than-normal mid-level temperatures (500 mb temperatures above
-6 degrees Celsius), subsidence,  and stability aloft. Although
local effects and daytime heating will drive the shower and
thunderstorm activity, the most likely scenario is for it to be
localized and short-lived. As mentioned in the previous discussion,
a tropical wave is forecast to move across the islands, pooling well
above normal tropical moisture by late Wednesday night and Thursday.
From the latest deterministic guidance of the GFS and ECMWF, PWAT
values are likely to increase up to 2.20 inches, well above
climatological normal, mainly in the low and mid levels.
Additionally, theres more agreement between global models, as
ensemble members are showing less variability between them, tending
to wetter conditions. In terms of instability, the latest guidance
continues to suggest areas of enhanced vorticity associated with the
wave, increasing the likelihood of an advective pattern across
windward sections of the islands, followed by afternoon convection.
As the tropical wave departs from the region, atypical conditions
for this time of the year may be expected due to another SAL with
low to moderate concentrations moving across the area, introducing a
slot of drier air in the low levels and limiting shower and
thunderstorm activity. Concentrations could lower by Saturday, with
conditions becoming more seasonal.

Overall, afternoon showers and thunderstorms can be expected each
day, particularly across interior and western/northwestern Puerto
Rico, though the flooding threat is low on Wednesday and Friday.
Saturday conditions should lead to ponding of water over roadways
and poorly drained areas. Meanwhile, the tropical wave on Thursday
should increase both the lightning and flooding threat, with the
potential of urban and small stream flooding across interior and
northwestern Puerto Rico and minor flooding across the U.S. Virgin
Islands and eastern/southeastern Puerto Rico.

Under an E-SE wind pattern, seasonal to warmer conditions are very
likely across the CWA, particularly by the end of the long-term
forecast. During the peak hours of the day, urban and low-lying
areas of the islands could easily reach 108 degrees Fahrenheit,
increasing the heat threat. Therefore, an elevated heat threat is
expected throughout the forecast period, though it may be limited on
Thursday given the weather forecast.

&&

.AVIATION...
(18Z TAFS)
Issued at 1242 PM AST Sun Sep 13 2026

Mainly VFR conds expected to prevail across all terminals thru the
next 24 hours. SHRA/TSRA will continue to develop across portions of
northern, central and western PR, resulting in VCTS at
TJSJ/TJPS/TJBQ thru 13/23Z. Brief MVFR conds are possible at TJBQ
btw 13/18-22Z. VCSH possible at TIST/TISX thru 13/22-23Z. E winds 12-
16 kt with gusts up to 26 kt through early this evening, becoming
light and VRB aft 13/23Z. Winds are expected to increase again to 12-
16 kt with gusts and sea breeze variations aft 14/13Z.

&&

.MARINE...
Issued at 1242 PM AST Sun Sep 13 2026

Moderate to fresh easterly trade winds will continue across the
regional waters tonight and through the next several days,
maintaining choppy seas of 4 to 6 feet. The frequency of showers and
isolated thunderstorms over the regional waters may increase at
times as surface perturbations and tropical waves move across the
region and interact with unstable weather conditions. Another
tropical wave is expected to bring additional moisture and an
increase in showers and thunderstorms from Thursday into Friday.

&&

.BEACH FORECAST...
Issued at 1242 PM AST Sun Sep 13 2026

Beach conditions will remain relatively similar through Tuesday,
with a moderate risk of rip currents mainly along the north-facing
beaches of Puerto Rico and in a few localized areas of the offshore
islands and U.S. Virgin Islands. A low risk is expected elsewhere.

The rip current risk is expected to increase again from Wednesday
through Friday as trade wind-driven seas affect more of the regional
beaches. Moderate risk conditions may expand along portions of the
northern and southern coasts of Puerto Rico, Culebra, and parts of
the U.S. Virgin Islands.

Showers and isolated thunderstorms may become more frequent over the
next several days as patches of moisture and tropical waves move
across the region. Brief heavy rainfall and lightning will be
possible at times. Beachgoers should monitor changing weather
conditions and move indoors if thunder is heard.

&&

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

$$

SHORT TERM/AVIATION...GRS
LONG TERM...MNG
MARINE/BEACH FORECAST...CVB