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322
FXCA62 TJSJ 160756
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
356 AM AST Wed Sep 16 2026

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

.KEY MESSAGES...
Issued at 357 AM AST Wed Sep 16 2026

- Showers and localized thunderstorms associated with the passage
  of a tropical wave are expected over the US Virgin Islands and
  Puerto Rico today into Thursday, increasing the potential for
  flooding and lightning risks.

- Breezy to locally windy conditions are likely along exposed
  coastal areas over the next few days.

- There is a moderate risk of rip currents through Thursday for
  some beaches across the US Virgin Islands and Puerto Rico.

- Limited extreme heat risks are expected today and Thursday, with
  elevated to significant hazard risks returning Friday into the
  weekend.

&&

.Short Term(Today through Friday)...
Issued at 357 AM AST Wed Sep 16 2026

Conditions for today are expected to be variable across the CWA. A
dry air mass will briefly pass through the forecast region in the
overnight hours bringing precipitable water (PWAT) values down to
1.3-1.6 inches across the US Virgin Islands and Puerto Rico.
However, these conditions will quickly shift as a mass of moist air
associated to a tropical wave to the east-southeast of the windward
Islands arrives to the US Virgin Islands in the early morning and to
Puerto Rico during the late morning into the afternoon hours
bringing PWAT values up to 2.0-2.4 inches. In addition, a low
pressure system in the high levels located to the west-northwest of
the region is still leading to areas of enhanced upper-level
divergence over the islands, providing a source of instability for
the development of convection. Nonetheless, this instability is
currently enhancing thunderstorm activity and upper lever clouds
over the western region of Puerto Rico and its offshore waters. This
enhanced cloudiness may act as a limiting factor for daytime heating
over the region and may inhibit the development of widespread shower
and thunderstorm activity during the afternoon. Model guidance
suggests that we will have moist low levels and dry mid levels which
may also act as a limiting factor for the development of deep
convective activity. Thus, local effects (i.e. diurnal heating,
orographic effects, and sea-breeze convergence), which serve as
mechanical lifting mechanisms, will play a crucial role for the
development of showers and thunderstorms, especially over the
western portion of Puerto Rico. Nonetheless, the atmospheric column
will become more thermodynamically favorable for the development of
convection during the afternoon hours as the thermodynamic graphs
project a moister column with mid-level temperatures between -7 and -
6 degrees Celsius. Furthermore, the surge in moisture accompanied
with the ESE wind flow may also favor the development of advective
showers during the morning hours with a focus over the US Virgin
Islands, windward waters, and eastern-southeastern sectors of Puerto
Rico. Breezy conditions are expected today which may also lead to
the development of streamers across the windward Islands and El
Yunque tropical rainforest. Thus, the excessive rainfall risk for
today is limited to elevated across the majority of the US Virgin
Islands and Puerto Rico with a focus over the western interior and
eastern Puerto Rico. This means that flooding in urban areas, roads,
small streams, and washes is possible, with a low chance of isolated
flash flooding. This elevated risk for excessive rainfall will
persist for the remainder of the short term forecast.

While the tropical wave will arrive to the area by this evening the
bulk of the moisture will be over the region throughout Thursday
with PWAT values going up to 2.46 inches. Model guidance suggest
that the atmosphere will remain thermodynamically favorable for the
development of deep convection as the thermodynamic graphs project
moist column and the mid-tropospheric temperatures will remain below
-6 degrees Celsius. While the upper-level low will move further
northwest of the region it will continue to promote upper level
divergence over the islands into Thursday, further enhancing
instability. Thus, the excessive rainfall risk for Thursday is also
limited to elevated across the majority of the US Virgin Islands and
Puerto Rico with a focus over the northwest of mainland Puerto Rico.
Given the enhanced rainfall activity over the past 72 hours, we
cannot rule out the potential for flash flooding and landslides.
With increased risk in higher terrain and across portions of the
west, western interior and northeastern interior of Puerto Rico,
where soil remains saturated from recent rainfall activity.

Lingering moisture will persist into Friday accompanied by
Saharan Dust. We will also see the arrival of an upper level ridge
to the south of the region bringing some additional stability. The
thermodynamic environment will be less favorable for the development
of deep convection with more stable mid levels. Nonetheless, we
forecast that the local effects coupled with available moisture will
still lead to the development of passing showers over the western
region of Puerto Rico during the afternoon hours. The heat risk for
Friday remains limited with the reduction in cloud coverage over the
region.This level of heat affects primarily those individuals with
extreme sensitivity to heat, especially when outdoors without
effective cooling and/or adequate hydration. We recommend that
individuals be cautions when pursuing outdoor activities and to
remain well hydrated.

&&

.Long Term(Saturday through Wednesday)...
Issued at 357 AM AST Wed Sep 16 2026

During the early part of the long-term period, an upper-level low
northeast of the Caribbean will induce above-normal low-level
moisture across the region, while a weak surface high pressure north-
northwest of the area draws an east-southeasterly wind flow over the
local area. Precipitable water (PWAT) values between 2.0 and 2.2
inches are expected, remaining slightly above normal for this time
of year. Despite this moisture, a mid-to-upper level ridge will
dominate the area, maintaining unfavorable upper-level conditions
and sustaining a seasonal pattern. Passing showers are expected
during the morning hours across the regional waters and windward
sections of the islands, followed by afternoon showers and isolated
thunderstorms over the interior and western sections of Puerto Rico,
as well as downwind from El Yunque, Puerto Rico`s smaller islands
and the US Virgin Islands. By Sunday, winds are expected to shift to
southeasterly, focusing afternoon convection primarily over the
interior and northwestern Puerto Rico. Weak steering winds around 10
knots will elevate the flood risk where persistent showers develop.

By Sunday night into early next week, the moist southeasterly flow
will pull more tropical moisture into the area, holding PWAT values
around 2.0 inches (seasonal to slightly above normal). The mid-to-
upper level ridge and the upper-level low to the northeast will both
remain in place, continuing the seasonal regime under east-
southeasterly winds. Morning showers will favor eastern and
southeastern portions of the islands, including the US Virgin
Islands, with afternoon convection concentrating across the interior
and northwestern quadrant of Puerto Rico. With 500 mb temperatures
hovering near -6C, isolated afternoon thunderstorms will remain
possible each day. Additionally, the moist southeasterly flow will
drive an elevated heat risk from Sunday onward, primarily impacting
heat-sensitive individuals without access to effective cooling or
hydration.

By mid-week, a tropical wave may approach the area, though forecast
uncertainty remains due to model disagreement, with the GFS bringing
the wave closer to the local area, while the ECMWF pulls it to the
north. Please continue to monitor the forecast for future updates as
guidance resolves.

&&

.AVIATION...
(06Z TAFS)
Issued at 357 AM AST Wed Sep 16 2026

Mainly VFR conds early today. -SHRA/TSRA will increase as a
tropical wave cross the region, leading to TEMPO MVFR conds across
all terminals at times. Light and variable winds increasing to
15-20 kt with higher and becoming more easterly through 16/23Z.
Light and VRB aft 16/23Z.

&&

.MARINE...
Issued at 357 AM AST Wed Sep 16 2026

Moderate to locally fresh easterly winds will prevail across the
region, influenced by a broad surface high and a passing tropical
wave. Small craft operators should exercise caution. Expect
increased shower and thunderstorm activity over the next day or so.
However, as the tropical wave shifts west of the area on Friday and
leaves behind lingering moisture, conditions will gradually improve,
while moderate easterly trade winds persist into the weekend.

&&

.BEACH FORECAST...
Issued at 357 AM AST Wed Sep 16 2026

A moderate risk of rip currents will continue today along much of
the north-facing coast of Puerto Rico, becoming more localized
Wednesday and Thursday. Another increase is expected along the
north coast on Friday, with generally lower risk conditions over
the weekend.

Afternoon thunderstorms may affect western Puerto Rico, while a
tropical wave will increase showers and thunderstorms today into
Thursday. Beachgoers should seek shelter immediately if thunder is
heard.

Hot and humid conditions will also persist. Stay hydrated and
limit prolonged sun exposure during the hottest part of the day.

&&

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

$$

EVE...CAM/LIS
MID...ICP/RVM

NWS SJU Office Area Forecast Discussion