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939
FXCA62 TJSJ 070803
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
403 AM AST Mon Sep 7 2026

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

.KEY MESSAGES...
Issued at 356 AM AST Mon Sep 7 2026

- An Extreme Heat Warning is in effect for Puerto Rico from 10 AM to
  5 PM AST, with a Heat Advisory also in effect for Vieques,
  Culebra, and the U.S. Virgin Islands. Although passing showers,
  t-storms and clouds can provide some relief, enough sunshine and
  moisture is expected for heat indices to reach dangerous
  levels.

- Afternoon convection is also forecast to develop mainly over the interior
  to W and NW Puerto Rico, while streamers can once again develop
  downwind of the local islands and El Yunque.

- Low to locally moderate concentrations of Saharan Dust will
  filter over the islands later today into overnight tomorrow.

- A low risk of rip currents will persist along all coastal areas
  today and tonight, before increasing to moderate tomorrow along
  the northern and eastern coastal areas of Puerto Rico, Culebra,
  Vieques, and the U.S. Virgin Islands.

- Drier and more stable conditions are forecast tomorrow, before a tropical
  wave increases moisture once again on Wednesday. Low to briefly
  moderate concentrations of Saharan Dust will also filter over
  the islands on Wednesday.


&&

.Short Term(Today through Wednesday)...
Issued at 356 AM AST Mon Sep 7 2026

During the overnight hours, a lingering moisture field from
yesterday`s tropical wave continued over the islands. Showers and
isolated t-storms continued moving across the region under an ESE
steering flow. Since midnight, radar estimated accumulations were
detected over interior to eastern PR, and over Vieques, Culebra and
the USVI. Stations have reported minimum temperatures in the upper
70s to low 80s, across urban and lower-elevation areas of Puerto
Rico, in the low 80s across Vieques, Culebra and the USVI, and in
the 60s across higher elevations of Puerto Rico. This lingering
moisture from yesterday`s tropical wave will continue over the local
islands, with Precipitable water (PWAT) values over 2 inches,
through at least tonight under an ESE steering flow. With above
normal moisture, ESE flow persisting, and satellite imagery not
suggesting widespread cloud cover, another day with high heat
indices is forecast. 925 mb temperatures are forecast to reach 2
standard deviations above normal while wind speeds remain seasonal
as a surface high gradually approaches the area. An extreme heat
warning will be in effect from 10 AM to 5 PM AST today for Puerto
Rico while a heat advisory will be in effect for the USVI, Vieques
and Culebra from 10 AM to 5 PM AST today. Although passing showers,
t-storms and clouds can provide some relief, enough sunshine and
moisture is expected for heat indices to rapidly rise during the day
and reach dangerous levels. These passing showers and isolated t-
storms can continue to reach windward sectors of the islands to
start the day under the ESE flow. Afternoon convection is also
forecast due to available moisture, diurnal heating, sea breeze
convergence and local orographic effects. This convective activity,
showers and t-storms, is forecast to develop mainly over the
interior to W and NW Puerto Rico, while streamers can once again
develop downwind of the local islands and El Yunque. This will all
result in a limited flooding risk over mainly those areas as locally
heavy rainfall is forecast. An upper level trough west of the region
will continue moving away from the eastern Caribbean, resulting in
upper level ridging over the local area. 500 mb temperatures will
reach high end seasonal to warmer than seasonal values while 850 mb
to 700 mb lapse rates increase to seasonal and above seasonal
values. Low to locally moderate concentrations of Saharan Dust will
also filter over the islands later today into overnight tomorrow.

Tomorrow, Tuesday, and Wednesday, a surface high-pressure will
continue moving closer to the region, tightening the local pressure
gradient and resulting in breezy, to locally windy, ESE flow,
especially along coastal areas. Drier air will filter in later
tonight into tomorrow with PWAT values decreasing to between 1.5
inches and 1.8 inches, below normal to normal values for this time
of the year. Available moisture will also be limited to mainly below
850 mb, as ridging continues to establish itself and drier air
becomes present above 850 mb. Local patches of moisture moving over
the region and afternoon convection over western PR are still
forecast to result in locally higher and generally seasonal PWAT.
Conditions will be more stable on Tuesday however, as 700 to 500 mb
lapse rates decrease to below-seasonal values and 500 mb
temperatures warm up to above seasonal values. Passing showers will
continue to reach windward sectors under an ESE flow. Although
diurnal heating and local effects will still result in afternoon
showers and t-storms, their coverage (over mainly interior to W-NW
PR) is forecast to be more limited on Tuesday compared to the rest
of the short term period. Current model guidance has PWAT values
decreasing even further, ranging between around 1 to 1.4 inches,
tomorrow night into the early overnight hours on Wednesday.

This decrease in moisture will be short-lived however, as a tropical
wave (currently approaching 50W) and its associated moisture field
move over the region from the east by the late Tuesday overnight
hours into Wednesday morning. This moisture field along with
trailing moisture will once again increase PWAT values to between
1.8 to around 2 inches over the region, with current model guidance
suggesting patches of lower moisture also reaching the region. 500
mb temperatures are also forecast to decrease to around -6 and -7C
on Wednesday while available moisture once again reaches the mid-
levels. This can support advective showers and t-storms over
windward sectors under a breezy, to locally windy ESE, flow and
afternoon showers (more widespread compared to Tuesday) and t-
storms, mainly for interior to W-NW PR as well as downwind of the
local islands and El Yunque. Although a limited flooding risk is
forecast, wind speeds, however, can serve to limit rainfall
accumulations. Low to briefly moderate concentrations of Saharan
Dust will also filter over the islands on Wednesday. A limited to
elevated heat risk is forecast to persist Tuesday and Wednesday as
ESE flow continues, however, 925 mb temperatures are forecast at
slightly above normal values.

&&

.Long Term(Thursday through Monday)...
Issued at 356 AM AST Mon Sep 7 2026

A surface high-pressure across the Atlantic will continue to promote
breezy to locally windy southeast winds on Thursday, becoming more
easterly Friday, and then east to northeasterly Saturday through
Monday. On Thursday, residual moisture from a tropical wave will
remain over the region, with precipitable water values around 1.9 to
2.2 inches, seasonal to near-above-average for this time of the year
based on climatology. Model guidance suggests that 500mb
temperatures will remain near -6 to -7 degrees Celsius, which will
likely support thunderstorm development. As a result, expect passing
showers across eastern Puerto Rico and the U.S. Virgin Islands early
in the morning, followed by afternoon convection across the western
interior to northwestern Puerto Rico. That activity will likely
cause ponding on roads and in poorly drained areas. At the moment,
those areas are being highlighted with a limited flood threat.

By the end of the week into Saturday, a drier weather pattern with
hazy skies will dominate. A drier air mass, accompanied by moderate
to locally high concentrations of Saharan dust particles, will
approach the northeastern Caribbean. Precipitable water values are
expected to drop below 1.5 inches, ranging from below normal to
nearly two standard deviations below normal. In terms of rainfall, a
few afternoon showers will remain possible across western Puerto
Rico, fueled by local effects, but shower activity should remain
limited, with no flooding concerns. Heat will be the primary hazard,
as warmer conditions are expected, along with the potential need for
heat-related products.

By Sunday into early next week, an upper-level low and an associated
lower-level disturbance may move closer to the northeastern
Caribbean, gradually increasing moisture and instability.
Precipitable water values are expected to rise to seasonal levels,
while cooler 500-mb temperatures and more favorable mid-level
conditions could further enhance instability and shower and
thunderstorm development, especially during the morning across
eastern areas and in the afternoon across the western interior and
northwestern quadrant of Puerto Rico.

Overall, residual moisture on Thursday will result in afternoon
showers and thunderstorms. However, hazy, drier, and warmer
conditions are expected to return by the end of the week and into
Saturday, followed by increasing instability from Sunday into early
next week.

&&

.AVIATION...
(06Z TAFS)
Issued at 356 AM AST Mon Sep 7 2026

Mainly VFR conditions are expected across the terminals. Passing -
SHRA/VCSH and isold-VCTS can continue moving over mainly the eastern
terminals. Locally induced SHRA/TSRA can result in brief MVFR
conditions at TJSJ/TJBQ btwn 07/17-21z where PROB30s are in effect.
Winds light and VRB to calm through 07/13z, then becoming E-ESE at
10 to 15 kt, with sea breeze variations and stronger gusts near
SHRA/TSRA.

&&

.MARINE...
Issued at 356 AM AST Mon Sep 7 2026

Moderate easterly winds will gradually strengthen tonight into
Tuesday as a surface high pressure strengthens across the central
Atlantic. Winds of 15 to 20 knots, with higher gusts, will become
increasingly widespread from Tuesday onward, resulting in choppy to
locally rough marine conditions. Seas will generally build to 4 to 6
feet across the offshore waters and local passages, with occasional
seas up to 8 feet possible by Tuesday night and through midweek.
Passing showers will continue across portions of the regional
waters, with locally higher winds and seas possible in and near
thunderstorms.

&&

.BEACH FORECAST...
Issued at 356 AM AST Mon Sep 7 2026

A low risk of rip currents will continue across all beaches through
today. Increasing easterly winds and building seas will result in a
moderate risk beginning tomorrow, Tuesday, mainly along the
northern and eastern beaches of Puerto Rico, Culebra, Vieques, and
the U.S. Virgin Islands. A low risk will persist along the
southern and western beaches of Puerto Rico through at least
tomorrow night. A moderate risk is forecast for most local
beaches by Wednesday night.

Heat will remain an additional concern for beachgoers. Showers and
isolated thunderstorms may also produce brief gusty winds and
lightning, especially during the afternoon.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for PRZ001>011.

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ012-013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION...MRR
LONG TERM...YZR
MARINE/BEACH...ICP

NWS SJU Office Area Forecast Discussion