348
FXCA62 TJSJ 211837
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
237 PM AST Fri Aug 21 2026

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

.KEY MESSAGES...
Issued at 237 PM AST Fri Aug 21 2026

- A Heat Advisory remains in effect until 5 PM AST due to above-
  normal temperatures and elevated heat indices across urban and
  coastal areas of PR and USVI. Similar conditions are expected
  to persist over the coming days.

- Limited showers possible across western PR this afternoon.

- Patches of moisture arriving Saturday and Sunday will likely
  increase the frequency of trade wind showers and afternoon
  convection, with a limited flood threat across western PR.

- A low risk of rip currents is expected through early next week,
  with moderate risk returning by midweek, along with breezier
  winds along most local beaches.

&&

.Short Term(This evening through Sunday)...
Issued at 237 PM AST Fri Aug 21 2026

Mostly fair weather conditions prevailed across Puerto Rico and the
U.S. Virgin Islands this morning, although hazy skies continued due
to suspended Saharan dust particles. Meanwhile, the combination of
warm temperatures and available low-level moisture has already
resulted in heat index values above 100 degrees across many urban
and coastal areas. Several stations across north-central, western,
and southwestern Puerto Rico have already reported heat index values
at or above 108 degrees. Therefore, limited to elevated heat impacts
are expected to persist through the remainder of the afternoon,
particularly across coastal and urban areas.

For the remainder of today, Saharan dust and a relatively stable air
mass will continue to dominate the local weather pattern.
Precipitable water values will generally remain between 1.4 and 1.6
inches, while very dry air in the mid-levels and relatively warm
temperatures aloft should continue to suppress widespread or deep
convective development. Nevertheless, daytime heating, local
effects, and sea-breeze convergence may be sufficient to generate a
few showers across portions of western and interior Puerto Rico this
afternoon. Any activity that develops should remain limited in
coverage and duration. Elsewhere, mostly fair and hazy conditions
should prevail. Breezy easterly winds will continue, especially
along coastal areas, with locally stronger gusts.

Tonight into Saturday, a gradual transition toward a somewhat wetter
trade-wind pattern is expected as the concentration of Saharan dust
diminishes and patches of low-level moisture approach from the east.
Precipitable water values are forecast to increase, periodically
reaching around 1.8 to 1.9 inches. However, moisture will remain
concentrated primarily below 700 mb. In addition, warm 500-mb
temperatures and generally weak low- to mid-level lapse rates will
continue to limit atmospheric instability.

As a result, more frequent passing showers are expected across the
U.S. Virgin Islands and eastern and southeastern Puerto Rico during
the overnight and morning hours as moisture patches move through in
the prevailing easterly flow. During the afternoon, the combination
of increased low-level moisture, daytime heating, and local
convergence should support additional shower development over the
interior and western portions of Puerto Rico. A few thunderstorms
cannot be ruled out with the strongest activity; however, the
relatively stable mid-level environment should limit its overall
intensity and longevity.

A similar variable trade-wind pattern is expected Sunday, with
patches of moisture supporting passing showers across windward areas
during the morning, followed by afternoon development mainly across
interior and northwestern Puerto Rico. Although locally moderate to
briefly heavy rainfall is possible, widespread heavy rainfall is not
anticipated. Despite the increase in moisture and shower activity
this weekend, hot conditions will persist. Heat index values of 108
to 111 degrees will remain possible each afternoon, particularly
across coastal and urban areas. Overall, conditions will transition
from today`s hot, hazy, and relatively dry pattern to a more
variable trade-wind shower regime this weekend.

&&

.Long Term(Monday through Friday)...
Issued at 258 AM AST Fri Aug 21 2026

The main long-term forecast concerns are persistent excessive heat
and the evolution of the mid-level ridge as a couple of weak
tropical disturbances move through the Caribbean. Confidence remains
moderate through Tuesday, then decreases somewhat for the latter
half of the week as model differences become more pronounced.

On Monday, the deterministic GFS suggests the mid-level ridge will
erode by the afternoon. If this occurs, reduced mid-level inhibition
combined with daytime heating and local effects should support a
better opportunity for afternoon convection across the interior and
western portions of Puerto Rico. Farther east, the U.S. Virgin
Islands and windward portions of Puerto Rico should experience
mainly isolated, fast-moving showers. Limited to elevated excessive
heat conditions will remain a concern, particularly across coastal
and urban areas.

By Tuesday, a weak tropical wave is forecast to approach the region.
However, the deterministic GFS and ECMWF differ on the strength of
the mid-level ridge and associated moisture. Nevertheless,
increasing moisture should support showers and isolated
thunderstorms across the U.S. Virgin Islands and eastern Puerto Rico
as early as Tuesday morning, followed by additional convection over
the interior and western portions of Puerto Rico during the
afternoon. Confidence in rainfall coverage and amounts remains
limited, however. The GFS is more aggressive in weakening the mid-
level ridge, while guidance also suggests increasing trade winds. A
faster low-level flow could limit rainfall duration and reduce the
potential for significant accumulations.

Any rainfall would be beneficial given the ongoing drought
conditions across portions of Puerto Rico and the U.S. Virgin
Islands, particularly if precipitation occurs within reservoir
watersheds. However, given the expected progressive nature of the
showers and current model uncertainty, confidence remains low that
this wave will provide meaningful drought relief.

Guidance generally favors the mid-level ridge rebuilding over the
region Wednesday, which should once again suppress widespread
convection while maintaining hot conditions. Another disturbance is
being monitored for Thursday, although current GFS guidance keeps
the bulk of its moisture and associated activity south of the local
islands over the Caribbean Sea. Another possible tropical wave may
approach Friday, but current guidance suggests limited organization
and moisture. Given the lead time and model variability, confidence
in the Thursday-Friday evolution remains low.

Overall, periods of excessive heat will remain one of the primary
hazards during the long-term period, particularly Monday and Tuesday
and potentially into Wednesday. Rainfall opportunities will increase
at times with passing disturbances, but confidence in significant or
widespread accumulations remains low.

&&

.AVIATION...
(18Z TAFS)
Issued at 237 PM AST Fri Aug 21 2026

Mainly VFR conds expected across all terminals thru the period.
Saharan dust will continue to promote HZ, with VIS generally P6SM
through 21/22Z. SHRA/TSRA development over W PR may result in
brief MVFR conds at/near TJBQ/TJPS aft 22/17Z. VCSH expected at
USVI/E PR terminals overnight into Sat AM. E-ESE winds 13-17 kt
with higher gusts, becoming lighter overnight.

&&

.MARINE...
Issued at 237 PM AST Fri Aug 21 2026

Subtropical Atlantic high pressure is sustaining moderate easterly
trade winds across the region. This pattern is expected to continue
through the weekend, with seas of 3 to 4 feet, occasionally reaching
5 feet across offshore waters and local Caribbean passages, and 2
to 4 feet across coastal waters. Occasionally breezy conditions may
persist across the more exposed waters, resulting in locally moderate
chops. Around Tuesday, winds are expected to become moderate to
fresh with the arrival of a wind surge, increasing seas to as high
as 6 feet. The frequency of showers may also increase.

&&

.BEACH FORECAST...
Issued at 237 PM AST Fri Aug 21 2026

A moderate risk of rip currents will prevail tonight along the
beaches of northern and eastern Puerto Rico, meaning life-
threatening rip currents are possible in the surf zones and
beachgoers should exercise caution. By Saturday into early next
week, with slightly lighter winds, the risk is forecast to drop to
low across most beaches of Puerto Rico, Vieques, Culebra, and the
U.S. Virgin Islands. However, remember that even when the risk is
low, life-threatening rip currents can occur near groins,
jetties, reefs, and piers. From Wednesday into the end of next
week, winds are expected to gradually become breezier, with the
risk returning to moderate levels and expanding to most local
beaches. Additional hazards will include heat across all areas and
isolated thunderstorms, particularly across the western beaches
of Puerto Rico during the afternoons.

&&

.FIRE WEATHER...
Issued at 237 PM AST Fri Aug 21 2026

Elevated fire weather conditions will continue through the remainder
of the afternoon, particularly across the southern coastal plains
and other fire-prone areas of Puerto Rico. Low RH values, breezy
E-ESE winds, and dry fuels will continue to favor rapid fire spread.
Conditions should gradually improve this evening as winds decrease
and RH values recover. Increasing moisture and shower activity
from Saturday onward should further reduce the fire weather threat.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION/FIRE...CVB
LONG TERM...CAM
MARINE/BEACH...YZR