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433
FXCA62 TJSJ 201702
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
102 PM AST Mon Jul 20 2026

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

.KEY MESSAGES...
Issued at 100 PM AST Mon Jul 20 2026

* A Heat Advisory has been issued for certain coastal and urban
  areas of Puerto Rico and the U.S. Virgin Islands based on recent
  observations. This afternoon, heat indices are expected to
  exceed 107F. The heat threat is projected to intensify further
  on Tuesday and Wednesday as increased tropical moisture returns.
  Additional Heat Advisories or even Extreme Heat Warnings may be
  issued for some lower elevation areas.

* Hazy skies associated with Saharan dust will persist through
  the rest of today, with conditions gradually improving later
  this afternoon.

* A shift to a more unsettled weather pattern is anticipated from
  Tuesday through Wednesday, with additional changes expected
  later in the weekend and early next week as environmental
  conditions become more favorable for showers and isolated
  thunderstorms, especially in the afternoons.

* There is also a slight risk of localized urban and small-stream
  flooding. Elevated fire weather conditions will persist today,
  as critically dry vegetation, low relative humidity, and breezy
  conditions continue to support rapid fire spread. Multiple fire
  incidents have already been reported across Puerto Rico,
  highlighting the ongoing fire danger.

* The prevailing easterly winds will maintain a moderate risk of
  life-threatening rip currents across many beaches of Puerto
  Rico and the U.S. Virgin Islands throughout much of the week.
  Beachgoers should swim near lifeguards, heed local beach
  warnings, and never swim alone.

&&

.Short Term(This evening through Wednesday)...
Issued at 100 PM AST Mon Jul 20 2026

A warmer-than-normal morning was observed across Puerto Rico and
the U.S. Virgin Islands, with minimum temperatures remaining in
the upper 70s to lower 80s across most coastal and urban
locations. These warmer overnight temperatures were likely
enhanced by moderate Saharan dust concentrations over the region,
which limited nocturnal cooling. By early afternoon, temperatures
had already climbed into the lower 90s at many urban and coastal
observing sites. Combined with the lingering low-level moisture
associated with yesterday`s tropical wave, the increasing
temperatures have produced heat indices exceeding advisory
criteria, prompting the issuance of a Heat Advisory for portions
of the coastal and urban areas of Puerto Rico and the U.S. Virgin
Islands. Surface winds were generally from the east at 10 to 20
mph, with occasional higher gusts. Visible satellite imagery and
local observations showed mostly sunny skies across much of the
region during the morning, while cumulus clouds developed across
the Cordillera Central and western Puerto Rico in response to
daytime heating, local topographic effects, and sea-breeze
convergence. In addition, trade wind streamers developed downwind
of Vieques, Culebra, St. Thomas, and St. Croix, producing isolated
showers over the local waters and adjacent coastal areas.

Although a drying trend is expected to develop later this
afternoon as a pocket of drier air advects into the region on the
easterly trade winds, local effects will continue to support
afternoon convection across the interior and western sections of
Puerto Rico. Daytime heating, sea-breeze convergence, and
orographic lifting will promote the development of scattered
showers, while trade wind streamers are expected to develop
downwind of the U.S. Virgin Islands. Most of these streamers
should remain over the local waters; however, those originating
from Vieques and Culebra may drift westward and produce passing
showers across eastern Puerto Rico.

The Saharan Air Layer will continue to exit the region this
evening and will gradually be replaced by an induced surface
trough moving westward across the northeastern Caribbean. This
feature will increase low-level moisture and enhance shower
activity from late tonight through Tuesday, with the greatest
coverage expected from mid-morning into the afternoon as local
effects become established. While localized ponding of water and
isolated urban and small-stream flooding cannot be ruled out under
the heaviest showers, the anticipated rainfall will provide much-
needed moisture to portions of Puerto Rico and the U.S. Virgin
Islands that continue to experience rainfall deficits. According
to the latest U.S. Drought Monitor, areas across the islands
remain classified under D0 (Abnormally Dry), D1 (Moderate
Drought), and D2 (Severe Drought).

A transition to a more unsettled weather pattern is expected on
Tuesday as an induced surface trough moves across the northeastern
Caribbean. At the same time, the mid-level ridge will weaken,
allowing moisture to deepen to near or slightly above
climatological levels. This will support more widespread shower
activity and isolated thunderstorms, particularly during the
afternoon across the interior, western, and southwestern portions
of Puerto Rico. Locally heavy rainfall may result in ponding of
water and isolated urban and small-stream flooding.

On Wednesday, low-level winds are forecast to veer to the east-
southeast, shifting the focus of afternoon convection toward
western and northwestern Puerto Rico. This wind shift will also
favor warmer temperatures across the northern and western coastal
and urban areas. As a result, heat indices may reach Heat Advisory
or even Extreme Heat Warning criteria in portions of the lower
elevations. At the same time, scattered afternoon showers and
isolated thunderstorms continue to develop across the interior and
western Puerto Rico.

&&

.Long Term(Thursday through next Sunday)...
Issued at 441 AM AST Mon Jul 20 2026

For the second half of the workweek, upper level lows will continue
to linger northeast and then east-southeast of the region. A surface
high over the Atlantic will reestablish itself and once again result
in up to breezy conditions, especially Friday and Saturday. Current
model guidance has Precipitable water (PWAT) values on a downward
trend to start the long term period as moisture from a surface
trough moves northeast and away from the region under an east-
southeast steering flow. PWAT values should stay within normal
values on Thursday, with locally above normal values being observed
during afternoon convection over mainly W-NW PR. A drier air mass
with below normal PWAT will then quickly move Thursday night into
early Friday, before another pulse of moisture reaches the area
Friday morning. More humid air masses will then move over the area
through the weekend as waves continue moving close to the region,
this will maintain a generally normal PWAT over the islands as
showers continue to be steered towards windward sectors and
afternoon convection (showers and possible t-storms) develop over
mainly the western half of Puerto Rico. At 500 mb, temperatures are
forecast to cool from above-normal values to near-seasonal levels,
around -6C to -7C, late Thursday onwards. Most available moisture
should remain below 800mb through at least Saturday while 250 mb
heights remain at seasonal values.

By late Sunday through Monday, current guidance suggests that a
tropical wave and wind surge will reach the area and elevate PWAT
values to above normal values and result in breezy to locally
windy E-SE flow. This will increase the flooding risk as quick
moving passing showers reach the region and afternoon convection
develops over mainly W-PR and downwind of El Yunque and the local
islands. As an upper trough moves over the region, available
moisture will also reach the upper levels. Warm to hot conditions
will persist through at least the weekend, with an elevated heat
risk forecast for coastal and lower elevations of the islands. In
terms of Saharan Dust, low to moderate concentrations will
continue to filter over the islands through the second half of the
week and early next week, with guidance suggesting lower
concentrations as the above mentioned moisture reaches the islands
late Sunday and Monday. Continue to monitor the forecast for
updates.

&&

.AVIATION...
(18Z TAFS)
Issued at 100 PM AST Mon Jul 20 2026

VFR conds across all TAF sites. HZ will gradually diminish
through 07/23Z. E-NE winds will continue, btwn 15 - 20 kt, gusty
winds up to 30 kt, and weakening btwn 5 - 10 kt. Expect VCSH
across TIST, TISX, and TJSJ from 21/06Z through 21/13Z.

&&

.MARINE...
Issued at 100 PM AST Mon Jul 20 2026

A high-pressure system over the central Atlantic is expected to
bring moderate to fresh east to east-southeast winds, along with
choppy seas, to the region through this evening. An induced
surface trough will enhance the likelihood of showers and
thunderstorms, as well as increase sea conditions and wind
strength near any thunderstorms on Tuesday and Wednesday.
Additionally, another tropical wave is forecast to move near the
Lesser Antilles by Thursday and approach the region by Friday.

&&

.BEACH FORECAST...
Issued at 100 PM AST Mon Jul 20 2026

Easterly trade winds will enhance wave-breaking action along many
beaches in Puerto Rico (PR) and the U.S. Virgin Islands (USVI)
this week, leading to a moderate risk of life-threatening rip
currents at north-, east-, and south-facing beaches through the
weekend. Beachgoers should swim near lifeguards, obey beach
warning flags, and never swim alone.

There is also a risk of extreme heat, especially from late
morning to afternoon, due to high temperatures, humidity, and
limited shade. To minimize heat-related illnesses, beachgoers
should stay hydrated, seek shade, and apply sunscreen regularly.

Starting Tuesday, afternoon showers and isolated thunderstorms are
expected, particularly in western Puerto Rico. While most beaches
should remain dry, beachgoers should be cautious of thunderstorms,
as lightning can strike far from rain. If thunder is heard, leave
the beach and seek shelter in a substantial building or hard-
topped vehicle.

&&

.FIRE WEATHER...
Issued at 100 PM AST Mon Jul 20 2026

A Fire Danger Statement remains in effect for the local fire
weather zones. Observations continue to support elevated fire
weather conditions across the southern half of Puerto Rico and the
U.S. Virgin Islands. Critically dry fuels and persistent soil
moisture deficits persist across portions of the islands, with
Keetch-Byram Drought Index (KBDI) values exceeding critical
thresholds. Multiple fire incidents have been reported across
Puerto Rico today, highlighting the heightened fire danger.
Combined with breezy easterly trade winds, any new fires that
develop could spread rapidly through dry vegetation, particularly
across Puerto Rico`s southern coastal plains. Residents and
visitors are urged to avoid outdoor burning and to exercise
extreme caution with open flames, discarded cigarettes, and any
activity that could generate sparks.

&&

.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.
&&

$$

MORNING SHIFT...CAM/MMC
EVENING SHIFT....MRR/YZR

NWS SJU Office Area Forecast Discussion