801
FXCA62 TJSJ 180711
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
311 AM AST Tue Aug 18 2026

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

.KEY MESSAGES...
Issued at 248 AM AST Tue Aug 18 2026

- Dry conditions will persist today as a drier air mass dominates
  the region, limiting the shower activity across the islands.

- A Heat Advisory is in effect from 10 AM to 5 PM AST for urban
  and coastal areas of Puerto Rico and the U.S. Virgin Islands.

- By tonight into Wednesday, a vigorous tropical (formerly Invest
  94L) will approach the islands, increasing the potential for
  showers and thunderstorms.

- A low to moderate risk of rip currents is expected across local
  beaches throughout the workweek.

&&

.Short Term(Today through Thursday)...
Issued at 248 AM AST Tue Aug 18 2026

Fair weather conditions prevailed overnight across Puerto Rico and
the U.S. Virgin Islands, as a dry, stable air mass continues to
dominate the region. Once again, it was another warm night across
the islands, with overnight lows in the upper 70s to low 80s across
lower elevations and in the 70s across higher elevations. Winds were
light and variable at 10 knots or less.

Another dry, warm day is expected today; however, a gradual
transition to a wetter, more unstable pattern is anticipated
beginning this evening into Wednesday as a vigorous tropical wave
(formerly Invest 94L) approaches the region. Precipitable water
(PWAT) values will remain below normal today at 1.35 to 1.50 inches,
limiting overall shower activity. Nevertheless, localized afternoon
convective activity remains possible across portions of western and
central Puerto Rico due to the combination of daytime heating and
local effects. As the day progresses, PWAT values will gradually
increase with the arrival of the tropical wave, increasing to above-
normal values near 2.25 inches by Wednesday morning.

This tropical wave is expected to interact with a deep-layer trough,
increasing the potential for moderate to heavy rainfall and
thunderstorms across the region. Recent model guidance continues to
show some uncertainty regarding how much of the moisture will
directly impact the islands, and the amount of rain. However, if the
current forecast materializes, this setup will favor periods of
moderate to locally heavy showers across the U.S. Virgin Islands and
eastern Puerto Rico, followed by additional afternoon activity
across interior and western Puerto Rico. Localized urban and small-
stream flooding is possible if the heavy rainfall develops. Stronger
thunderstorms could also produce frequent lightning and locally
strong, gusty winds. Any rainfall associated with this wave would be
beneficial given ongoing dry conditions across portions of the
region, providing welcome relief. Please continue to monitor
forecast updates closely today as conditions evolve.

Following the wave`s passage by Thursday, conditions will rapidly
dry out as a drier, stable air mass establishes over the islands.
This air mass, accompanied by low concentrations of Saharan dust,
will persist through the weekend.

An elevated heat risk will affect coastal and urban sections of
Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands today,
where a Heat Advisory remains in effect from 10 AM to 5 PM AST.
These conditions significantly increase the risk of heat exhaustion
and heat stroke, especially for young children, the elderly, and
anyone spending extended time outdoors. Stay hydrated, limit direct
sun exposure, and take frequent breaks in the shade or air
conditioning. Brief relief from the hot temperatures is expected on
Wednesday if the forecast materializes and cloudiness with
widespread rain persists over the region.

&&

.Long Term(Friday through Tuesday)...
Issued at 248 AM AST Tue Aug 18 2026

Drier to seasonal conditions may persist this weekend, with
uncertainty increasing by the end of the period. The wind pattern
may vary, as a surface high building over the Western Atlantic will
likely promote E-ENE winds on Friday, becoming from the E-ESE Sunday
onwards as a weak tropical wave approaches the local area. Friday will
likely be the "driest" day of the period, as model guidance suggests
a drier airmass, along with concentrations of Saharan Dust,
filtering into the region. Based on the latest deterministic
guidance of the GFS and ECMWF, PWAT values should drop to below
climatological normals (between 1.3 and 1.5 inches), as moisture
content drops particularly in the mid levels (below 40%).
Additionally, the proximity of a mid-level ridge (north of the
Hispaniola) should lead to warmer-than-normal mid-level temperatures
(500 mb temperatures up to -4 degrees Celsius), subsidence and
stability aloft. Although daytime heating combined with local
effects should trigger afternoon convection over the western side of
Puerto Rico and local island streamers, rainfall accumulations will
likely be limited under this pattern, resulting in no flooding
threat. By Saturday, moisture content is likely to increase as a
perturbation in the trades moves over the CWA, with PWAT values up
to 1.80 inches, mainly observed in the 850 - 700 mb layer. Based on
the latest model guidance, passing showers should move over windward
sections of the islands, during the morning, with afternoon
convection over interior and western Puerto Rico. With Saharan Dust
concentrations diminishing, conditions look more favorable for
showers, without ruling out isolated thunderstorms, over the
aforementioned areas. Nevertheless, rainfall accumulations should
lead to ponding of water over roadways and poorly drained areas. A
similar pattern is expected on Sunday as lingering moisture and
winds veering from the E-ESE may enhance deep convection over the
northwestern side of Puerto Rico. Monday`s forecast remains
uncertainty as global model solutions suggests different outlook.
From the latest GFS solution, a tropical wave moving over the
Central Atlantic may become a cyclonic system and move northeast of
the CWA, while the ECMWF`s solution suggests that this system
shifting northward, far away of the region. Therefore, uncertainty
will remain high for the beginning of the next workweek.

Seasonal to warmer-than-normal conditions are very likely in the
long-term, with Sunday being the "hottest" day of the week as winds
shift from the E-ESE. With the available moisture content, heat
indices will very likely exceed 100 degrees Fahrenheit, particularly
over urban and low-lying areas. Therefore, the heat threat will
remain elevated to significant throughout the forecast period.
Residents and visitors should stay hydrated and avoid prolonged
outdoor activities.

&&

.AVIATION...
(06Z TAFS)
Issued at 248 AM AST Tue Aug 18 2026

Mainly VFR conds expected across all TAF sites. Generally fair
weather conditions will prevail across the region with aftn SHRA
across western PR. Light winds at 10 kt or less through
18/13Z. Winds E-ESE at 10-15 kt with sea breeze variations and
ocnl gusts up to 25 kt through 18/23Z. SHRA/VCSH expected to
increase aft 18/23Z at TJSJ, TIST, TISX as a vigorous tropical
wave approaches the islands.

&&

.MARINE...
Issued at 248 AM AST Tue Aug 18 2026

Light to moderate easterly trade winds will continue to
prevail across the region over the next few days. Seas peaking up to
5 feet today will gradually ease to 2 to 4 feet later in the
workweek. By late tonight into Wednesday, an approaching tropical
wave will draw in deep moisture, increasing the potential for
showers and thunderstorms across most local waters. This will
elevate localized lightning hazards and bring gusty winds over the
marine zones through midweek. Also, expect hazy skies across the
northeastern Caribbean due to suspended Saharan dust, with higher
concentrations after the wave passage.

&&

.BEACH FORECAST...
Issued at 248 AM AST Tue Aug 18 2026

A low to moderate risk of rip currents is expected through
most of the workweek as seas continue to subside to around 2-3 feet
across the coastal waters of the islands. However, remember that
even when the risk is low, life-threatening rip currents can occur
near groins, jetties, reefs, and piers. The lightning potential will
increase on Wednesday due to thunderstorm activity associated with
the arrival of a tropical wave. Low to moderate concentrations of
Saharan dust will keep sky hazy before the wave passage, with higher
concentrations expected from Thursday onward.

&&

.FIRE WEATHER...
Issued at 248 AM AST Tue Aug 18 2026

Critical fuel conditions persist across the islands, with extreme
drought affecting portions of eastern Puerto Rico and the southern
coastal plains. Dry conditions will persist across the region with low
relative humidity, stressed vegetation, and dry fuels continuing
to support elevated fire danger.

The greatest concern remains across southern and southwestern Puerto
Rico, where fuels are exceptionally dry and recent fire activity has
been frequent. A gradual increase in moisture is expected later in
the afternoon, as a vigorous tropical wave approaches the islands.
Therefore, a Fire Danger Statement is in effect from 10 AM to 5
PM AST for the southern and southwestern plains of Puerto Rico.
Residents and visitors are strongly urged to avoid outdoor
burning, as conditions will easily support wildfire ignition and
rapid spread.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.

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

AM...None.
&&

$$

EVE SHIFT...DSR/MNG
MID SHIFT...GRS