543
FXCA62 TJSJ 261838
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
238 PM AST Thu Jun 26 2025

.KEY MESSAGES...

* Moderate concentrations of Saharan dust particles will promote
  hazy skies through at least late tonight, low concentrations
  on Friday. Another round is possible early next week.

* Better chance of showers and isolated thunderstorms on Friday
  and Saturday, mainly across sectors of western Puerto Rico each
  afternoon.

* A tropical wave is expected to approach Puerto Rico and the
  U.S. Virgin Islands by late Saturday into Sunday.

* Breezy to locally windy conditions will continue to promote
  choppy seas and moderate risk of life-threatening rip currents.

&&

.SHORT TERM...Tonight through Saturday...

Mostly fair weather conditions prevailed this morning across the
forecast area under hazy, partly cloudy skies. A few light showers
were observed, resulting in trace to very light rainfall accumulations
across the eastern half of Puerto Rico and Culebra. A moderate
layer of Saharan dust remains in place and is expected to persist
at least through late tonight. A surface high-pressure system over
the Atlantic continues to promote a northeasterly, breezy to
locally windy flow, particularly over the surrounding waters and
coastal areas of Puerto Rico and the U.S. Virgin Islands. Surface
weather stations have reported maximum wind gusts ranging from the
upper 20s to lower 30s mph again today. Daytime maximum
temperatures have reached the upper 80s to low 90s across urban
and coastal areas, while the mountainous regions experienced highs
in the upper 70s to low 80s.

The short-term forecast remains mostly unchanged. This afternoon,
there is a 50% to 60% chance of rain across western Puerto Rico.
However, if showers do develop, they are expected to be brief and
produce minimal accumulations. Tonight, mostly quiet conditions
are expected to prevail, with low temperatures dropping into the
70s to low 80s near the coast, and 60s to 70s in the higher
elevations. Rain chances tonight into Friday morning from 10 to
30% across eastern areas of Puerto Rico, USVI and surrounding
waters.

By tomorrow, Friday, the greater concentrations of Saharan dust
will shift westward, resulting in improved air quality across the
region, with only low concentrations lingering. At the same time,
a gradual increase in low-level moisture is expected. Latest
guidance indicates precipitable water values rising to between
1.50 and 1.65 inches (near seasonal averages) with localized
values potentially reaching up to 1.85 inches during the afternoon
in western Puerto Rico. As a result, variable weather conditions
are expected, with partly cloudy skies and passing showers in
eastern windward areas early in the day, followed by afternoon
showers driven by local effects and available moisture. Winds will
have a more northeasterly component from the surface up to 500
mb, focusing afternoon convection mainly over the southwestern
portions of Puerto Rico. From Friday night into Saturday, similar
weather conditions are anticipated. However, winds at the lower
levels are expected to veer from the east to southeast. Aloft,
conditions will become more unstable and conductive to shower
development due to the proximity of an upper-level trough,
although moisture levels are expected to remain near average.
Saturday will likely feature passing showers early, followed by
scattered to numerous showers across the interior and northwestern
parts of Puerto Rico in the afternoon. Temperatures at 500 mb
will range from near to slightly above average, around -5C to
-6C, which could support the development of a few thunderstorms
during the afternoon convection. The flood risk for Friday and
Saturday will range from limited to elevated in the western
interior and southwestern areas during the afternoon. This means
there is a possibility for ponding of water on roads and in poorly
drained areas, as well as urban flooding and localized stream
rises. A limited heat risk will likely prevail through the
forecast period across urban and coastal areas. This level of heat
affects primarily those individuals extremely sensitive to heat,
especially when outdoors without effective cooling and adequate
hydration.

.LONG TERM...Sunday through Thursday...
.PREV DISCUSSION... /issued 457 AM AST Thu Jun 26 2025/

A somewhat unstable and wet weather pattern is expected to unfold
across the region beginning this weekend. A retrograding mid- to
upper-level trough/low over the Mona Passage will continue to move
westward over the Bahamas. At the same time, a tropical wave
approaching from the east will bring a notable surge in tropical
moisture across the forecast area. This interaction will enhance
the potential for showers and thunderstorms, particularly Sunday
afternoon into the evening hours, with the greatest risk for
flooding rains concentrated during that period. Residents and
visitors should monitor for localized flooding, especially in
areas with poor drainage or near steep terrain.

Following this unsettled period, conditions are expected to
stabilize as a mid-level ridge builds and holds over the northeast
Caribbean by early next week. This feature will establish a trade
wind inversion, promoting subsidence and a drier atmosphere
aloft, which will significantly limit deep convective development
through midweek. Additionally, a Saharan Air Layer (SAL) is
forecast to arrive behind the tropical wave, resulting in hazy
skies, degraded air quality, and reduced visibility from Monday
through at least Tuesday. While a tropical wave will be moving
southward well away from the area, the local weather will remain
influenced by pockets of moisture embedded within easterly trade
winds, occasionally producing brief showers across windward
coastal regions, especially during nighttime and early morning
hours. Afternoon convection will remain localized, mainly
affecting western Puerto Rico due to diurnal, sea breeze and
topographic influences.

Temperatures are expected to trend above normal, especially under
the influence of the SAL, which will reduce overnight cooling and
result in warmer minimum and maximum temperatures across the
islands.

&&

.AVIATION...
(18Z TAFs)

VFR conditions across all TAF sites. Expect hazy skies with
visibilities around 6SM due to a SAL, which will limit rainfall
activity until 27/10Z when the dust particles move out of the
region. Winds will remain from the E at 15 to 18 knots with gusty
winds decreasing at 26/23Z, and increasing again by 27/13Z across
all TAF sites.

&&

.MARINE...

A surface high-pressure system over the Atlantic will support
moderate to fresh easterly winds, resulting in choppy seas across
the local Atlantic and Caribbean waters. The presence of suspended
Saharan dust will continue to reduce visibility through late
tonight. A trade wind perturbation is expected to arrive around
Friday, increasing the potential of isolated to scattered showers
and thunderstorms, followed by another tropical wave late Saturday
into Sunday.

&&

.BEACH FORECAST...

Wave heights of around 3 to 5 feet are expected across the nearshore
waters, and around 4 to 6 feet across the offshore Atlantic and
Caribbean waters, as well as the local passages, driven by breezy
to locally windy conditions. As a result, a moderate risk of rip
currents is expected tonight and over the next few days along most
beaches of Puerto Rico, including Vieques, Culebra, and the U.S.
Virgin Islands. A moderate risk indicates that life-threatening
rip currents are possible in the surf zone. Beachgoers are therefore
encouraged to exercise caution.

&&

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

$$

SHORT TERM...YZR
LONG TERM...CAM
AVIATION...LIS